Using integrated circuits in chemical instrumentation - Analytical

REPORT FOR ANALYTICAL CHEMIST5

Using Integrated Circuits in Chemical Instrumentation JOHN S.SPRINGER Fairchild Semiconductor Mountain View, Calif. 94040

YEARS AGO articles about analytical chemistry dealt with new complexing agents, new separation methods, and other laboratory operations with only an occasional paper on instrumental analysis. Until about 1955 an operational amplifier was a fairly exotic device, not something the ordinary laboratory could obtain or even know how to use. The computer age had only begun to exercise its influence on laboratory methods. When the transistorized operational amplifier was developed, this powerful electronic tool was suddenly available t o everyone. The symposium on operational amplifiers which appeared in ANALYTICAL fl___.__^I__. :.. -~~,. ~ i n f i i m i h i n r 111 LYVJ ( I ) , permps marked the introduction of modern electronics to the laboratory. Now the journals contain many articles dealing with new instruments. The illustrations are laden with integrators, differentiators, followers, all made from operational amplifiers, and in the past few months, digital circuits have begun to appear also. , Many researchers have remained 1 close enough t o the 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 “integrated circuits,” and this article will attempt t o show just bow the integrated circuit can be used by the analytical chemist.

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Advantages of Integrated Circuits

M a n y chemists may be asking themselves now, “Why should I he concerned with an integrated circuit? M y job is analytical chemistry; let the electronic engineer take care of the electronics.” The

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 t o 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, t h a t the chemist can use electronics in the form of a n integrated circuit, without being a n electronic engineer. The integrated circuit is a self-contained package, a black box with a few inputs and outputs. T h e chemist can use i t without being concerned with how i t does what it does. B u t the integrated circuit can make i t possible for t h e chemist t o 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. T h e price of an op amp dropped nearly a n order of magnitude; the integrated circuit ( I C ) 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 a t 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 h a r e 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 t h a t the entire circuit is made a t one tinie. The resistors and transistors are made by diffusing impurities such as boron, phosphorus, and arsenic into a flat piece of silicon crystal.

T h e 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. T h e exposed photo resist is washed away, and the impurities are diffused through the uncoated regions. T h e process is repeated several times with different impurities to build up the transistors, A layer of SiOp is then deposited over the entire surface, A masking operation is used to etch holes in the SiOa to make connections to the devices, and a h minum strips are deposited to make the wires and the connections. The completed circuit is coated with SiOn 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 t h a t 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 a t once, the same process can produce a few transistors or a hundred transistors on a single chip a t about the same cost. The difference in cost between a small circuit and a complex one is mainly due to the greater area required b y 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. T h e effect of reduced yield is, of course, increased unit price. Figure l a is a photomicrograph of a chip containing over 100 transistors. Figure I b shows a wafer containing many integrated circuits.

Linear Integrated Circuits Linear Integrated Circuit means an amplifier, generally a n 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 2 1 5 V, and the power supplies usually need not be highly regulated. The power consumed by an I C op amp is very low; some types, especially designed for battery-powered instrumentation, use only a few microwatts of power. The new I C op amps have very low drift with temperature and time and can withstand short circuits on the output without damage. Of course the I C 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 t10 V I , that it can supply only a few milliamps of current a t the output, and t h a t its input impedance is usually less than 1 Mn. The low current capabilities are due to the difficulty of dissipating the heat produced by highpower devices. T h e impedance property is the principal limitation of the device in chemical instrumentation, but there are ways around the problem. T h e low-input impedance is a result of the technological limitations of the integrated circuit fabrication, but, as the state of the a r t pro-

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

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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 package is called a hybrid. Hybrids with very high-input impedances are frequently identified as “FET (Field Effect Transistor) Input H y brid Operational Amplifier.” They cost a little more than the amplifier alone. For most applications the extra input impedance is not required. Characteristics of Linear Integrated Circuits

The first rule in buying an operational amplifier is not t o overbuy. Amplifiers are available with a wide range of characteristics and prices. Listed below are some of the specifications which are important to the user when selecting the proper amplifier. 1. Gain. The gain of the amplifier is the constant A in the characteristic equation eoUT = --A (el e z ) . 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 lo4. A voltage difference on the input of 24A

V will produce an output of 1 V. The gain falls as the frequency a t which the amplifier is operated increases. A curve similar to t h a t in Figure 2 is usually shown in the manufacturer’s data sheet. When feedback is used to set the gain of the amplifier in a particular configuration, the gain ‘us. frequency curve will be flat a t the set value until the “open loop gain” is intersected. Then the gain will fall off with frequency. T h e gain of the amplifier can never exceed the open loop gain. 2. Input Impedance. This figure varies widely from one device to another. The inexpensive amplifiers generally have impedances on the order of 750,000 a. More expensive amplifiers and hybrids have higher impedances. The impedance needed depends on the configuration in which the amplifier is to be used. High impedances are required when very low currents are involved, because the proper operation of the circuit depends on the assumption that current flowing into the amplifier input is negligible. The reciprocal of the input impedance times the gain is approximately the current (in amps) which will flow into the amplifier input. 3. Ofjset Voltage. The offset voltage is the voltage difference between the two inputs required to produce 0 volts a t the output. Ideally it is zero. Virtually all integrated circuit op amps have provision for a “balance” control which allows the user to null out the offset voltage. 4. Bias Current. The bias cursent is the minimum current which must he supplied a t an input t o make the amplifier work. T h e input impedance figure holds only after this minimum current has been supplied. The bias current is only a few nanoamps and frequently can be ignored. When this small current 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. The currents supplied t o the two amplifier inputs should always be made equal. This means t h a t resistors on the two inputs should have about the same value. If only the inverting input is to be used, the noninverting input should be

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

grounded through a resistor whose value is equal to the parallel combination of all the resistors tied t o the inverting input, as shown in Figure 3. 5 . Slew Rate. I n dc applications the slew rate is usually not critical. The slew rate is the speed a t which the output can change voltage. For most amplifiers this number is about 2 Vlpsec. 6. Input Voltage Range. T h e inputs to an amplifier may be damaged if the voltage on them exceeds either supply voltage or if the difference between the two inputs exceeds some value, usually about 10

v.

7 . Frequency Compensation. Most of the older op amps, such as the PA709 ( p for microminiaturisation), require the addition of a resistor and two capacitors t o stabilize 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 part numbers containing “709” or “101.” The newer amplifiers, “741,” are fully compensated internally and are no more expensive than the older type. If an amplifier breaks into oscillation 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 a n extremely powerful tool. The fol-

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source, for negligible balance “drift” with temperature. Almost instant cooling from 1000°C to ambient for shortest “turn around” time a n d maximum instrument utility. The magnetic transition temperature calibration system (patent applied for) provides T calibration in the sample space, not just somewhere near it. In fact, the TGS-1 is the only rapid scanning TGA system providing sample temperatures you can believe. The Cahn RG Electrobalance@ we use is famous for its sensitivity, accuracy and ruggedness. Others use it, too-but Perkin-Elmer keeps it cool and it does its best. With the TGS-1, data can be recorded simultaneously with the DSC. A derivative circuit for DTG is included. Moreover, with the UU-1 programmer you can have a completely independent TGA system at moderate cost.

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PERKIN-ELMER

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

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lowing circuits illustrate some of the uses t h a t can be made of it: 1. Summer (Figure 4). The summer is perhaps the most common application of an operational amplifier. The output is equal to the sum of all the input voltages in proportions determined by the resistors. If all resistors are equal, then the output is the straight sum of the inputs. 2. Integration (Figure 5 ) . I n the integrator, current trickles from the input voltage, e I s , through resistor R and on t o 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 integration of the input voltage. If a constant voltage is applied t o the input, the output voltage will increase a t a constant rate. A steadily increasing volt,age is called a “ramp.” The integration constant is 1/RC, where RC is the “time constant.” T h e ramp can be reset to zero by shorting the capacitor 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 constant 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. Differentiators are very susceptible to electrical noise. Noise response can often be improved by placing a small capacitor (C,) across the feedback resistor. 4. Follower (Figure 7 ) . The voltage follower is one way to increase impedance in a circuit. The

output voltage is identical to, or “follows,” the input voltage. Current can be drawn from the output, but negligible current is consumed a t the input. A circuit like this is frequently used in conjunction with an electrode system. The potential of a n electrode may be measured or used to perform operations without drawing any current from the electrode. The follower is a n example 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 logarithmic 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 transistors. While i t is possible to get two matched transistors by manually testing and sorting, it is also possible to buy the transistors on a single chip as an integrated circuit. Several manufacturers sell matched transistors in one package. A savings in cost can also be realized by purchasing an integrated circuit containing two operational amplifiers. 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 improved linearity) can be achieved by using an FET input amplifier in place of A l . An FET input, unfortunately, will also increase the drift of the circuit with tempera-

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ture (FET’s are notoriously temperature sensitive), An alternative method, which raises input impedance less than an FET, but which improves temperature stability markedly, is the use of a temperature-compensated preamplifier. The pA727, for example, contains its own oven which holds it a t a constant temperature. 6. Current-to-Voltage Converter (Figure 9 ) . This circuit converts a current into a proportional voltage. The inverting input of the amplifier is a t virtual ground. The circuit is useful with polarographic cells, as shown in Figure 10. I n applications like this, i t is once again important to have a high input 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 exhibits a very high impedance, an FET input may be called for (see circuit 7 ) .

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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 transistors in the circuit above. There are two types of FET’s. A junction FET has a gate impedance of 1010 0 ; an MOSFET has a gate impedance of 1OI2 or more. Special wiring precautions must be taken with FET input stages, and it is probably safer to purchase a hybrid amplifier than to construct one from an op amp and two field-effect transistors. The principal disadvantages of FET’s are t h a t they tend to introduce noise into the circuit and that they drift with temperature 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 operations used in instrumentation, They all require only an inexpensive operational amplifier and a few additional components. I n the last section of this paper, these “building blocks” will be used in some examples of instrumentation.

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

Chemists have been using computers for years, but only recently have they begun to use digital build28A


ing blocks in their own designs. Digital circuits tend to be used in two different kinds of applications. One is simply to generate logic functions for control purposes. (Example: Turn off titration when rroff” button is pushed or when inflection point is crossed or when buret is empty.) The other application is in data processing; these circuits tend to be quite complexing and are not usually used in analytical instruments except for counting and digital readouts. Digital data processing is used when high accuracy and speed must be obtained and when the instrument is to be connected directly to a computer. 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 successor 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 a t ground, and the logic high is a few volts positive. The term 1 will be used to refer to a logic higk 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 AXD, OR, and NOT. Consider the expression AND. What are the characteristics of a logic gate whose equation is Z = A AND B ? This equation means the output, Z, will be high if, and only if, both inputs A and B are high. The logic symbol and truth table are shown in Figure 12. If Z = A OR B, then Z will be high if A or B or both are high. The “OR” that is used in English language is generally the Exclusive OR; it implies one or the other of two alternatives, but not both. The logic OR is an inclusive OR; it implies that a t least one of the possible alternatives is true. The OR operation is indicated by a plus sign. Note the distinction between the symbol for the OR gate,

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Repolt for Analytical Z =AB

Figure 13, and t h a t for the A N D gate. If 2 = N O T A, then 2 and A are always opposite or "complementary." The N O T function is simply inversion of a signal. "NOT" io symbolized by a bar over the negated expression, as illustrated in Figure 14. The symbol for the inverter cousists of two parts. The triangle represents the amplification of the signal which is present in any IC logic gate. Amplification prevents deterioration of signal quality through gates. The amplifier symbol is followed by a little circle a t the output. It is this circle which indicates inversion. It may be thought of as meaning "active level low"Le., when the input conditions are satisfied, the output is low instead of high. Several logic symbols are shown in Figure 15 t o illustrate the use of the circle. Real I C gates consist of two parts. First the logic is performed (AND or OR) and then amplification occurs to return the logic levels to their specified values. Amplification 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 AND, __ or NAND) and a c t i v e l o w OR (which is N O T OR, or NOR). Truth t a b l e s f o r t h e s e gates are shown in Figure 16. Note t h a t N A N D is a 0 if all inputs are 1's and NOR is a-1, if all inputs are 6's. Identities. T h e identity rulesTor logic combinations are given below. Note the similarity to the arithmetical operations expressed with the same symbols.

1. Double inversion X=X 2. Distributive A(B+C)=AB+AC 3. Associative A+ (B+C) = (A+B) + C (AB)C=A(BC) 4. DeMorgan's laws

Z=Amd B

Figure 12. Logic symbol, equation, and truth table for AND function

Figure 13. Logic symbol, equation, and truth table for OR function

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Figure 14. Logic symbol, equation, and truth table for NOT function

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The last two are extremely important rules. Note t h a t A B is not the same as B. "Neither A NOR B" is not the same as "NOT A OR NOT B." DeMorgan's laws are shown symbolically in Figure 17. Digital integrated circuits are produced in several families. The two most important are RTL (Resistor-Transistor Logic) and T2L (Transistor-Transistor Logic). The basic gate function in R T L is N O R ; in TIL,NAND. ~

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

R T L was formerly the least expensive type of integrated circuit logic, As of early 1970, this is no longer true, but it is worth discussing anyway as an introduction to I C logic because of its widespread use in designs of the past few years. The devices are used in two common packages, a round one, about the size of a transistor, with eight leads on it, called "TO-5," and a rectangular one with two rows of 7 leads each, called "Dual In-Line" or "DIP." I n both cases, several materials are used, with plastic or epoxy the least expensive. The most commonly used gates in R T L are the two input NOR gates and the buffer inverter.

ANALYTICAL CHEMISTRY, VOL. 42,

NO. 8, JULY 1970

Figure 16. Logic symbols and truth tables for gates

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


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These devices can be purchased in small quantities for less than 506 per gate. Fairchild makes a single inverter (pL900) in a TO-5 package, and hlotorola produces a DIP package with six inverters in i t ( N C 7 8 9 P ) . The inverters are called “buffer inverters” because they haye high “fanouts,” a subject discussed below. All members of the R T L family use the same power supply voltage, T’cc, of $3.6 V i 10%. The devices can be interconnected to form a n y logic function. T h e function 2 = A(B C) D(B C) is implemented in Figure 18 with NOR gates. T h e two equivalent forms of

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have been used to make the logic function of a particular gate clearer. Fanout and Delau. There are two important considerntions of digital logic which may be illustrated 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 , The outputs of gates 4, 5, and 6 drive only one input each The input and output load and drive characteristics are expressed by the manufacturer in terms of unit loads. Typically a n input to a n y R T L gate is three unit loads and an output is 15 unit loads. This means that one output can be connected to five inputs. If more than five inputs are connected to one output, the logic levels of that output may deteriorate and the succeeding gates may become confused about whether they are receiving logic 1’s or logic 0’s. The inverters are designed to have very high fanouts. For example, the pL900 has a fan-

out of 80 unit loads. Hence the name “buffer inverter.” The device buffers a signal by providing a very high fanout. Another characteristic of logic circuits is delay time. The gates operate in only a few nanoseconds, so delay times are not usually critical 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 path. A circumstance like this is called a race, because two signals are going from the same place to the same place over more than one path. The signal traveling through gate 2 has one less delay than that traveling through gates 5 and 3, and hence will arrive a t gate 7 sooner. The result will be a momentary 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 that all delay paths are equal (this is “cheating”), and the other way is to use a sequential circuit, as discussed later. Wired-OR

I n RTI, it is permissible to connect 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 outputs are connected together is low, then the common line will be low. The voltage from any gates which are 1’s \Till simply be grounded throGgh the output of the gate which is a 0. The connection would be better called a wired-AKD because 0.11 outputs must be high for the common line to he high. When this arrangement is used in R T L ,

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two

connect only one of the IC packages to the power supply. The V c c of the other packages is left unconnected. Only a few gates should be connected in this manner. T h e wired-OR is illustrated in Figure 19. Multivibrators

Important classes of circuits are those whose outputs are 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), “bist a bl e mu 1ti vi bra to r .” An R -5’ flip-flop can be made from tn.0 cross-coupled KOR gates connected 2s shown in Figure 20. If a pulse is applied to input S, and R is a 0, then Q will become a 1. Note that -the circuit “latches” s-o that Q remains a 1 e w n after the pulse has disappeared. If a pulse


33A

Figure 21. Use of the R-S flip-flop t o eliminate contact bounce from a me. chanical switch

OUTPUT

@E+= TRIGGER

TRIGGER POINTX

[‘

{: [A

OUTPUT

TIME

-

8k 4k 2k R

10 5.0

g

Ik

2.0 1.0

.5

d

5

.2 .1 .05 * 02

.01

.01

[email protected] .50 1.0 2.0 5.0 10

c CUF) 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 useful. The most common application 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. nlechanical switches always bounce open and close many times whenever they are switched. I n high-speed logic circuits, these bounces appear as a series of pulses and may cause many 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 ignored. The R-S flip-flop can be made to 34A

e

reset automatically after a predetermined 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 accomplished by integrating a voltage on a capacitor until it becomes sufficiently high to be considered a 1 by a gate. The circuit is shown in Figure 22. Reference (10) discusses this circuit and several others using the %input KOR gate. The period of the pulse depends on the value of RC, the time constant. The resistor should not exceed 10 K n , because i t must be able to supply current to the gate input and also charge the capacitor. A capacitor can also be added t o the S input of the flip-flop, producing an astable multivibrator, or oscillator, as shown in Figure 23. If the outputs are buffered, a good square wave can be obtained, The two periods for the oscillator (the low time and the high time) are determined by the two RC networks. They can differ somewhat, but best operation is obtained if the two time periods are equal. Once again, the resistors should not exceed 10 K in value. If very unsymmetric oscillations are required, an astable may be used to trigger a monostable (Figure 24). The astable multivibrator is used principally 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 synchronous flip-flop is just like the R-S flipflop described above, except that it can change state only when it receives a clock pulse. Such a circuit 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-I< type. The operation of a J-K flip-flop is given in the table in Figure 25. The table shows that if both J and K are O’s, then nothing happens when a clock pulse is received. If J is a 1 and K is a 0 the flip-flop “sets” orbecomes a 1.- If K is a 1 and J is a 0, the flip-flop resets. V i to this point i t behaves exactly like the R-S flip-flop with J = S and


mg:

+”cc

Figure 23. cillator

Astable multivibrator or os-

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

OUTPUT CLOCK COMPLEMENTARY OUTPUT

1 0 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 before the clock pulse and Q* is the new output

0 CP 0 P

0 CP

Q ”1923

j

0

0 0 TOGGLE 1 0 1 0 1 0

MC7WP

1 1 hOCHANGL

Figure 26. Fairchild pL923 and Motorola MC790P RTL J-K flip-flops. The P and CD inputs are asynchronous presets, which clear the outputs irrespective of the clock input. The S input is an active low (written J and the C input is an active low K (K). The active low clock indicates that the outputs change state when the clock goes from high t o low

a measured GC stability


-J

-

C

A

I -J

-

-c

INPUT PULSES

T

- - 1

L_

T

K = R except t h a t it is clocked. The difference is in the last two states of the table. If J and K are both l’s, the device will toggle, or switck to the opposite of its current state on receiving a clock pulse. Examination of the R-S flip-flop circuit will show t h a t if both R and S are 1’s the behavior of the circuit is undefined. It may end up in either state. Hence R = S = 1 is not allowed. B u t for the JK-flip-flop this condition is allowed and is defined to mean toggle. The JK flip-flop can be purchased in a TO-5 package (Fairchild pL923) or a dual JK flip-flop is available in a DIP package (Motorola MC790P). These R T L flipflops are “negative edge triggered” --i.e., they are clocked by a transition from 1 to 0 on the clock input. The R T L flb-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 P is a preset. If it goes high, the flip-flop will immediately set (Q = 1 ; 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. T h e 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 0, which is - to on every other input pulse. The toggling ripples through the series of flip-flops, resulting in a count of the pulses in natural binary ordering. A “ripple” counter is easy to build and inexpensive, but for some applications it is too slow-80 nsec are required for a pulse to get through

a=

I

J

:

L

T

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

one flip-flop. This is the delay between the time t h a t 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 output is 0.4 psec, so the counter cannot operate faster than 2.5 MHz. If greater speed is required, a synchronous counter is needed. A synchronous 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 previous stages are 1’s. Hence a synchronous counter- can be made by using gates to control the J and K inputs on the flip-flops and clocking all flip-flops together as in Figure 28. I n the circuit shown, flip-flop A will always toggle because J = K = 0. Flip-flop B will toggle when Q A a 1 (GA= 0 ) , and C will toggle onlyTf both A and B are 1’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 highspeed applications, however, it is better to use the circuits described later under “hlSI.” Several applications notes are available which describe the operation and uses of R T L logic. They may be obtained a t no cost from the manufacturers (principally Motorola and Fairchild). Transistor-Transistor Logic and Diode-Transistor Logic

Although R T L has been widely used in the past, it suffers from some


-__--

_.

3

Figure 27. Binary counter with ripple carry

36A

A

CP

CP

+K

c

----~

distinct disadvantages. The primary drawback is low noise immunity. The low noise immunity is due to the fact t h a t the output of a gate in a low state is only 0.1 V below the maximum voltage allowed for a low a t an input. (maximum “low” 0.4V voltage a t an output) VIL (maximum voltage a t 0.5 V an input guaranteed to be a 0) TiOL

A noise spike of only 100 mV in an R T L system can cause a 0 to be interpreted as a 1. Noise can-be introduced from &itching transients, from other equipment in the area, or from “cross-talk” (interference between adjacent wires). T2L and D T L have much superior noise margins (400 mV) . The industry has switched largely to the use of T2 logic and consequently 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 “hlSI” or Medium Scale Integration. D T L and T2 logic are not compatible with R T L , so the designer must choose which type to use a t the outset, and then use it throughout the system. D T L and T 2 L are compatible with each other, however. D T L or T 2 L circuits should be used if noise immunity is likely t o be a problem, if RISI blocks are to be used, or if the instrument must interface directly with equipment using T2 logic, such as the PDP-81. The basic gate function in both D T L and T T L is KAND. Gates are available with anything from two to eight inputs. I n addition, there are inverters and J-K flipflops. The devices are sold in 14 or 16 pin DIP’Sand use a power supply of 5.0 V k 105%. They are both

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Circle NO. 95 on Readers' Service Card

38A


750 S F U L T O N AVE , M T V E R N O N . N.V 10SSO A OlVlSlON OF PEPI. I N C


quired, an M S I counter is much more satisfactory than one made l&A from discrete flip-flops. T h e MSI product is more reliable and in3 x w + iv EQUIVALENTCIRCUITFOR INPUTS 0.8rnA( 10.8rnA volves no design problems. A counter may be constructed t o count in straight binary up t o 1111, Current Sinking Logic or in Binary Coded Decimal (BCD) up to 1001. A four-bit binary counI n R T L , current flows from gate TO OUTPUT STAGE outputs into gate inputs, so a dister is sometimes called “hexadeciconnected input is a logical 0 bemal,” and a four-bit B C D counter INPUTS is called a “decimal” counter. T h e cause there is no current flowing B C D counter, which goes to zero into it. I n T2L,current flows out of Figure 29. Currents associated with after count nine, is generally prefthe inputs and into the outputs. TTL logic. Currents into inputs are erable. T h e outputs are current sinks rather leakage of reversed biased junctions Four-bit B C D counters can be than sources. Consequently, a dispurchased as “up-counters,” which connected input in T Z L is a logical count in one direction only; (‘up1, because no current, can flow out down counters,” which can be reof it. Typical numbers for the curversed; and presettable counters, rent in T2L gates are given below, which can be loaded with some iniand illustrated in Figure 29. tial number other than zero. InexI I L (total current out of 1.6 mA pensive counters use “ripple carry’’ inputs which are low) which means t h a t the change in one I I I j (current into each 60 p A counter from 9 to 0 clocks the next Figure 30. Derivation of function of Figure 18 using NAND gates input which is high) counter. More expensive counters use “carry lookahead” similar to lor, (maximum current 20 mA the gating system illustrated in into output when Figure 28 to speed up the R T L output is low) binary counter. MSI decoder/ l o a (maximum current 1.2 mA drivers are available which receive out of output when the four lines from a B C D counter output is high) and convert the result to some output which can drive a display sysGenerating Logic Functions with NAND Figure 31. Implementation and repretem. This output generally is one Logic functions can be generated sentation of R-S flip-flop using NAND of two types. as easily v i t h SAND logic as with gates T h e most common readout sysKOR. I n Figure 30 the circuit of tem uses “Xixie” tubes (Nixie is a Figure 18 is implemented with BURROUGHS 85150 registered trademark of the BurNAND gates. T h e derivation of roughs Corp.) . The decoder/driver the S\TAKD logic equations is shown turns “on” (grounds) one of the 10 also. CLOCK outputs which corresponds t o the R-S flip-flops are made with decimal number on its inputs. T h e - 1 *11OV croas-coupled gates, as in R T L , but REST selected number lights up. A newer the inputs are active low instead of type of readout is the seven-seghigh (Figure 3 1 ) . AIultivibrators cannot be reliably made with T2 ment lamp. Decoder/drivers are also available for these. Figure 32 gates. Instead, multivibrators can a t top illustrates a display system - - - -KCOLC be purchased as monolithic inteREST using R T L parts and a Nixie tube. grated circuits (Fairchild 9601 and Figure 32 a t bottom shows a seven9602). segment display using T2L MSI. Figure 32. Top! RTL decade counter and decoder/drrver for gas-filled lamp I n conjunction with the counter Medium Scale Integration (MSI) display. Bottom: TTL MSI system for and decoder, it is frequently de>IS1 is defined as a monolithic seven-segment display sirable to use a four-bit latch. A device containing more than five latch is a flip-flop used to store inor six gates on a chip. Most of them formation. I t is placed between the are rather complex circuits for procounter and the decoder so t h a t the cesqing digital data. These include counter can be reset without demultiplexers, shift registers, arithmetic units and the like. T h e ?\IS1 stroying the displayed information. When the latch is enabled, the outproducts frequently used in instruputs simply follow the inputs. mentation are counters and deWhen i t is disabled, the outputs are codcr/drivers. “locked into place” and can no When high-speed counting is re-

current sinking logic. The main difference between D T L and T T L is t h a t D T L is less expensive and T T L is faster. Everything t h a t is said below about T T L applies t o DTL also.

:t

I

1


39A


ENABLE

REST

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 (E) on the latch is lowered for a moment and then raised t o a 1 again. This will enter the count and store it. The stored count is decoded by 9960

-6V VOLTAGE TRANSFER CHARACTERISTIC

longer change. Generally the latch will be enabled only for a brief moment a t 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 R T L system using all three components. The 9958, 9959, and 9960 may also be used with T2 systems, but this is not advisable; T2 MSI should be used in a T2 system, There are many useful MSI and T2L 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 obsolete 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

-1.0

-3.0

-5.0

VIN

1.0

- INPUTVOLTAGE - mV

3.0

5.0

Figure 34. Voltage transfer characteristic. The voltage comparator has an output of logical 1 or 0 depending on the relative magnitudes of the two inC2 put voltages

Digital systems in instrumentation are frequently controlled by the relative magnitudes of two analog signals. T o accomplish this, a comparator is used. The comparator is like an operational amplifier with a digital output. If an operational amplifier were operated with no feedback resistor, then as soon as the voltage on the +input exceeded the voltage on the -input, the output would jump to a positive saturation voltage of about +10 V. Similarly, if the -input were greater than the +input, the output would saturate a t -10 V. A comparator is made by clamping the output levels so t h a t the output saturates a t logic levels instead of + l o 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. T h e greater the gain of the comparator, the lower the “threshold” voltage and the steeper the linear region. The Fairchild pA710, illustrated in Figure 34, is typical. Some Small Systems

Figure 35. Top: analog differentiator. Bottom: digital differentiator (edge detector) 40A

0

Differentiators (Figure 3 5 ) . Two kinds of differentiators are shown. The analog differentiator produces


an output voltage proportional t o the rate of change of the input, as long as the input is changing with a frequency less than w. The additional components R1 and Cz reduce noise by limiting the gain a t 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 R T L gate. (T2L gates don’t like to have capacitors on their inputs.) T h e example shown in Figure 35 uses a pL900, which is convenient because it has the required resistor on the chip. Pulse Counter (Figure 3 6 ) . Pulses over 100 mV in magnitude and of a t least 40 nsec duration can be detected by the comparator (pA710) and the comparator output 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 pA733 and the video amplifier output to the comparator. The two diodes, comparator input, protect the input from high voltages. Pulse Height Detector (Figure 3 7 ) . The system in Figure 37 allows only pulses with magnitudes between V L and V Dto 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 mono&able. If, however, the input pulse exceeds the upper limit set by V u , then the second comparator sets the R-S flip-flop, which places a 1 on the output gate. This will h o l d t h e output a t 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 photon counting to reject low-level noise pulses and high-energy pulses from gamma rays. Voltage-to-Frequency Converter (Figure 3 8 ) . The voltage-to-fre-

Coleman Model 44 provides direct linear absorbance at half the cost of similar spectrophotometers Now, for the first time, you can get direct linear absorbance readout in a spectrophotometer in t h e low price range. T h e new COLEMAN Model 44 UV-Visible-NIR Meter Spectrophotometer provides the easy, precise readout formerly available only in instruments costing twice as much. I t offers a wavelength range of 325 to 825 nm. Linear absorbance readings and linear recorder output eliminate the inconvenience of working with logarithmic scales or charts; save time for the analyst; reduce the possibility of visual errors. Results are shown clearly on the large, 7” wide meter scale. Because the scale is linear, all parts of the meter-even the high absorbance rangeare easy t o read. Stable meter reading assures fast, accurate readout. Built-in conversion circuit permits instant switching from A t o Coleman Instruments Division, The Perkin-Elmer Corporation, 42 Madison Street, Maywood, Illinois 60153

70T, and vice versa. For new speed and ease in sample handling, Model 44 can be used with the new AutofillTM Cell Assembly-permits you t o introduce a sample, read, flush out, and insert the next sample in seconds. Model 44 carries on the Coleman tradition of low-cost, high-precision, easy-to-use spectrophotometers. This rugged, compact, solid-state instrument is ideal for industrial research, quality control, school laboratories, water quality tests, air pollution measurement, analytical determinations of impurities or trace elements in iron, steel, aluminum or copper; and for testing electroplating equipment. Write t o d a y for new Model 44 brochure, complete with facts on the unique new AutofillThc Cell Assembly. Send for Bulletin A-334.

PERKIN-ELMER

(312) 345-7500 Circle NO. 31 on Readers’ Servics Card


41A


IN IN

Y DECODERIDRIVER

5

V

DECODERlDRlVER

2..2 k w

R5

Figure 36. This system will detect pulses over a few mV and count them

4

OOUl

{ R 3.3k 6

I Figure 38. Voltage-to-frequency converter TO COUNTER RESET

TO COUNTER

CO~IPARATOR 2

w

D1

42A

h

\I

elNo COMPARATOR

-

-loV

Figure 37. Window input for counter

quency converter is useful for transmitting signals over wires or radio carriers. The circuit shown has an output of a series of pulses whose frequency is directly proportional to the input voltage. The input voltage produces a current through R3 which is integrated on capacitor C1. The input voltage must be negative in order to produce a positive signal a t the amplifier output. If the input voltage is constant, the output will be a positive going ramp. The second amplifier is used as a comparator to compare the ramp with a voltage set by the divider R5 and Rs. For the resistances shown, this will be about 10 V. The output of the second amplifier is saturated a t $12 V initially and the transistor is turned off. When the ramp reaches 10 V, the amplifier saturates a t -12 V and the transistor turns on, forcing current from R5 into the integrator input. The 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. The ramp 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 +10 17. The output is a series of pulses from + l o to ground and back. The

O.1Uf

-I%

1, G l e DECADE COUNTER

i--c(E

LATCH

I

Figure 39. Analog-to-digital converter

frequency of the pulses depends on how long it takes the integrator to reach +lo V, which is proportional to the input voltage.

Analog-to-Digital

Converter

(Figure 39). The analog-to-digital converter has many similarities to the previous circuit. A reference voltage, derived from 10 V Zener diode D , is integrated to produce a smooth ramp voltage. During the integration, pulses from an astable multivibrator are counted. When the ramp reaches the input voltage, a comparator turns on, setting the R-S flip-flop, which stops the pulses into the counter. The comparator a t the same time enables the latch (9959) briefly to enter and store the count. The R-S flip-flop drives current into the integrator to discharge it and return the ramp 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. The number decoded and displayed is a digital representation of the input voltage. The resistor and capacitor shown produce a ramp whose slope is 10 V/ sec. See (11) for a lengthy discu,,'w o n of these circuits. Electroniete,. (Figure 4 0 ) . 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 t o make a

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Circle No. 122 on Readers' Service Card


0

43 A


IN

OUT

F o r fo =

lkHz

,

R1

- -

R2

=

403R

R3

=

637kR

c1 cz 0.0w Figure 40. Electrometer for use with high-impedance sources

very high-impedance (300 Ma) low-noise highly stable input. The second amplifier is used t o provide a voltage gain of 1 V/59 mV. The impedance is high enough t o 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 t o about 100°C. An FET input stage on a n op amp can be used directly with a glass electrode, but temperature drift is significant unless very closely matched FET’s are used. [Such devices may be available a t low cost in the first part of 1970 ( 1 2 ) . ] A very simple FET electrometer for glass electrodes is discussed ( I S ) .

31.8kR

I

‘U

FREQUENCY

SELECTION OF COMPONENT VALUES

1. Select %, fg, Q.

2. C h m e a r b i t r a r y capacitance C .

Figure 41. Tuned amplifier or active filter. Only frequencies around Fo are amplified by this circuit. A. is the gain of the circuit at the center frequency and Q is a measure of the bandpass

/”

LIGHT SOURCE

CHOPPER

FLAME

-- - - - - -

----

Define

- - - --

k =hf0c

H=VQ

4. C a l c u l a t e C I - C z - C 1

R1‘K R2

1 (29-k Cln

LU

R 3 ’ T

DETECTOR

~

0.

3.

4.

TUNED ~

AMPLIFIER

RECORDER ~

Active Filter (Tuned Amplifier) (Figure 41). Sometimes i t is desirable to amplify a signal only of a certain frequency, while suppressing other frequencies. A collection of resistors and capacitors around a n op amp can make a tuned amplifier with moderate gain (5-20) , for frequencies rvithin a certain band, and low gain ( < I ) for other frequencies. A simple active filter (Buttervorth response) is shown in Figure 41, along with the rules for selecting component values. Q should be as high as possible t o get a narrow band pass, but very high values will dictate negative resistances. It is difficult t o get a Q greater than nbout 25 with this configuration. Active filters are quite complex, but a good discussion appears in ( 1 4 ) . Several recent articles in Electronics have dealt with them, also (15, 1 6 ) . Figure 42 illustrates the use of the tuned amplifier t o improve signal/noise in a flanie photometer. Multipliers and Dividers. The 44A

Figure 42. Spectroscopic system with tuned amplifier. Light is chopped by segmented rotating disk. Amplifier is tuned t o chopping frequency and can greatly enhance signal/noise ratio

nonlinear operations of multiplication and division are difficult t o perforni accurately. If high precision is required, analog multipliers can be purchased as hybrids. They are called “quarter-square multipliers.” T o build a circuit which can multiply and divide, one can form logs, add or subtract them, and then form the antilog. For high accuracy, a circuit like the log amplifier in Figure 8 should be used, but if accuracy of only a few percent is needed and if the circuit will be operated at 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 relationship of the base-emitter junction in an NPN silicon transistor. The discussion of this kind of circuit in (17) should be consulted for in-


formation on the biasing and offset schemes. Amplifiers 1, 2, and 3 form the logs of X,Y , and 2. Amplifier 5 inverts one of the logs. The logs are summed by amplifier 4, lvitli log Y having twice the weight of the others. Amplifier 6 takes the antilog. K i t h proper biasing and selection of transistors, an accuracy of 1.107. should be obtainable in the output.

Digital-to-Analog

Converter

(Figure 44). A digital-to-analog converter is used to obtain an analog voltage from a binary number. This is frequently accomplished by connecting each bit of the binary number to a resistor whose Yalue is inversely proportional t o 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

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complete second double beam channel to the IL 253. It’s quick and easy. Just plug in six components. No outside boxes, each addition is an integral part of the unit. Then analyze samples for two elements simultaneously by absorption and/or emission on either channel. Or, for the highest possible precision use the new channel as an internal standard. The IL 253 provides all this and a great deal more at a cost no higher than some single beam, meter readout, economy model instruments. For instance, you get such well established IL features as the only

standard AA monochromator fitted with step-variable curved slits; the patented Counter-Flo JetT for greatly increased efficiency of nebulization, plus automatic ignition with “fail-safe” circuitry. Best of all the IL 253’s innovative design eliminates the need to buy an expensive second unit to handle increased demands. It provides the highest sensitivity, precision and stability in a single channel, dual beam absorption/emission spectrophotometer available, plus the unique “Two Tomorrow ADD-vantage.” For further information send today for Data 253.

lnstrurnentation Laboratory Inc.

-

113 HartwellAvenue, Lexington, Massachusetts 02173 Circle No. 70 on Readers’ Service Card

'

n

SPECTRONtC 20

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ter. It's easy to remember the Spectronic 20 . . . the Easy Reader. Over 85,000 users have already chosen the Spectronic 20. Our new Catalog 33-266 explains why. Write for a copy. On request we'll arrange a demonstration of the Spectronic 20 in your lab at your convenience. Call or write Analytical Systems Division, Bausch & Lomb, 1207-22 Linden Avenue, Rochester, New York 14625. Circle No. 13 on Readers' Service Card

Report for Analytical C h e m s

converted by connecting a 1K resistor to the most significant bit, 2 K t o the next, 4K t o the next, 8K to the least significant bit. Then the currents through the resistors are propoitional t o the weights of the corresponding bits. Unfortunately this simple scheme is not accurate because the logic levels are not exactly specified. They vary somewhat from unit t o unit. The p-4722, used in Figure 44, is a n accurate current switch. The binary digits are used t o switch currents onto the output line, with a unit weight of 10 pA-e.g., a binary number of 00001001 (= 9) would give a n output of 90 p A . The part is moderately expensive, but is accurate t o 0.08% a t 25°C.

SE4021 X 47k

hefirst time. But the simpler the ystem, the fewer the possibilities or error or malfunction. One last caution: Your inte;rated circuits should be purchased rom the manufacturer’s distributor n your area. Cut-rate deals from liscount merchandisers generally urn out to be bad deals. The parts vi11 not only not meet specifications, hey may be functional rejects and lot work a t all. Distributors can Jso usually supply you with appliations help, or refer you to inormation to aid you. References

1) Symposium on Operational Amplifiers, ANAL. CHEM., 35, 1770-1829 (1963). 2) “Applications Manual for Computing ilmplifiers for Modeling, hleasuring, h,!anipulating 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). 1) “Fairchild Semiconductor Linear Integrated Circuits Applications Handbook,” Fairchild Semiconductor, 313 Fairchild Drive, Mountain View, Calif. 94940 (1967).

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Circle No. 80 on Readers’ Service Card

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( 5 ) Jerry Eimbinder, Ed., “Designing with Linear Integrated Circuits,” Wiley, New York, N. Y., 1969. (6) Willard, Merit, and Dean, “Instrumental Methods of Analysis,” Van Yostrand and Co., Princeton, 1965. (7) Malmstadt, Enke, and Toren, “Electronics for Scientists,” Benjamin, New York, N . Y., 1963. ( 8 ) Malmstadt and Enke, “Digital Electronics for Scientists,” Benjamin, New York, 3.Y., 1969. (9) George Lauer and R. -4.Osteryoung, ASAL. CHEM., 4 0 ( l o ) , 30 -4 (August 1968). (10) Donald Lancaster, “Using New Low-Cost Integrated Circuits,” Elecfron. TYorld, hlarch, p 50 (1966). (11,) Hermann Schmid, “AID Conversion,” Electron. Design, 25, 49 (Dec. 5, 1968) ,and 26,57 (Dec. 19, 1968). (12) R. Christensen, and D. Wollensen, “Matching FET’a by Design Is Faster and Cheaper Than by Pick and Choose,” Electron., 42 (25), 114 (Dec. 8, 1969). (13) R. C. Dennison. “Solid State DH hfeter,” Pop. Electron., 29 ( 5 ) , -33 ( S o v . 1968). (14) “Handbook of Operational Amplifier Active R C Networks,” Burr-Brown Research Corp., 1966. (15) Michael Hills, “Active Filters: Part 13, Sarroiving the Choice,” Election., 42 (221, 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, -1s.~~. CHEM.,41, 1560 (1969). (19) H.C . .Tones e t al., ANAL. CHEM., 4 1 , 7 7 2 (1969). (20) Kepco. Inc., “Power Supply Handbook,” Publ. No. 146-1131: Kepco, Inc.. 131-38 Sanford Ave., Flushing, N.Y. 11352.

Tlie list helow gives the nameq addresses and product lines o l the principal manufacturers of integrated circuits surh as t h o k used in ihis ai-Cicle. The list is not intended t o be complete, hut does include all the large companies which have extensive appiications information available. A S A L O G DET’ICEB High-quality operational amplifiers 221 Fifth Street Cambridge, Mass. 02142 FAIRCHILD SELIICOSDUCTOR R T L , DTL, T T L , op amps 464 Ellis Street hlountain VLew, Calif. 94040 hlOTOROL.1 SE>IICOSDL‘CTOR R T L , DTL, T T L , op amps Box 20912 Phoenix, driz. 85036 NATIOS.4L SEXTICOSDUCTOR T T L , op amps 2975 San Ysidro Way Santa Clara, Calif. 95051 PHILBRICK RESE.iRCHES, I S C . Hybrid amplifiers Allied Drive a t Rte. 128 Detlliam, Alass. 02026 SIGNETICS C O R P O R I T I O X DTL, T T L 811 East Arques Sunny\.ale, Calif. 94086 SYLTANI.1 E L E C T R O S I C COMPONESTS DTL. TTL 1100 Slain Street Buffalo, N.Y. 14209 TEXAS I S S T R U M E N T S INCORPORATED R T L , T T L , op amps P.O. Box 5012 Dallas, Tex. 75222 Names, adrlwsses, and products of many other companies can be Eound in magazines such as Electronic Products and Electronic Design.

Using integrated circuits in chemical instrumentation - Analytical

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