Using Integrated Circuits in Chemical Instrumentation - ACS Publications

REPORT FOR ANALYTICAL CHEMISTS

Using Integrated Circuits in Chemical Instrumentation J O H N S. S P R I N G E R Fairchild S e m i c o n d u c t o r Mountain View, Calif. 9 4 0 4 0

A FEW YEARS AGO articles

about

analytical chemistry dealt with new complexing agents, new separation methods, and other laboratory operations with only a n occasional paper on instrumental analysis. Until about 1955 an operational amplifier was a fairly exotic device, not something t h e ordinary laboratory could obtain or even know how to use. T h e computer age had only begun t o exercise its influence on laboratory methods. When t h e transistorized operational amplifier was developed, this powerful electronic tool was suddenly available t o everyone. T h e symposium on operational amplifiers which appeared in ANALYTICAL C H E M I S T R Y in 1963 (1), perhaps marked t h e introduction of modern electronics t o the laboratory. N o w the journals contain m a n y articles dealing with new instruments. T h e illustrations are laden with integrators, differentiators, followers, all made from operational amplifiers, and in the past few months, digital circuits have begun t o appear also. M a n y researchers have remained close enough t o t h e electronics industry to feel the breezes created by the whirlwind of activity of the past few years. Most of this dramatic activity has centered around " i n tegrated circuits," a n d this article will a t t e m p t t o show just how t h e integrated circuit can be used by the analytical chemist.

(a) Figure 1. (a) Photomicrograph of chip containing over 100 transistors. (b) Wafer containing many integrated cir-

Advantages of Integrated Circuits

M a n y chemists m a y be asking themselves now, " W h y should I be concerned with a n integrated circuit? M y job is analytical chemistry; let t h e electronic engineer take care of t h e electronics." T h e

(&)

Integrated circuits provide the analytical chemist with versatile powerful building blocks with which he can build up very complex systems with ease. This approach to system design frees the analyst to spend his time on decisions about methods to use and analysis of results

electronic engineers have taken care of the electronics very well; so well, in fact, that the chemist can use electronics in the form of an integrated circuit, without being an electronic engineer. The integrated circuit is a self-contained package, a black box with a few inputs and outputs. The chemist can use it without being concerned with how it does what it does. But the integrated circuit can make it possible for the chemist to do things which would not have been possible for him before. You can decide what kind of a function you need and you can implement it, simply and cheaply. The big difference between the older tube-type operational amplifier and the transistor version was cost. The price of an op amp dropped nearly an order of magnitude; the integrated circuit (IC) op amp has resulted in the same change. A modern operational amplifier can be purchased for as little as $4.00 in single unit quantities. This is surely within the reach of every laboratory. The digital circuits provide logic functions which operate at very high speed and cost very little. All the difficult problems of logic design, such as good noise margins and high speed over large-temperature ranges have been taken care of within the IC. The user has only to connect the functional blocks together. The integrated circuit is inexpensive for the simple reason that the entire circuit is made at one time. The resistors and transistors are made by diffusing impurities such as boron, phosphorus, and arsenic into a flat piece of silicon crystal.

The silicon is first coated with a polymeric substance called "photo resist." Then the photo resist is exposed, under a mask, much like a photographic negative. The exposed photo resist is washed away, and the impurities are diffused through the uncoated regions. The process is repeated several times with different impurities to build up the transistors. A layer of Si0 2 is then deposited over the entire surface. A masking operation is used to etch holes in the S1O2 to make connections to the devices, and aluminum strips are deposited to make the wires and the connections. The completed circuit is coated with Si0 2 for protection. The circuit is on the order of 0.1 by 0.1 in. and there are hundreds of them made simultaneously on a single 2-in. diameter piece of silicon called a "wafer." After testing, the individual circuits are cut apart so that each circuit is on a single "chip." The chips are mounted in packages, and wire bonds are made from the chip to the pins on the package. Because everything is made at once, the same process can produce a few transistors or a hundred transistors on a single chip at about the same cost. The difference in cost between a small circuit and a complex one is mainly due to the greater area required by a larger circuit. A large area increases the probability of a defect on the chip, and hence reduces the yield of good circuits on the wafer. The effect of reduced yield is, of course, increased unit price. Figure la is a photomicrograph of a chip containing over 100 transistors. Figure 16 shows a wafer containing many integrated circuits.

Linear Integrated Circuits

Linear Integrated Circuit means an amplifier, generally an operational amplifier. The basic theory and construction of the operational amplifier are discussed in many publications {2-7). Especially valuable discussions may be found in {2) and (7). The integrated circuit op amp differs from the transistorized and tube versions in several respects. The tube models required well-regulated high voltages; IC's require only low voltages, generally ± 1 5 V, and the power supplies usually need not be highly regulated. The power consumed by an IC op amp is very low; some types, especially designed for battery-powered instrumentation, use only a few microwatts of power. The new IC op amps have very low drift with temperature and time and can withstand short circuits on the output without damage. Of course the IC op amp is very much smaller than its discrete component counterpart. The disadvantages of the IC op amp are that it generally can operate only over a limited voltage range (about ± 1 0 V), that it can supply only a few milliamps of current at the output, and that its input impedance is usually less than 1 MO. The low current capabilities are due to the difficulty of dissipating the heat produced by highpower devices. The impedance property is the principal limitation of the device in chemical instrumentation, but there are ways around the problem. The low-input impedance is a result of the technological limitations of the integrated circuit fabrication, but, as the state of the art pro-

ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970

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23 A

Report for Analytical Chemists

1 0 _ 4 V will produce an output of 1 V. The gain falls as the frequency at which the amplifier is operated increases. A curve similar to t h a t in Figure 2 is usually shown in the manufacturer's d a t a sheet. When feedback is used to set the gain of the amplifier in a particular con figuration, the gain vs. frequency curve will be flat at the set value until the "open loop gain" is inter sected. Then the gain will fall off with frequency. T h e gain of the amplifier can never exceed the open loop gain. Figure 2. Typical curve showing drop in gain with increase in frequency

gresses, high impedances are being achieved. Several high-impedance amplifiers are on the market, but they are still quite expensive. There are several devices available which consist of an operational amplifier chip and some additional parts to raise the input impedance, all contained in one package. A component like this, which contains more than one chip in a single pack age is called a hybrid. Hybrids with very high-input impedances are frequently identified as " F E T (Field Effect Transistor) I n p u t H y brid Operational Amplifier." They cost a little more t h a n the amplifier alone. For most applications the extra input impedance is not re quired. Characteristics of Linear Integrated Circuits

The first rule in buying an opera tional amplifier is not to overbuy. Amplifiers are available with a wide range of characteristics and prices. Listed below are some of the speci fications which are important to the user when selecting the proper amplifier. 1. Gain. The gain of the ampli fier is the constant A in the charac teristic equation βουτ = —Α. (θι — e 2 ). The number applies for a dc voltage difference a t the input and for no feedback on the amplifier. The amplifier is rarely used in this "open loop" configuration. The gain is expressed in decibels, which is 20 times the log of the gain. If the open loop gain of the amplifier is 80 db, then the gain is 10 4 . A voltage difference on the input of 24 A ·

2. Input Impedance. This figure varies widely from one device to another. T h e inexpensive ampli fiers generally have impedances on the order of 750,000 Ω. More ex pensive amplifiers and hybrids have higher impedances. T h e impedance needed depends on the configuration in which the amplifier is to be used. High impedances are required when very low currents are involved, be cause the proper operation of the circuit depends on the assumption that current flowing into the ampli fier input is negligible. The recip rocal of the input impedance times the gain is approximately the cur rent (in amps) which will flow into the amplifier input. 3. Offset Voltage. The offset voltage is the voltage difference be tween the two inputs required to produce 0 volts at the output. Ideally it is zero. Virtually all in tegrated circuit op amps have pro vision for a "balance" control which allows the user to null out the offset voltage. 4. Bias Current. T h e bias cur rent is the minimum current which must be supplied a t an input to make the amplifier work. T h e in put impedance figure holds only af ter this minimum current has been supplied. T h e bias current is only a few nanoamps and frequently can be ignored. When this small cur rent is not negligible it should be supplied by a large resistor from the power supply, so t h a t the circuit will not have to supply the current. T h e currents supplied to the two amplifier inputs should always be made equal. This means t h a t re sistors on the two inputs should have about the same value. If only the inverting input is to be used, the noninverting input should be


grounded through a resistor whose value is equal to the parallel com bination of all the resistors tied t o the inverting input, as shown in Figure 3. 5. Slew Rate. In dc applications the slew rate is usually not critical. The slew rate is the speed at which the output can change voltage. For most amplifiers this number is about 2 V//usec. 6. Input Voltage Range. T h e in puts to an amplifier m a y be dam aged if the voltage on them exceeds either supply voltage or if the dif ference between the two inputs ex ceeds some value, usually about 10 V. 7. Frequency Compensation. Most of the older op amps, such as the μΑ709 (μ for microminiaturiza tion), require the addition of a re sistor and two capacitors to sta bilize the device. Without these components the amplifier will break into high-frequency oscillation. The values to use for the resistor and capacitors are given on the data sheets by the manufacturer of the amplifier. The most common amplifiers of this type have p a r t numbers containing "709" or "101." The newer amplifiers, "741," are fully compensated internally and are no more expensive t h a n the older type. If an amplifier breaks into oscil lation even though "compensated," the device is probably being used with large resistances which have upset the compensation somehow. Frequently the oscillations can be stopped by placing a few picofarads of capacitance across the feedback resistor. Basic Operational Amplifier Circuits

The operational amplifier is an extremely powerful tool. T h e fol-

Figure 3. For impedance matching, the resistors at the amplifier inputs should be equal


lowing circuits illustrate some of the uses that can be made of it: 1. Summer (Figure 4). The sum mer is perhaps the most common application of an operational am plifier. The output is equal to the sum of all the input voltages in pro portions determined by the resis tors. If all resistors are equal, then the output is the straight sum of the inputs. 2. Integration (Figure 5). In the integrator, current trickles from the input voltage, eIN, through re sistor R and on to capacitor C. The output voltage adjusts itself to remove an equal quantity of charge from the other side of the capacitor. The result is an inte gration of the input voltage. If a constant voltage is applied to the input, the output voltage will in crease at a constant rate. A steadily increasing voltage is called a "ramp." The integration con stant is 1/RC, where RC is the "time constant." The ramp can be reset to zero by shorting the ca pacitor momentarily. A circuit like this can be used to generate a polarographic sweep. 3. Differentiator (Figure 6). The differentiator is very similar to the integrator, with the resistor and capacitor reversed. A con stant input voltage produces zero output voltage. A step change on the input will produce a spike on the output. This function is useful for detecting inflection points, as in an automatic titration. Dif ferentiators are very susceptible to electrical noise. Noise response can often be improved by placing a small capacitor (C2) across the feedback resistor. 4. Follower (Figure 7). The voltage follower is one way to in crease impedance in a circuit. The

output voltage is identical to, or "follows," the input voltage. Cur rent can be drawn from the output, but negligible current is consumed at the input. A circuit like this is frequently used in conjunction with an electrode system. The potential of an electrode may be measured or used to perform operations with out drawing any current from the electrode. The follower is an ex ample of an electrical "buffer." 5. Log Amplifier (Figure 8). The logarithmic amplifier is useful in two kinds of applications. The first are those in which a logarith mic characteristic is desired, as, for example, in converting transmittance to absorbance. The second group of applications are those in which a very wide range of signal levels must be handled. It has been proposed that spectroscopists make use of log A, because peaks with low absorbance can be more easily seen without pushing high absorbance peaks off scale. The circuit shown requires two amplifiers and two matched transis tors. While it is possible to get two matched transistors by manually testing and sorting, it is also pos sible to buy the transistors on a single chip as an integrated circuit. Several manufacturers sell matched transistors in one package. A sav ings in cost can also be realized by purchasing an integrated circuit containing two operational ampli fiers. A dual op amp costs less than two single op amps. The log circuit is one in which impedance is important. A higher input impedance (resulting in im proved linearity) can be achieved by using an FET input amplifier in place of A\. An FET input, un fortunately, will also increase the drift of the circuit with tempera-

Figure 4. Op amp used to obtain weighted sum of voltages 26 A

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Figure 5.

Integrator

Figure 6.

Differentiator

Figure 7.

Voltage follower

ture (FET's are notoriously tem perature sensitive). An alterna tive method, which raises input im pedance less than an FET, but which improves temperature sta bility markedly, is the use of a temperature-compensated pream plifier. The μΑ727, for example, contains its own oven which holds it at a constant temperature. 6. Current-to-Voltage Conver ter (Figure 9). This circuit con verts a current into a proportional voltage. The inverting input of the amplifier is at virtual ground. The circuit is useful with polarographic cells, as shown in Figure 10. In applications like this, it is once again important to have a high in put impedance on the amplifier. The bias current should be supplied externally to minimize the currents actually flowing into the amplifier input from the cell. If the cell ex hibits a very high impedance, an FET input may be called for (see circuit 7).


Figure 8.

Figure 9.

Logarithmic amplifier

Current-to-voltage converter

Figure 10. Current-to-voltage converter for polarographic cell

7. FET Input (Figure 11). The field-effect transistor is extremely useful when very high impedances are required. The leads shown as inputs are the "gates" of the tran sistors in the circuit above. There are two types of FET's. A junc tion FET has a gate impedance of ΙΟ10 Ω; an MOSFET has a gate im pedance of ΙΟ12 Ω or more. Special wiring precautions must be taken with FET input stages, and it is probably safer to purchase a hy brid amplifier than to construct one from an op amp and two field-effect transistors. The principal disad vantages of FET's are that they tend to introduce noise into the cir cuit and that they drift with tem perature changes. (If matched FET's are used in the circuit shown, the temperature drifts cancel.) The functions shown above form the basic collection of analog op erations used in instrumentation. They all require only an inexpen sive operational amplifier and a few additional components. In the last section of this paper, these "building blocks" will be used in some ex amples of instrumentation.

Figure 11. FET input stage for op amp Digital Integrated Circuits

Chemists have been using com puters for years, but only recently have they begun to use digital build 28 A ·


ing blocks in their own designs. Digital circuits tend to be used in two different kinds of applications. One is simply to generate logic func tions for control purposes. (Ex ample: Turn off titration when "off" button is pushed or when in flection point is crossed or when buret is empty.) The other appli cation is in data processing; these circuits tend to be quite complexing and are not usually used in analyt ical instruments except for count ing and digital readouts. Digital data processing is used when high accuracy and speed must be ob tained and when the instrument is to be connected directly to a com puter. An excellent reference to the use of digital electronics is Malmstadt's new book (8). Digital-integrated circuits were the first type of integrated circuit produced. The simplest logic gate consists of two transistors and three resistors and was a natural succes sor to the mass-produced transistor. Digital circuits are binary—they can exist in only two states: on or off. The inputs and outputs have one of two possible voltage levels. The logic low is about at ground, and the logic high is a few volts positive. The term 1_ will be used to refer to a logic high and 0 will mean logic low in this paper. Logic Expressions. There are only three logic expressions. From these (really from only two of the three) any logic function can be generated. The three expressions are AND, OR, and NOT. Consider the expression AND. What are the characteristics of a logic gate whose equation is Ζ = A AND B? This equation means the output, Z, will be high if, and only if, both inputs A and Β are high. The logic symbol and truth table are shown in Figure 12. If Ζ = A OR B, then Ζ will be high if A or Β or both are high. The "OR" that is used in English language is generally the Exclu sive OR; it implies one or the other of two alternatives, but not both. The logic OR is an inclusive OR; it implies that at least one of the possible alternatives is true. The OR operation is indicated by a plus sign. Note the distinction be tween the symbol for the OR gate,

Report for Analytical Chemists Z = AB

Figure 13, and t h a t for the A N D gate. If Ζ = N O T A, then Ζ and A are always opposite or "complemen t a r y . " The N O T function is simply inversion of a signal. " N O T " is symbolized by a bar over the ne gated expression, as illustrated in Figure 14. The symbol for the inverter con sists of two parts. T h e triangle rep resents the amplification of the sig nal which is present in any I C logic gate. Amplification prevents de terioration of signal quality through gates. The amplifier symbol is fol lowed by a little circle at the out put. I t is this circle which indi cates inversion. I t m a y be thought of as meaning "active level low"— i.e., when the input conditions are satisfied, the output is low instead of high. Several logic symbols are shown in Figure 15 t o illustrate t h e use of the circle. Real I C gates consist of two parts. First the logic is performed (AND or OR) and then amplifica tion occurs to return the logic levels to their specified values. Amplifi cation always results in inversion,

so real I C gates always come out active level low. The two gating functions commonly used are active level low A N D (which is N O T A N D , or N A N D ) and active~low OR (which is N O T OR, or N O R ) . T r u t h tables for these gates are shown in Figure 16. Note t h a t N A N D is a 0 if all inputs are l's and N O R is a 1, if all inputs are O's. Identities. The identity rules for logic combinations are given below. Note the similarity to the arithmeti cal operations expressed with the same symbols.

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Β Ι Ζ

0 0 1 1

0 1 0 1

0 0 0 1

Figure 12. Logic s y m b o l , equation, and t r u t h table for AND function

Ζ - A +B Α

Β Ι Ζ

0 0 1 1

0 1 0 1

0 1 1 1

Figure 13. Logic symbol, equation, and t r u t h table for OR f u n c t i o n

1. Double inversion

Έ= Χ 2. Distributive A (B-\-C) = AB + AC 3. Associative A + (B + C) = (A + B) + Ç (AB)C = A{BC) 4. DeMorgan's laws A+B=AΒ AB = A + B The last two are extremely im p o r t a n t rules. N o t e t h a t A -\- Β is not the same as A + B. "Neither A N O R B" is not the same as " N O T A OR N O T B." DeMorgan's laws are shown symbolically in Figure 17. Digital integrated circuits are produced in several families. T h e two most important are R T L (Re sistor-Transistor Logic) and T 2 L (Transistor-Transistor Logic). T h e basic gate function in R T L is N O R ; in T 2 L, N A N D . Resistor-Transistor Logic

John Springer received his bache lor's degree in chemistry from the College of Idaho in 1967 and his master's degree in analytical chem istry from Oregon State University in 1969. He is currently employed at Fairchild Semiconductor as a digital systems engineer. His re sponsibilities include applications support and logic design of digital integrated circuits.

Α

R T L was formerly the least ex pensive t y p e of integrated circuit logic. As of early 1970, this is no longer true, but it is worth discuss ing a n y w a y as an introduction to I C logic because of its widespread use in designs of the past few years. T h e devices are used in two common packages, a round one, about the size of a transistor, with eight leads on it, called " T O - 5 , " and a rec tangular one with two rows of 7 leads each, called " D u a l I n - L i n e " or " D I P . " I n both cases, several materials are used, with plastic or epoxy the least expensive. T h e most commonly used gates in R T L are the two input N O R gates and the buffer inverter.

ANALYTICAL CHEMISTRY, VOL. 4 2 , NO. 8, JULY 1970

Ζ - A A I Ζ

0 1

1 0

Figure 14. Logic s y m b o l , equation, and t r u t h table for NOT f u n c t i o n

OUTPUT IS LOW IF ALL INPUTS ARE HIGH

OUTPUT IS HIGH IF BOTH INPUTS ARE LOW

OUTPUT ISLOW I F Ε AND G ARE HIGH AND F IS LOW OUTPUT IS LOW IF H OR I OR J IS HIGH

Figure 15. symbols

Illustration of several logic

A B C ΙΖ

A B C Ι Ζ

0 0 0 0 1 1 1 1

0 0 0 0 1 1 1 1

0 0 1 0 1 0 1 0 0 1 1 0 0 0 0 0 1 0 1 0 0 1 1 0

0 0 1 0 11 10 1 1 1 1 0 0 1 0 1 1 1 0 1 1 1 0

Figure 16. Logic symbols and tables for gates

truth


Figure 17. Logically equivalent gates, by De Morgan's laws

Z • IA"~B) (C t i l (E -i-F) Ζ • ΙΛ_Β) (C D) IE FI Ζ - A • Β +C +D +Ε +F

Figure 18. Implementation of a function with NOR gates. Ζ = A(B + C) + D(B + C)

Ζ - Α +Β »C +D +Ε *'F

Figure 19. RTL

These devices can be purchased in small quantities for less t h a n 50i per gate. Fairchild makes a single inverter (juL900) in a T O - 5 pack age, and Motorola produces a D I P package with six inverters in it (MC789P). T h e inverters are called "buffer inverters" because t h e y h a v e high "fanouts," a subject discussed below. All members of the R T L family use the same power supply voltage, Vcc, of + 3 . 6 V ± 1 0 % . T h e de vices can be interconnected to form any logic function. T h e function Ζ — A (B + C) + D(1T+ C) is implemented in Figure 18 with N O R gates. T h e two equivalent forms of NOR

and

have been used to m a k e the logic function of a particular gate clearer. Fanout and Delay. There are two i m p o r t a n t considerations of digital logic which m a y be illus trated by the example above: fanout and delay times. Notice t h a t the output of gate 1 must drive the inputs of two other gates, gate 2 and gate 5. T h e outputs of gates 4, 5, and 6 drive only one input each. T h e input and o u t p u t load and drive characteristics are expressed by the manufacturer in terms of unit loads. Typically an input to any R T L gate is three unit loads and an output is 15 unit loads. This means t h a t one output can be con nected to five inputs. If more t h a n five inputs are connected to one out put, the logic levels of t h a t o u t p u t m a y deteriorate and the succeeding gates m a y become confused about whether they are receiving logic l's or logic 0's. T h e inverters are de signed to have very high fanouts. F o r example, the ^1.900 has a fan-

out of 80 unit loads. Hence the name "buffer inverter." T h e de vice buffers a signal by providing a very high fanout. Another characteristic of logic circuits is delay time. T h e gates operate in only a few nanoseconds, so delay times are not usually criti cal in instrumental applications. I n the circuit above, there is one gate between gate 1 and gate 7 along one path, and two gates along the other p a t h . A circumstance like this is called a race, because two signals arc going from the same place to the same place over more t h a n one path. T h e signal traveling through gate 2 has one less delay t h a n t h a t traveling through gates 5 and 3, and hence will arrive a t gate 7 sooner. T h e result will be a mo m e n t a r y spurious output, commonly called a "glitch." These glitches are frequently not important, but the designer should be aware of the possibility of their occurrence. There are two ways to eliminate them. One way is to ensure t h a t all delay paths are equal (this is " c h e a t i n g " ) , and t h e other w a y is to use a sequential circuit, as discussed later. Wired-OR

In R T L it is permissible to con nect together the outputs of several gates. Such a combination is called a wired-OR, but this is a misnomer. If any one of the gates whose out puts are connected together is low, then the common line will be low. T h e voltage from any gates which are 1 's will simply be grounded through the output of the gate which is a 0. The connection would be better called a w i r c d - A N D be cause all outputs must be high for the common line to be high. When this arrangement is used in R T L ,

Figure 20. made from gates

Use of the wired-OR with

R-S two

(Set-Reset) flip-flop cross-coupled NOR

connect only one of the I C packages to the power supply. T h e Vcc of the other packages is left uncon nected. Only a few gates should be connected in this manner. The wired-OR is illustrated in Figure 19. Multivibrators

I m p o r t a n t classes of circuits are those whose outputs arc fed back to their inputs. These circuits are multivibrators or flip-flops. The simplest kind of multivibrator is an R-S flip-flop (Reset-Set), "bi stable multivibrator." An R-S flip-flop can be made from two cross-coupled N O R gates connected as shown in Figure 20. If a pulse is applied to input S, and R is a 01, then Q will become a 1. N o t e t h a t the circuit "latches" so t h a t Q remains a 1 even after the pulse has disappeared. If a pulse


.

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Figure 2 1 . Use of the R-S flip-flop to eliminate contact bounce from a me chanical switch

Figure 22. Monostable multivibrator or one-shot from R-S flip-flop and RC charging circuit. Top: circuit; middle: timing diagram; bottom: graph for se lection of R and C

is applied to input R, Q will return to a 0 and once again latch up. This simple circuit is extremely use ful. T h e most common applica tion is starting some operation (for example, an integration) with one pulse and stopping it with another. Another important application is the elimination of contact bounce in a switch. Mechanical switches al ways bounce open and close m a n y times whenever they are switched. I n high-speed logic circuits, these bounces appear as a series of pulses and m a y cause m a n y problems. A solution is to use the mechanical switch to apply voltage to the S or R input of the flip-flop as in Figure 21. Then the first contact of the switch will set or reset the latch and the extraneous pulses will be ig nored. The R-S flip-flop can be made to 34 A ·

reset automatically after a prede termined time. A pulse on the S input will cause the output to go to a 1 momentarily, and then lapse back to 0. Such a device is called a "monostable multivibrator" or a "one-shot." The reset is accom plished by integrating a voltage on a capacitor until it becomes suffi ciently high to be considered a 1 by a gate. T h e circuit is shown in Fig ure 22. Reference (10) discusses this circuit and several others using the 2-input N O R gate. T h e period of the pulse depends on the value of RC, the time con stant. T h e resistor should not ex ceed 10 Kil, because it m u s t be able to supply current to the gate input and also charge the capacitor. A capacitor can also be added to the S input of the flip-flop, produc ing an astable multivibrator, or os cillator, as shown in Figure 23. If the outputs are buffered, a good square wave can be obtained. T h e two periods for the oscillator (the low time and the high time) are determined by the two RC net works. T h e y can differ somewhat, but best operation is obtained if the two time periods are equal. Once again, the resistors should not ex ceed 10 Κ in value. If very unsymmetric oscillations are required, an astable m a y be used to trigger a monostable (Figure 24). T h e asta ble multivibrator is used princi pally as a clock in a digital circuit. Sequential Circuits

A more complex kind of feedback in a digital circuit is used to make synchronous flip-flops. A synchron ous flip-flop is just like the R-S flipflop described above, except t h a t it can change state only when it re ceives a clock pulse. Such a cir cuit is valuable for eliminating glitches, because the input to the flip-flop can be given time to settle before the device is clocked. Most synchronous flip-flops are of the J-K type. T h e operation of a J-K flip-flop is given in the table in Fig ure 25. T h e table shows t h a t if both J and Κ are 0's, then nothing h a p pens when a clock pulse is r e c e d e d . If J is a 1 and Κ is a 0 the flip-flop "sets" or becomes a 1. If Κ is a 1 and J is a 0, the flip-flop resets. Up to this point it behaves exactly like the R-S flip-flop with J — S and


Figure 23. cillator

Astable multivibrator or os

ONE SHOT

ASTABl£

Figure 24. Periodic short pulses, fre quently needed for clock functions, can be made by triggering a monstable mul tivibrator with an astable multivibrator

J Κ Q I Q0 0 0 0 0 0 1 1 0 1 0 0 0 1 1 0 10 0 1 10 1 1 1 1 0 1 1 1 1 0

Figure 25. Symbol for and operation of a J-K flip-flop. Q is the output be fore the clock pulse and Q* is the new output

j

κ;

0

0 i TOGGLE

1 01 0 1 1

1

Q

1 0 NO CHANGE

Figure 26. Fairchild /tL923 and Motor ola MC790P RTL J-K flip-flops. The Ρ and CD inputs are asynchronous presets, which clear the outputs irrespective of the clock input. The S input is an ac tive low (written J and the C input is an active low Κ (Κ). The active low clock indicates that the outputs change state when the clock goes from high to low


Figure 27.

Κ = R except that it is clocked. The difference is in the last two states of the table. If J and Κ are both l's, the device will toggle, or switch to the opposite of its cur rent state on receiving a clock pulse. Examination of the R-S flip-flop cir cuit will show that if both R and S are l's the behavior of the circuit is undefined. It may end up in either state. Hence R = S = 1 is not allowed. But for the JK flip-flop this condition is allowed and is de fined to mean toggle. The JK flip-flop can be purchased in a TO-5 package (Fairchild yu,L923) or a dual JΚ flip-flop is available in a D I P package (Mo torola MC790P). These RTL flipflops are "negative edge triggered" —i.e., they are clocked by a transi tion from 1 to 0 on the clock input. The RTL flip-flops are shown in Figure 26 with everything active low; setting the flip-flop requires a low on pin 1, clearing a low on pin 3. If both pins 1 and 3 are low, then the device will toggle. The pin labeled Ρ is a preset. If it goes high, the flip-flop will immediately set (Q = 1 ; Q= 0) regardless of the inputs or clock. The most common application of these devices is for counting. If the output of one flip-flop is connected to the clock input of the next, a binary counter can be made, as shown in Figure 27. The input pulses will cause the first flip-flop to toggle on each pulse. The second flip-flop will toggle every time the first changes from 1 to 0, which is on every other input pulse. The toggling ripples through the series of flip-flops, resulting in a count of the pulses in natural binary order ing. A "ripple" counter is easy to build and inexpensive, but for some appli cations it is too slow—80 nsec are required for a pulse to get through 36 A .

Figure 28. Binary counter with carry lookahead logic. each stage J = K = 1 when all previous stages are l's

Binary counter with ripple carry

one flip-flop. This is the delay be tween the time that the clock changes and the output changes. If the counter is to be useful, the count must ripple through all the flip-flops before a new clock pulse is received. For the five-stage counter above, the total delay from the first clock input to the last out put is 0.4 /xsec, so the counter can not operate faster than 2.5 MHz. If greater speed is required, a syn chronous counter is needed. A syn chronous counter is made by letting each flip-flop know immediately if it is going to have to change state on the next pulse without having to wait for the previous stage to change. Examination of natural binary ordering shows that a given stage must change only if all pre vious stages are l's. Hence a syn chronous counter can be made by using gates to control the J and Κ inputs on the flip-flops and clocking all flip-flops together as in Figure 28. In the circuit shown, flip-flop A will always toggle because «7 = K = 0. Flip-flop Β will toggle when QA is a 1_ (QA = 0), and C will toggle only if both A and Β are l's. This process can be repeated indefinitely by using larger and larger NOR gates. The maximum delay is now only one flip-flop and one gate for any size of counter. For very high speed applications, however, it is better to use the circuits described later under "MSI." Several applications notes are available which describe the opera tion and uses of RTL logic. They may be obtained at no cost from the manufacturers (principally Moto rola and Fairchild). Transistor-Transistor Logic and Diode-Transistor Logic

Although RTL has been widely used in the past, it suffers from some


For

distinct disadvantages. The pri mary drawback is low noise im munity. The low noise immunity is due to the fact that the output of a gate in a low state is only 0.1 V be low the maximum voltage allowed for a low at an input. VOL (maximum "low" 0.4 V voltage at an output) VIL (maximum voltage at 0.5 V an input guaranteed to be a 0) A noise spike of only 100 mV in an RTL system can cause a 0 to be interpreted as a 1. Noise can be in troduced from switching transients, from other equipment in the area, or from "cross-talk" (interference be tween adjacent wires). T 2 L and DTL have much superior noise mar gins (400 mV). The industry has switched largely to the use of T 2 logic and conse quently most of the new circuits being produced are in this family. The new integrated circuits which can be of use in instrumentation are the very complex functions known as "MSI" or Medium Scale Integra tion. DTL and T 2 logic are not com patible with RTL, so the designer must choose which type to use at the outset, and then use it throughout the system. DTL and T 2 L are com patible with each other, however. DTL or T 2 L circuits should be used if noise immunity is likely to be a problem, if MSI blocks are to be used, or if the instrument must in terface directly with equipment using T 2 logic, such as the PDP-8I. The basic gate function in both DTL and TTL is NAND. Gates are available with anything from two to eight inputs. In addition, there are inverters and J-K flipflops. The devices are sold in 14 or 16 pin DIP's and use a power supply of 5.0 V ± 10%. They are both


current sinking logic. The main difference between DTL and TTL is that DTL is less expensive and TTL is faster. Everything that is said below about TTL applies to DTL also. Current Sinking Logic

In RTL, current flows from gate outputs into gate inputs, so a dis connected input is a logical 0 be cause there is no current flowing into it. In T 2 L, current flows out of the inputs and into the outputs. The outputs are current sinks rather than sources. Consequently, a dis connected input in T 2 L is a logical L, because no current can flow out of it. Typical numbers for the cur rent in T 2 L gates are given below, and illustrated in Figure 29. IIL (total current out of 1.6 mA inputs which are low) Tin (current into each 60 μΑ. input which is high) IOL (maximum current 20 mA into output when output is low) Ion (maximum current 1.2 mA out of output when output is high) Generating Logic Functions with NAND

Logic functions can be generated as easily with NAND logic as with NOR. In Figure 30 the circuit of Figure 18 is implemented with NAND gates. The derivation of the NAND logic equations is shown also. R-S flip-flops are made with cross-coupled gates, as in RTL, but the inputs are active low instead of high (Figure 31). Multivibrators cannot be reliably made with T 2 gates. Instead, multivibrators can be purchased as monolithic inte grated circuits (Fairchild 9601 and 9602). Medium Scale Integration (MSI)

MSI is defined as a monolithic device containing more than five or six gates on a chip. Most of them are rather complex circuits for pro cessing digital data. These include multiplexers, shift registers, arith metic units and the like. The MSI products frequently used in instru mentation are counters and de coder/drivers. When high-speed counting is re

quired, an MSI counter is much more satisfactory than one made from discrete flip-flops. The MSI product is more reliable and in volves no design problems. A counter may be constructed to count in straight binary up to 1111, or in Binary Coded Decimal (BCD) up to 1001. A four-bit binary coun ter is sometimes called "hexadeci mal," and a four-bit BCD counter is called a "decimal" counter. The BCD counter, which goes to zero after count nine, is generally pref erable. Four-bit BCD counters can be purchased as "up-counters," which count in one direction only; "updown counters," which can be re versed; and presettable counters, which can be loaded with some ini tial number other than zero. Inex pensive counters use "ripple carry" which means that the change in one counter from 9 to 0 clocks the next counter. More expensive counters use "carry lookahead" similar to the gating system illustrated in Figure 28 to speed up the RTL binary counter. MSI decoder/ drivers are available which receive the four lines from a BCD counter and convert the result to some out put which can drive a display sys tem. This output generally is one of two types. The most common readout sys tem uses "Nixie" tubes (Nixie is a registered trademark of the Bur roughs Corp.). The decoder/driver turns "on" (grounds) one of the 10 outputs which corresponds to the decimal number on its inputs. The selected number lights up. A newer type of readout is the seven-seg ment lamp. Decoder/drivers are also available for these. Figure 32 at top illustrates a display system using RTL parts and a Nixie tube. Figure 32 at bottom shows a sevensegment display using T 2 L MSI. In conjunction with the counter and decoder, it is frequently de sirable to use a four-bit latch. A latch is a flip-flop used to store in formation. It is placed between the counter and the decoder so that the counter can be reset without de stroying the displayed information. When the latch is enabled, the out puts simply follow the inputs. When it is disabled, the outputs are "locked into place" and can no

Figure 29. Currents associated with TTL logic. Currents into inputs are leakage of reversed biased junctions

Figure 30. Derivation of function of Figure 18 using NAND gates

Figure 3 1 . Implementation and repre sentation of R-S flip-flop using NAND gates

Figure 32. Top: RTL decade counter and decoder/driver for gas-filled lamp display. Bottom: TTL MSI system for seven-segment display


·

39 A


Figure 33. Complete counting system using RTL components. Pulses on the clock input are counted by the 9958 decade counter. When the count has been completed, the enable (£) on the latch is lowered for a moment and then raised to a 1 again. This will enter the count and store it. The stored count is decoded by 9960

longer change. Generally the latch will be enabled only for a brief mo ment at the completion of a count, so that the display shows only the final value reached by a count, not the counting process itself. Figure 33 shows an RTL system using all three components. The 9958, 9959, and 9960 may also be used with T 2 systems, but this is not advisable; T 2 MSI should be used in a T 2 system. There are many useful MSI and T 2 L products on the market, and much literature is available from manufacturers regarding their use. Manufacturers' applications notes are, in fact, the only good way to learn how to use MSI products, as there are no texts on the subject. Any book written would become ob solete before it could be published. Excellent applications notes are available from Fairchild, Signetics, Motorola, and Texas Instruments. Systems applications are shown {9). Interface Analog/Digital

Figure 34. Voltage transfer character istic. The voltage comparator has an output of logical 1 or 0 depending on the relative magnitudes of the two in put voltages c2

Digital systems in instrumenta tion are frequently controlled by the relative magnitudes of two ana log signals. To accomplish this, a comparator is used. The compara tor is like an operational amplifier with a digital output. If an opera tional amplifier were operated with no feedback resistor, then as soon as the voltage on the -(-input exceeded the voltage on the —input, the out put would jump to a positive satura tion voltage of about -f-10 V. Sim ilarly, if the —input were greater than the -(-input, the output would saturate at —10 V. A comparator is made by clamping the output levels so that the output saturates at logic levels instead of ± 1 0 V. The -(-input must be greater than the —input by some minimum level, a few millivolts, before the output switches. There is a small linear range, as indicated on the transfer function in Figure 34. The greater the gain of the comparator, the lower the "threshold" voltage and the steeper the linear region. The Fairchild Â710, illustrated in Fig ure 34, is typical. Some Small Systems

Figure 35. Top: analog differentiator. Bottom: digital differentiator (edge de tector) 40 A ·

Differentiators (Figure 35). Two kinds of differentiators are shown. The analog differentiator produces


an output voltage proportional to the rate of change of the input, as long as the input is changing with a frequency less than w. The addi tional components Ri and C% re duce noise by limiting the gain at high frequencies. For frequencies greater than w, the circuit acts like an integrator. The digital differentiator is an "edge detector." It produces a short pulse when the input switches logic levels. It is made by replacing the amplifier of the analog differentiator with a digital gate, preferably a comparator or an RTL gate. (T 2 L gates don't like to have capacitors on their inputs.) The example shown in Figure 35 uses a ^L900, which is convenient because it has the required resistor on the chip. Pulse Counter (Figure 36). Pulses over 100 mV in magnitude and of at least 40 nsec duration can be detected by the comparator (μΑ710) and the comparator out put pulses are counted and decoded. Such a system can form part of a photon counting scheme. The photomultiplier should be connected to a high-speed ("video") amplifier such as the μΑ733 and the video amplifier output to the comparator. The two diodes, comparator input, protect the input from high voltages. Pulse Height Detector (Figure 37). The system in Figure 37 al lows only pulses with magnitudes between VL and Vv to be counted. A pulse greater than VL triggers the first comparator. When the pulse ends, and the comparator returns to 0, a pulse is generated by the monostable. If, however, the input pulse exceeds the upper limit set by Vn, then the second comparator sets the R-S flip-flop, which places a 1 on the output gate. This will hold the output at 0 and prevent the pulse from the monostable from getting through to the counter. The R-S flip-flop is reset on completion of the input pulse by the second monostable. The capacitors should be as small as possible, as their pulse widths limit the speed of operation of the circuit. This system might be used in pho ton counting to reject low-level noise pulses and high-energy pulses from gamma rays. Voltage-to-Frequency Converter (Figure 38). The voltage-to-fre-


Figure 36. This system will detect pulses over a few mV and count them Figure 38. Voltage-to-frequency converter

Figure 37. Window input for counter

quency converter is useful for t r a n s mitting signals over wires or radio carriers. T h e circuit shown has an output of a series of pulses whose frequency is directly proportional to the input voltage. T h e input voltage produces a current through Rs which is integrated on capacitor C\. The input voltage must be negative in order to produce a positive signal at the amplifier output. If the input voltage is constant, the output will be a positive going r a m p . T h e second amplifier is used as a comparator to compare the r a m p with a voltage set by the divider Rs and R6. For the resistances shown, this will be about 10 V. The output of the second amplifier is saturated at + 1 2 V initially and the transistor is turned off. When the r a m p reaches 10 V, the amplifier saturates a t —12 V and the transistor turns on, forcing current from Rs into the integrator input. T h e integrator input must be a t virtual ground, so the noninverting input of the second amplifier goes to virtual ground when the transistor turns on. T h e r a m p is discharged until it reaches ground, a t which time the second amplifier changes state again, turning off the transistor and the discharge current. Then its input returns to + 1 0 V. T h e output is a series of pulses from + 1 0 to ground and back. T h e 42 A

·

Figure 39.

Analog-to-digital converter

frequency of the pulses depends on how long it takes the integrator to reach + 1 0 V, which is proportional to the input voltage. Analog-to-Digital Converter (Figure 39). T h e analog-to-digital converter has many similarities to the previous circuit. A reference voltage, derived from 10 V Zener diode £>i is integrated to produce a smooth r a m p voltage. During the integration, pulses from an astable multivibrator are counted. When the r a m p reaches the input voltage, a comparator turns on, setting the R-S flip-flop, which stops the pulses into the counter. T h e comparator at the same time enables the latch (9959) briefly to enter and store the count. T h e R-S flip-flop drives current into the integrator to discharge it and return the r a m p to 0. When


the ramp reaches 0, the second comparator pulses, resetting the flip-flop (which turns off the discharge) and resetting the counter to all 0's. The higher the input voltage, the longer the integration must last, and hence, the more counts are accumulated by the counter. T h e number decoded and displayed is a digital representation of the input voltage. The resistor and capacitor shown produce a r a m p whose slope is 10 V / sec. See (11) for a lengthy discussion of these circuits. Electrometer (Figure 40). Chemical instrumentation frequently requires the measurement of potentials from high impedance sources, such as a glass electrode. The circuit shown uses a 727-741 pair in a follower configuration to make a


For f„ • 1kHz Q = 20 AQ-10

CJ • Cj-O.OUif Figure 4 0 .

E l e c t r o m e t e r f o r use w i t h h i g h - i m p e d a n c e sources

Rj =31.8kQ R? = 403Ω R, " 6371(0

very high-impcdancc (300 ΜΩ) low-noise highly stable input. The second amplifier is used to provide a voltage gain of 1 V / 5 9 mV. The impedance is high enough to use with some glass electrodes and most other electrode systems. Resistor R 4 is used to set the temperature of the internal "oven" of t h e 727 to about100°C. An F E T input stage on an op amp can be used directly with a glass electrode, but temperature drift is significant unless very closely matched F E T ' s are used. [Such de vices m a y be available at low cost in the first p a r t of 1970 {12).] A very simple F E T electrometer for glass electrodes is discussed (13). Active Filter (Tuned Amplifier) (Figure 41). Sometimes it is de sirable to amplify a signal only of a certain frequency, while suppressing other frequencies. A collection of resistors and capacitors around an op amp can m a k e a tuned amplifier with moderate gain (5—20), for fre quencies within a certain band, and low gain ( < 1 ) for other fre quencies. A simple active filter (Butterworth response) is shown in Figure 41, along with the rules for selecting component values. Q should be as high as possible to get a narrow band pass, but very high values will dictate negative resis tances. I t is difficult to get a Q greater t h a n about 25 with this con figuration. Active filters are quite complex, but a good discussion a p pears in (H). Several recent articles in Electronics have dealt with them, also (15, 16). Figure 42 illustrates the use of the tuned amplifier to improve sig nal/noise in a flame photometer. Multipliers and Dividers. The 44A

·

SELECTION OF COMPONENT VALUES 1. Select AQ, f „ , Q. 2. Choose arbitrary capacitance C.

Figure 4 1 . T u n e d a m p l i f i e r or active fil ter. Only f r e q u e n c i e s a r o u n d F„ are a m p l i f i e d by t h i s c i r c u i t . A0 is t h e gain of t h e c i r c u i t at t h e center f r e q u e n c y and Q is a m e a s u r e of t h e b a n d p a s s

3. Define: k - 2 j i f 0 C 4

·

Η •

4. Calculate:

AQ/Q

Cj • C? - C R K

- - L Hk

l

R K

2

.

'

•

(2Q - H)k 2Q R 3" k

Figure 4 2 . Spectroscopic s y s t e m w i t h t u n e d a m p l i f i e r . Light is c h o p p e d by seg m e n t e d r o t a t i n g disk. A m p l i f i e r is t u n e d t o c h o p p i n g f r e q u e n c y and can greatly en hance s i g n a l / n o i s e ratio

nonlinear operations of multiplica tion and division are difficult to per form accurately. If high precision is required, analog multipliers can be purchased as hybrids. T h e y are called "quarter-square multipliers." To build a circuit which can multiply and divide, one can form logs, add or subtract them, and then form the antilog. For high accu racy, a circuit like the log amplifier in Figure 8 should be used, but if ac curacy of only a few percent is needed and if the circuit will be operated a t constant temperature, a simple arrangement like t h a t used in Figure 43 can be used. This circuit takes advantage of the logarithmic current voltage re lationship of the base-emitter junc tion in an N P N silicon transistor. T h e discussion of this kind of circuit in (17) should be consulted for in

ANALYTICAL CHEMISTRY, VOL. 4 2 , NO. 8, JULY 1 9 7 0

formation on the biasing and offset schemes. Amplifiers 1, 2, and 3 form the logs of X, Y, and Z. Am plifier 5 inverts one of the logs. The logs are summed by amplifier 4, with log Y having twice the weight of the others. Amplifier 6 takes the antilog. With proper biasing and selection of transistors, an ac curacy of ± 1 0 % should be obtain able in the output. Digital-to- Analog Converter (Figure 44). A digital-to-analog converter is used to obtain an ana log voltage from a binary number. This is frequently accomplished by connecting each bit of the binary number to a resistor whose value is inversely proportional to the weight of the bit, and then summing the currents from all the resistors in a current-to-voltage converter. For example, a four-bit number could be


converted by connecting a IK resis tor to t h e most significant bit, 2K to the next, 4/v to the next, 8K to the least significant bit. Then the cur rents through the resistors are pro portional to the weights of the cor responding bits. Unfortunately this simple scheme is not accurate be cause the logic levels are not exactly specified. T h e y v a r y somewhat from unit to unit. T h e ju.A722, used in Figure 44, is an accurate current switch. The binary digits are used to switch cur rents onto the output line, with a unit weight of 10 μΑ—e.g., a binary number of 00001001 ( = 9) would give an output of 90 μΑ. T h e p a r t is moderately expensive, but is ac curate to 0.08% a t 25°C.

Figure 43.

Formation of nonlinear function XY2/Z with log and antilog circuits

Development of a Polarographic Function Generator

As a demonstration of the appli cation of the material which has been presented in this paper, a polarographic function generator is developed below. T h e system is suggested by t h a t of Bezman and M c K i n n e y (18). T h e first step in designing the function generator is to decide ex actly w h a t functions it should per form. Below are listed some func tions t h a t are desirable in polarography. 1. 2. 3. 4. 5. 6.

Constant dc level Positive going r a m p Negative going r a m p Triangle wave Staircase function External signal impressed upon any of the above

I n addition, some controls on these functions should be included. 1. Slope of positive r a m p 2. Slope of negative r a m p 3. Step size and rate of staircase function 4. Automatic reset a t a preset voltage 5. Stop and hold T h e basic method is indicated schematically in Figure 45. T h e box labeled Fl is a triangle wave generator on which the -fslope and —slope can be independently set. W i t h the two slopes set at opposite extremes, the "special case" of a sawtooth wave results.

Figure 44. Digital-to-analog converter used to generate polarographic sweep

T h e block labeled F2 is a stair case generator. Controls are pro vided to produce a staircase in either direction and to set the step size and the step rate. Block F2, is just a dc level to pro vide an initial starting point for the output. Finally there is an external input so the user can attach some other function, such as a sine wave for ac polarography. T h e outputs of all these functions are added together and amplifier Al produces some output voltage with respect to the voltage on the refer ence electrodes. A separate circuit is used to detect the time at which the output exceeds preset limits. This limit detector is used to control the function generators. I t m a y simply t u r n e\ 7 erything off or, if a triangle wave is desired, reverse the direction of the functions. Detail of the Triangle Generator. T h e triangle wave is generated by integrating either a positive or a

negative current on amplifier .43 in Figure 46. T h e currents come from the + 1 5 - and — 15-V power sup plies through the variable resistors, R-\- and R—. T h e lower the values of the variable resistors, the greater the slope of the r a m p . T h e inte grating capacitor is a large metalized polycarbonate t y p e which ex hibits good characteristics for longterm integrations. T h e two transistors, Q l and Q2, along with the associated diodes are level-shifting circuits to t r a n s form the T T L logic levels to 15-V swings. A logic 0 on the base of Ql provides + 1 5 V to the integra tor; a logic 0 on the base of Q2 provides —15 V. T h e level shifters are controlled by R-S flip-flops. N o w it is necessary to decide when the flip-flops should be set and when they should be reset. T h e -f-flip-flop should be set (ac tive low) if the system is in auto matic mode and the negative limit has been reached or if the "start up"


·

47 A


button is pushed ( = low = u).

R+ = (L+)

+ (STOP)

S+ = (auto • L—) + (U)

R= = (L-)

+ (STOP)

Similarly,

S~^ = (auto'L+) + 0) Resets should occur if voltage limits are reached or if the "stop" button is pushed.

48 A

·

Note the use of the wire-OR on a DTL NAND gate to generate the proper logic on the set input. Detail of Staircase Generator (Figure 47). The staircase generator is made with the digital-to-ana-

Figure 45.

Organization of function generator

Figure 46.

Ramp generator and control logic


log converter shown in Figure 44, and the voltage to frequency converter of Figure 38. The latter circuit, called a VCO (voltage controlled oscillator) is used as a variable frequency clock for the counters. As the voltage at the input to the VCO increases, the frequency of the output pulses increases. The output pulses are


counted in two four-bit counters ( = 2 8 = 256). T h e eight output lines drive the μΑ722 current gen erator. E a c h time there is a clock pulse, the current from the Â722 increases by one unit. T h e current is converted to a voltage by ampli fier A4, T h e feedback resistor sets t h e "gain," thus determining the step size of the staircase. If the stair case function is not turned on, the counters receive no clock pulses. T h e gates between t h e counters and the Â722 are Exclusive OR gates (Z = AB + AB). If the con trol line, C, is set to a 0, then the counter outputs go directly to the 722. If C = 1, the counter outputs are inverted. Inverting the outputs makes the count descend instead of ascend. Count 000 0 0 00 1 0 0 10 00 1 1

(0) (1) (2) (3)

1 1 1 0 (14) 1 1 1 1 (15)

Count 1111 1110 110 1 1100

(15 ) (14) (13) (12)

0001 00 00

(1) (0)

T h e result is t h a t the staircase goes down instead of u p . T h e C line is controlled by the up and down flip-flops which also control the r a m p . T h e "single s t e p " switch allows the operator to step one unit on command. T h e flip-flop is used to eliminate contact bounce. D T L gates are used in the flip-flop to use up the two left over in a package already present. T h e final instrument is a versatile machine indeed. I t can a u t o matically sweep a voltage range in either direction, a t a n y rate, either smoothly or in steps. I t can be preset to any starting voltage and a n y stopping voltage. If the step function is used, a digital represen tation of Ε applied is available. T h e active p a r t s required are four operational amplifiers (2 packages), 2 comparators, 8 T 2 L N A N D gates (2 packages), 4 D T L N A N D gates (1 p a c k a g e ) , 2 T T L N O R Gates (1 p a c k a g e ) , 1 current generator, 2 counters, and 3 discrete transistors. I t should be pointed out t h a t this instrument has been developed on paper only as an exercise in system

Figure 47.

Staircase generator

design. Such an instrument has not, to the best of m y knowledge, actually been constructed and tested. Conclusion

Integrated circuits provide the analytical chemist with versatile, powerful building blocks with which he can build up very complex sys tems with ease. T h e black-box a p proach to system design frees the analyst to spend his time on de cisions about methods of approach and analysis of results without h a v ing to waste great quantities of time and money trying to obtain a satisfactory instrument. This paper has been an a t t e m p t to pre sent enough background on applica tions of integrated circuits t h a t the analyst can put implementation of instruments in the back of his mind, confident t h a t , when the time comes, the electronic design will be a simple task. Some Notes on Construction

Integrated circuits are sold in several different grades and pack ages. T h e t y p e to buy for instru mentation is usually commercial or industrial grade (0 to 75°C, as op posed to military grade, —55 to + 125°C). Ordinarily, the D I P package is most suitable. F o r some IC's, epoxy or plastic packages are

available and these are the least ex pensive. Sockets can be purchased for all types of packages, b u t t h e sockets are frequently more expensive t h a n the I C itself, so they are not ordi narily used. T h e most satisfactory method for connecting IC's is on a printed circuit board. If only "one of a k i n d " is to be constructed, then patch boards are commercially available with rows of prepunched holes spaced for D I P packages. T h e circuits are soldered in directly. Operational amplifiers tend to "couple"—i.e., interact through powerline connections. To elimi nate this effect, which causes oscilla tions, the power supply pins on every second or third amplifier should be bypassed to ground through a 0.01 μΐ disk capacitor. Digital circuits are less often sensi tive to power supply coupling, but an occassional bypass capacitor is a good idea in a digital system also. Power supplies to the systems should be regulated against overvoltages. Most laboratory supplies provide such protection. There are m a n y sources of information on building your own regulated power supply for the finished product. One of the best is a handbook p u b lished by Kepco (30). T h e main consideration in design ing a system is simplicity. E x t r a


·

49 A

R e p o r t for Analytical

Chemists

time spent in design and extra money spent on complex MSI cir cuits are usually more than bal anced by the savings in package count and interconnection com plexity which result. Even though a system is carefully designed and thought out, it is still like a com puter program : I t won't work right the first time. But the simpler the system, the fewer the possibilities for error or malfunction. One last caution: Your inte grated circuits should be purchased from the manufacturer's distributor in your area. Cut-rate deals from discount merchandisers generally turn out to be bad deals. The parts will not only not meet specifications, they may be functional rejects and not work at all. Distributors can also usually supply you with appli cations help, or refer you to in formation to aid you.

THE

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References

(1) Symposium fiers,

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4-70MCB MC&B MANUFACTURING CHEMISTS 2 9 0 9 Highland A v e n u e N o r w o o d , Ohio 4 5 2 1 2 S e n d m e " S a f e t y in H a n d l i n g Hazardous Chemicals." Name Company Address City State

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ANAL.

on Operational Ampli CHEM.,

·

ANALYTICAL CHEMISTRY,

1770-1829

ANAL. C H E M . , 40 (10), 30 A

(August

1968). (10) Donald Lancaster, "Using New Low-Cost Integrated Circuits," Electron. World, March, ρ 50 (1966). (11) Hermann Schmid, " A / D Conver sion," Electron. Design, 25, 49 (Dec. 5, 1968) and 26, 57 (Dec. 19, 1968). (12) R. Christensen, and D. Wollensen, "Matching F E T ' s by Design Is Faster and Cheaper Than by Pick and Choose," Electron., 42 (25), 114 (Dec. 8, 1969). (13) R. C. Dennison, "Solid State p H Meter," Pop. Electron., 29 (5), 33 (Nov. 1968). (14) "Handbook of Operational Ampli fier Active R C Networks," Burr-Brown Research Corp., 1966. (15) Michael Hills, "Active Filters: Part 13, Narrowing the Choice," Electron., 42 (22), 106 (Oct. 27, 1969). (16) Roland J. Turner, "Feedback Sharpens Filter Response," ibid., 42 (20), 102 (Sept. 29, 1969). (17) Bill Ehrsarn, "Building Logarithmic Amplifiers the Easy Way," ElectroTech., 82 (3), 62 (Sept. 1968). (18) R. Bezman and P. S. McKinney, ANAL. C H E M . , 41, 1560 (1969). (19) H- C. Jones et al., ANAL. C H E M . ,

41,772 (1969). (20) Kepco, Inc., "Power Supply Hand book," Publ. No. 146-1131, Kepco, Inc.. 131-38 Sanford Ave., Flushing, Ν. Υ. 11352.

The list below gives the names, addresses, and product lines of the principal manufacturers of integrated circuits such as those used in this article. The list is not intended to be complete, but does include all the large companies which have extensive applications information avail able. High-quality operational amplifiers ANALOG DEVICES 221 Fifth Street Cambridge, Mass. 02142 FAIRCHILD SEMICONDUCTOR RTL, DTL, TTL, op amps 464 Ellis Street Mountain View, Calif. 94040 MOTOROLA SEMICONDUCTOR RTL, DTL, TTL, op amps Box 20912 Phoenix, Ariz. 85036 NATIONAL SEMICONDUCTOR TTL, op amps 2975 San Ysidro Way Santa Clara, Calif. 95051 PHILBRICK RESEARCHES, INC. Hybrid amplifiers Allied Drive at Rte. 128 Dedham, Mass. 02026 SIGNETICS CORPORATION DTL, TTL 811 East Arques Sunnyvale, Calif. 94086 SYLVANIA ELECTRONIC COMPONENTS DTL, TTL 1100 Main Street Buffalo, N.Y. 14209 TEXAS INSTRUMENTS INCORPORATED RTL, TTL, op amps P.O. Box 5012 Dallas, Tex. 75222 Names, addresses, and products of many other companies can be found in magazines such as Electronic Products and Electronic Design.

Circle No. 80 on Readers' Service Card 50 A

35,

(1963). (2) "Applications Manual for Comput ing Amplifiers for Modeling, Measur ing, Manipulating and Much Else," Philbrick Researches, Inc., Allied Drive at Route 128, Dedham, Mass. 02026 (1966). (3) "Handbook of Operational Amplifier Applications," Burr-Brown Research Corp., P.O. Box 11400, Tucson, Ariz. 85706 (1963). (4) "Fairchild Semiconductor Linear Integrated Circuits Applications Hand book," Fairchild Semiconductor, 313 Fairchild Drive, Mountain View, Calif. 94940 (1967).

(5) Jerry Eimbinder, Ed., "Designing with Linear Integrated Circuits," Wiley, New York, Ν . Υ., 1969. (6) Willard, Merit, and Dean, "Instru mental Methods of Analysis," Van Nostrand and Co., Princeton, 1965. (7) Malmstadt, Enke, and Toren, "Elec tronics for Scientists," Benjamin, New York, Ν . Υ., 1963. (8) Malmstadt and Enke, "Digital Elec tronics for Scientists," Benjamin, New York, Ν . Υ., 1969. (9) George Laucr and R. A. Osteryoung,

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Using Integrated Circuits in Chemical Instrumentation - ACS Publications

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