Electronics, Instrumentation, and Microcomputers - Analytical

Instrumentation

F. J. Holler

C. G. Enke S. R. Crouch Department of Chemistry Michigan State University East Lansing, Mich. 48824

Department of Chemistry University of Kentucky Lexington, Ky. 40506

H. V. Malmstadt

J. P. Avery

Pacific and Asia Christian University Box YWAM Kailua-Kona, Hawaii 96740

Department of Chemistry University of Illinois Urbana, III. 61801

Electronics, Instrumentation, and Microcomputers Principles and Practice for the Microcircuit Age

Tiny and inexpensive integrated circuits now perform many functions that required full racks of equipment only a few years ago. The miniaturization and economies thus introduced have revolutionized the types of scientific instruments available commercially, the form and structure of instruments, and the modes of instrument maintenance and repair. It would seem likely that the subject of scientific instrumentation, i.e., the body of knowledge and experience that supports the effective use and design of instruments, would also be profoundly affected. There are, in fact, several reasons why this should be so. First, the basic building blocks of electronic circuits have changed; a single integrated circuit (IC) now performs functions that earlier required dozens or thousands of transistors and other discrete components to achieve. Thus an understanding of the functions or processes performed by ICs is now more relevant than comprehension of transistor characteristics or discrete circuit design. Second, ICs perform functions on data encoded in all three domains— analog, time, and digital—and provide all possible conversions among these domains. Many recent domain conversion ICs utilize all three domains. The resulting mixture of domains used in most modern instruments makes the

earlier division of electronics into analog and digital less useful on the systems level. Third, ICs are incredibly inexpensive; the cost of the circuit board, connectors, power supply, and case usually exceeds the cost of the

ICs in an instrument by many times. This inversion of the economic factors from those of discrete device electronics has profoundly affected instrument design and maintenance practices, which suggests that a review of

Figure 1. Integrated circuit The most popular package for the integrated circuit chip is the dual in-line package (DIP) shown here. The metal contact "legs" are on 0.1-in. centers in two rows 0.3 in. apart. These are connected to the chip by fine gold wires. The actual IC occupies a very tiny fraction of the total package volume. For most ICs, the packaging is also the majority of the cost

ANALYTICAL CHEMISTRY, VOL. 54, NO. 2, FEBRUARY 1982 · 367 A

experiments and equipment for learning and design laboratories is in order. Fourth, large-scale integrated (LSI) circuits perform sophisticated operations such as analog multiplication, phase-lock detection, digital computation, active filtering, and correlation, to name just a few. These once-exotic techniques are no longer too expensive to be used in common instrumentation; therefore, their underlying principles are increasingly important. Fifth, the microcomputer has not only entered the scientific laboratory, but elements of microcomputer technology are being used in all kinds of instruments, from elegant spectrometers to pH meters. Computer operations and applications, along with digital data processing and control, are now an important part of the science of instrumentation. The nature and consequences of the impact of microcircuits on the principles and practice of chemical instrumentation are thus profound. Electronics is still an important tool, but the subject of instrumentation should no longer focus on electronic devices and circuits. The increased importance of understanding advanced data processing and control operations is fortunately counterbalanced by the decreased need to know the details of discrete level circuit design. Thus ICs allow and encourage a shift away from the "devices first" approach of traditional electronics presentations to a more "top-down" or function-centered conceptualization of modern instrumentation. Arguments for such a new perspective on electronics and scientific instrumentation are presented here along with a description of a learning system that demonstrates its practicality. The Functional Nature of Microcircuits AU the basic electronic circuit devices can be fabricated on the surface of a silicon chip and interconnected with vapor-deposited conducting paths to produce the amazingly powerful, compact, and inexpensive devices we call microcircuits or ICs. We rarely see the chips themselves, except in pictures. As shown in Figure 1, they are buried in a plastic or ceramic case that supports the chip and its connections. The first microcircuits were very expensive. It was the rapidly growing consumer market that motivated progress on the technological "learning curve" and enabled the introduction of automated mass production. Even today, the first ICs of a given type are very expensive, but the price drops rapidly as the production quantities go into the thousands and millions. For economy then, the circuits that are integrated must be gen-

erally useful; i.e., they must accomplish a commonly desired function. As a result, we now have an extensive "library" of generally useful functions available in inexpensive IC form. At first, only very basic functions were produced in IC form—logic gates, flip-flops, and crude operational amplifiers—but that was a momentous beginning. Today, as a result of the increased sophistication of electronic products and improvements in IC technology, the level of circuit complexity available in IC form is astounding. Examples of small-scale integration (SSI), medium-scale integration (MSI), and large-scale integration (LSI) are shown in Figure 2. In these examples, and in all ICs, the IC is named for the operation that it performs. The package provides connections for the data input(s), the data output(s), and the power supply. Intermediate points in the circuit are not available unless contact is needed to some supporting component. The base diagram (pinout) does not indicate the circuit of an IC, but rather gives only the symbol for the operation performed. For SSI devices, a "circuit diagram" of the chip is sometimes given in data sheets provided by the manufacturer, but this is oftenjust an incomplete transistor circuit that is similar in design concept to the IC. For an MSI or LSI device, the only diagram provided is a functional or block diagram that shows the IC as a combination of SSI (or MSI) level functions. From a practical standpoint, then, the normal user cannot study the circuitry of an IC or even analyze how its function is achieved in terms of its circuit design. However, to appreciate the use of ICs in instrumentation, it is important to understand the nature of the functions performed and the performance limitations (appropriate signal levels, output limits, sources of inaccuracy, etc.). Furthermore, to implement an IC in a new or custom application, the practical considerations for its effective use (power requirements, loading, etc.) are also important. The pedagogical argument for the need to study the equivalent transistor circuit of an IC in order to understand and/or apply its function is reminiscent of early descriptions of transistor characteristics that began with the analogous behavior of vacuum tubes. Today, the functional building blocks of electronics are neither vacuum tubes nor transistors, but undissectable functional level integrated circuits. This has led to the classification of electronics circuits (combinations of ICs) by their function, i.e., the type of operation(s) performed. Starting at the high level of the operation performed, an application can then be

368 A · ANALYTICAL CHEMISTRY, VOL. 54, NO. 2, FEBRUARY 1982

explored to whatever level of detail is necessary. An example of this "top-down" approach to electronics is the operation of taking the integral of a signal. At the highest level, integration can be understood as a mathematical operation performed on the encoded quantity. On the level of the principles of electronic operations, digital integration with a counter involves a different set of principles from those for analog integration with a capacitor and operational amplifier. Finally, on the circuit level, the limitations of a particular analog integrator will depend on the electrical characteristics of the specific integrating capacitor and operational amplifier used. By contrast, the organization of traditional electronics books has often been based on the classification of devices according to their structure and characteristics. This forces one to enter the subject at the device level and work up through circuits to operations. If one is to develop a structure based on the operation performed, a useful classification scheme for the operations is required. Fortunately, there is one—the concept of data domains based on the ways in which data are encoded and transformed in electronic systems (1). Digital, Analog, and Time Domain Microcircuits Information in an electronic instrument is encoded as some characteristic of an electrical signal. For example, the voltage from a photocell in a flame photometer is related to, and can be interpreted in terms of, the concentration of Na in the sample as illustrated in Figure 3. Each of the different ways data can be encoded is a data domain. A transformation from one domain to another occurs whenever there is a change in the characteristic by which the data are encoded. (There is always an associated change in the units of the encoding characteristic.) This provides a useful classification scheme because the number of domains by which data can be electrically encoded is limited. These domains fall into three categories: analog, time, and digital. Data in the analog domains are represented by the magnitude of one of four electrical quantities: charge, current, voltage, or power. The output signal of a pH electrode (voltage magnitude) or of a flame ionization detector (current magnitude) are examples of signals encoded in the analog domains. In the time domain, the information is encoded in the time relationship of the signal variations, not in the amplitudes of the variations. For example, the rate of occurrence of pulses generated by a Geiger tube is related to the

Figure 2. Small-, medium-, and large-scale integration The equivalent circuit of one of the four gates in a SN74LS00 Schottky TTL NAND gate (an SSI device) is shown in (a). Because the circuit exists on a single chip, methods of construction and interconnection are often used that cannot be represented exactly by circuits of interconnected discrete components. The circuits of MSI devices such as the magnitude comparator shown in (b) are not given; a connection scheme for SSI gates that would perform the equivalent function is given. The pin diagram only identifies the input and output connections. For LSI devices such as the dual slope analog-to-digital converter (ADC) shown in (c), the circuit is a combination of MSI level functions. This circuit also demonstrates the common mixture of analog, digital, and time domain operations in the same IC. The pinout of the Intersil 7106 ADC IC, which also includes the readout driver circuits, is shown as an example. Reprinted from Reference 2, with permission


Taking Advantage of Low-Cost Microcircuits

Figure 3. Data domains for a simple flame photometer The bold labels indicate the data domains in the measurement process. The flame, photocell, and voltmeter are interdomain converters. The flame converts the concentrations of Na and other sample species into related photon fluxes. The filter selects the photon-encoded information related to the Na concentration. The photocell converts this data to the voltage domain, which is converted by the voltmeter to the position of the pointer on the scale

level of radiation, The encoding quantity is the frequency of the pulses rather than their charge or current amplitude. Pulse-width and phase angle are additional examples of time domains. Digital domain information is contained in two-level signals (HI/ LO) that encode a specific integer. Thus, digital data are actually in numerical form. Since the desired result of a measurement is a number related to the measured quantity, measurements have always been digital; the digitizing step used to be done manually. Many modern instruments use signals encoded in all three categories of data domains. The signals from input transducers (devices that convert chemical or physical quantities into related electrical signals) are often in an analog domain. One or more interdomain conversion ICs, such as analog-to-digital converters, convert this information to the domain of the output display. Signal-processing ICs, such as amplifiers or integrators, perform intradomain operations on the data. Thus, a very useful description of an instrument can then be made in terms of the interdomain conversions and intradomain processes that it incorporates. Furthermore, all circuits and devices can be categorized according to the operation or transformation they perform. The data domains concept reduces the tens of thousands of electronic devices available today to a manageable number of categories and provides a high-level language for the description and analysis of complex instrumentation systems. There are many advantages to the early introduction of data domains in

the study of the electronics of instrumentation. It establishes at the outset the relationship between digital and analog electronic techniques and allows the full range of modern measurement techniques to be introduced and used throughout the study. The outline of a text (2) and a lab experiment book (3) that follow this plan is given in Table I. The order of topics is not a violent departure from recent practice, but the data operations orientation and top-down flavor are evident in several places. The basic foundation of data transformations among all three domains is completed relatively early, while all the important analog, time, and digital test instruments are introduced and conceptually explained. As the subject unfolds, techniques that involve mixed data domains like logic-controlled analog switches, monostable RC timers, and dual-slope ADCs fit in comfortably. The device-level topics of transistor amplifiers and logic families may be studied if desired, but they are not prerequisite to later topics. On the other hand, it is very important to introduce the microcomputer, its involvement in modern "intelligent" instruments, and the elegant data-processing techniques that the microcomputer can provide. A corresponding laboratory follows the same outline. In fact, the order of topics was developed first in the teaching/learning laboratory; only from its success there did the parallel text evolve. It is our experience that a topical sequence that works well in the lab can be an interesting and pedagogically sound basis for a text, but that many rationally organized texts do not support an effective laboratory.

The integrated circuit revolution has dramatically altered the economics of electronic system design. The IC costs in a system are now so low that they constitute less than 10% of the total system cost. In the period between 1959 and 1977 the per function cost of electronic circuits declined by a factor of 200 000 (4). As costs continue to fall, it is clear that IC costs will soon become a negligible fraction of total system cost. Unfortunately, there has been no similar revolution in connectors, packaging costs, mechanical components, power supplies, and other system components that support the electronic circuitry. Thus as IC prices have fallen, support component costs have assumed a larger and larger fraction of total hardware costs. A similar trend has ocurred in the reliability of electronic systems. As ICs have become more and more reliable, their failure rates have assumed a smaller and smaller fraction of total system failures, and the blame for instrument "down-time" has shifted toward mechanical components, power supplies, etc. Impact of Microcircuits in the Laboratory. The changing economics are having an increasing effect on scientific instrumentation in the research and teaching laboratories. The revolution in microcircuits has been accompanied by their incorporation into virtually every type of scientific instrument from balances to Fourier transform NMR spectrometers. Purchasers of scientific instruments are finding much more power and convenience available at the same or lower costs than in the past. At the same time, the electronic functions performed on scientific data are becoming more and more hidden from the user, making it imperative that we study these operations and functions to ascertain their effects—desired and otherwise. Scientists who design instruments to perform "state-of-the-art" experiments need to take advantage of the capabilities and characteristics of the latest generation of electronics devices and transducers. The teaching/learning laboratory has thus assumed a central position for understanding the principles of the advanced measurement and control concepts now being used in laboratory instruments. A Laboratory Breadboarding Frame. To be economical, it is necessary that a teaching/learning laboratory take advantage of inexpensive, high-volume circuits and test equipment developed for other (commercial) uses and that it minimize the use of specialized (and thus low-volume) teaching equipment. Therefore, we


Table 1. Outline of Text, "Electronics and Instrumentation for Scientists" and Lab Book, "Experiments in Electronics, Instrumentation and Microcomputers" Chapter

Name

Lab experiments

Text topics

1

"Electrically Encoded Information"

Data domains, analog and digital meters

2

"Periodic Waveforms and the Oscilloscope"

Information in ac signals, reactance, ac meters, oscilloscope

3

"Power Supplies"

Rectification, filtering, regulation,

4

"Input Transducers and Measurement Systems"

5

Breadboarding techniques, voltage, resistance and current measurements with DMM, voltage divider and current splitter, loading errors Oscilloscope and function generator operations, low-pass and high-pass filters, ac measurements with the DMM Characteristics of diodes and transformers, rectifiers, filters, and regulators Frequency and period measurements, basic digital devices, counting, transducers

"Operational Amplifiers and Servo Systems"

batteries Measurement principles, gating, counting, time domain measurements, transducers Servo system concepts, follower, summer, integrator

6

"Programmable Analog Switching"

Switch types, response to transients, RC timers, analog sampling

7

"Solid-State Switches and

Transistors, switching devices, amplifier circuits

Transient RC response, RC timers, relays and solid-state analog switching, programmable gain amplifier, and digitalto-analog converter Bipolar junction and field-effect transistor

Amplifiers" 8

"Linear and Nonlinear Op Amp Applications"

Waveshaping, oscillators, active filters, am modulation, lock-in amplifier

9

"Frequency, Time, and the Integrating D V M "

Universal counter, Schmitt trigger, integrating DVMs, fm modulation

11

"Logic Gates, Flip-Flops, and Counters" "Digital Devices and Signals"

12

"Microcomputers"

Gates, latches, flip-flops, counters, shift registers Logic families, output types, digital signal techniques Structure, numerical operations, memory, I/O, peripherals, languages Ports, clocks, converters, systems

10

13

"DACs, ADCs, and Digital I/O"

14

"Optimized Measurement and Control Systems"

have developed an efficient, practical, and inexpensive experimental patchwiring system that supports the experiments listed in Table I. The patchwiring system uses commercially available solderless wire sockets called "breadboards" for wiring and testing the functions and systems being studied. The breadboard frame shown in Figure 4 connects readily to an inexpensive open frame power supply and supports five interchangeable breadboard sockets of the "SK-10" type (E & L Instruments). This frame can be easily constructed from readily available components (5).

Signal-to-noise enhancement, rate measurements, control, optimization

Modularity is provided on the level of the solderless breadboard socket. Power is provided to the breadboard sockets through their connection to the frame. This same connector provides easy connection of the breadboard circuits to external signal sources such as function generators and to external test instruments such as multimeters, oscilloscopes, and frequency meters. This same arrangement allows the interconnection of signals among the various breadboard circuits so that complete systems can be neatly developed. Three categories of prearranged breadboard sockets (called "job


Comparison measurements, op amp followers, current-to-voltage converters, inverters, summers, integrators and differentiators, op amp characteristics

characteristics, SCRs and TRIACs, switching speed, transistor amplifiers Difference and instrumentation amplifiers, waveshaping, analog multiplier, active filters, tuned amplifiers and oscillators, lock-in amplifier Counters, timers and frequency meters, charge-to-count, voltage-to-frequency, and dual slope converters, phase-locked loop Logic gates, latches, flip-flops, counters, counter gating, shift-registers Logic families, open collector and tristate gates, digital signal conditioning BASIC programming, execution time and speed, BASic-machine language interface, interrupts I/O ports, timers, DAC and ADC interfacing, sample-and-hold, data logger Noise-reduction techniques, quantizing noise, multichannel averager and analyzer, boxcar integrator, Fourier transforms, aliasing, smoothing

boards") are used in the system: 1) prewired boards that perform specific unaltered functions, such as logic level indication or reference voltage generation; 2) IC job boards that are loaded with ICs and components, but have only the power connections prewired; and 3) utility job boards that have only power connections to the power buses. Typical job boards from categories 1 and 2 are shown in Figure 5. The prewired job boards (such as the reference job board in Figure 5) are patchwired once by the instructor and not altered. Since high function ICs are used, wiring is simple, and the resulting function boards are much less

Figure 4. Breadboarding frame In conjunction with an external power supply, the frame provides + 5 V, ±15 V, and common connections to as many as five breadboard cards plugged into its connectors. Input and output from the cards are via five BNC connectors and five pairs of banana jacks that allow ready connection of function generators, oscilloscopes, digital multimeters, and other test instruments to the ICs on the cards. PC board tabs on each card mate with a PC board socket on the interconnection mother board to allow bussed signal and power connections

Figure 5. Operational amplifier job board and reference job board The op amp job board (top) contains three dual BIFET op amps, an adjustable offset op amp, a generalpurpose op amp, a comparator, four analog switches, and 10 precision resistors for op amp applications. The reference job board (bottom) contains a crystal oscillator and switch-selectable frequency divider, a voltage reference source that provides four fixed voltages (+10 V, —10 V, + 1 V, —1 V) and two continuously variable voltages (+10 to —10 V and + 1 to —1 V), a noise generator, and a dual BIFET op amp for summing signals with random noise

expensive than special-purpose printed circuit boards would be. With the preloaded IC job boards, students do all the patchwiring (except power connections) of inputs and outputs and interconnections of ICs. This saves student time and avoids damage to IC leads by insertion and removal. Prewiring power and common connections is more efficient and prevents mistakes that cause the IC to perform the "fuse" function. Minimizing the complexity of connections focuses attention on the functions rather than the wiring. Utility job boards are used for special "single-use" ICs, for awkward components (photocells, thermistors, and other transducers), and for special advanced projects. Altogether, 13 job boards are used at present in the system as shown in Table II. For economy, some of the more advanced job boards, which are used only infrequently, can be shared among stations or reconfigured when new functions are needed. Seven job boards are needed frequently. A Complete Laboratory Station. A complete laboratory station consists of the breadboarding frame with its power supply and job boards, commercially available test equipment, and a microcomputer. All of this equipment, except for the breadboard frame (which must be assembled by the user) is standard hardware, available from competitive suppliers. A dual trace oscilloscope, a 3V2-digit digital multimeter, and a function generator (sine, square, triangle) are the standard test equipment. In addition a counter/timer/frequency meter and a digital panel meter are constructed for each station from inexpensive evaluations kits (Intersil, Inc.). A complete lab station is pictured in Figure 6. The cost of each station, with careful purchasing, can be just over a thousand dollars (not including the microcomputer and the characteristic curve tracer mentioned below). The microcomputer is used for experiments in programming, interfacing, and data acquisition and processing. Since it is possible to avoid having all students do these experiments at the same time, a single microcomputer can serve a group of four or five stations. A microcomputer that satisfies the needs of the laboratory experiments is the AIM-65 computer available from Rockwell International. Its built-in printer, display, standard keyboard, and BASIC interpreter, as well as its user I/O port, give it all the essential functions at low cost. A characteristic curve tracer provides those who would like to learn the details of semiconductor device characteristics with an efficient way to obtain characteristic curves. One unit can be shared by many experimenters. (continued on page 382 A)


Figure 6. Complete laboratory station The complete laboratory station pictured consists of an oscilloscope, a function generator, a digital multimeter, the breadboarding frame, a digital panel meter, and a counter/timer/frequency meter

Table II.

Job Boards

Name

1.

Reference

2.

Binary switch and indicator

3.

Op amp

4. 5. 6.

Basic gates Flip-flop Counter

7.

MSI gates

8.

Advanced gates Signal

9. 10.

conditioning Advanced

11.

analog circuits Power supply

12.

Interface

13.

Utility

Category and description

Prewired circuits: precision time base, noise generator, voltage reference source Prewired circuits: eight logic level debounced switches, two momentary debounced switches, 10 LED indicators ICs: eight op amps, one comparator, one quad analog switch, two resistor arrays. ICs: basic TTL logic gates (seven ICs) ICs: flip-flops and 4-bit shift register (six ICs) ICs and prewired display: two decade counters, latch and 1-decade display ICs: magnitude comparator, adder, decoder, multiplexer, arithmetic logic unit ICs: CMOS gate, tristate buffer, Schmitt trigger buffer/ driver, open collector NAND, LS NAND ICs: op amp, comparator, timer, monostable multivibrators, driver, relay, photoFET ICs: multiplier, active fitter, phase-locked loop, op amps, lamp ICs and components: regulator, power diodes, zener diodes, filter components ICs: DAC, ADC, sample-and-hold, monostable multivibrator, ribbon cable from computer I/O port Empty

Several inexpensive curve tracers that utilize an external XY oscilloscope are available commercially, some in kit form. New Instrumentation Is More Than Electronics The microcircuit revolution necessitates a change in the teaching/learning laboratory. Because modern circuits come in fully functional IC packages, it is less important for the "normal" user to learn about semiconductor device physics, construction of amplifiers, amplifier biasing techniques, and several other topics that were impor-

tant with discrete transistors. (These topics are still of importance for those working with high-frequency electronics and high-power systems). It is our belief that the IC revolution should prompt two changes. First, there should be an emphasis on the basic principles of measurement and control concepts, including voltage dividers, loading errors, RC filters and time constants, feedback control, null comparison measurements, etc. Because of the new microcircuits, these important principles need not be boring or taught without reference to their applications. The modern operational


amplifier, with its nearly ideal characteristics, provides an excellent vehicle with which to develop measurement and control concepts. Loading errors are dramatically and readily demonstrated with modern 372-digit multimeters. Integrated circuit timers of the 555 variety provide a fascinating tool for learning about capacitor charging and discharging curves, RC time constants, and level comparison. The power supply regulator, the operational amplifier, and the phase-lock loop ideally illustrate the principles of feedback and its many applications in measurement and control. Integratedcircuit active filters superbly demonstrate the frequency response of RC circuits, the characteristics of resonant circuits, etc. These are but a few examples of the way in which highfunction ICs use and demonstrate basic processes and allow us to focus on the principles rather than the implementation. Second, the microcomputer and high-function ICs give us an exceptional opportunity for studying in a practical manner advanced instrumentation concepts, including signalto-noise enhancement as in Figure 7, digital control as in Figure 8, correlation techniques, Fourier transformation methods, sampling and aliasing, and many other topics. Thus there is an increasingly strong need for an instrumentation course to include some statistics, linear systems analysis, control theory, noise and fluctuation phenomena, and computer science as in Figure 9. In our experience, these advanced instrumentation concepts need not be couched in mystique or advanced mathematics, and their omission leaves the student without the knowledge needed to properly operate a computerized data acquisition system as in Figure 10, or even a digital oscilloscope as in Figure 11. These important concepts and principles will (continued on page 386 A)

Figure 7. Computer boxcar integrator A programmable timer is used to generate a variable delay between the trigger signal and the acquisition of the sample. Through program control a specified number of samples can be taken at any given delay, the delay can be scanned at the desired rate, and the time increment between points can be varied. Reprinted from Reference 2, with permission

Figure 8. Dc motor controller with position encoder The motor speed and direction are controlled by the CPU through the DAC. Clockwise (CW) rotations cause the counter to increment, and counterclockwise (CCW) rotations reduce the count. For clockwise rotation, the LO - » HI (dark - • light) trigger at the D FF occurs when D is HI, so Q is HI (count up). For counterclockwise rotation, D is LO (dark) when the LO — HI trigger occurs, so Q is LO and the counter reverses. The encoding wheel shown produces two counts per rotation. The position of whatever the motor is driving can be read from the counter contents within half a revolution of the motor shaft. Reprinted from Reference 2, with permission

Figure 9. The structure of a basic digital computer The central processing unit acquires and executes the instructions. The program is stored in a section of memory reserved for that purpose. The CPU supervises communication along the shared data bus by using the address bus to specify the source or destination of the data. The program counter supplies the address of the instruction to be executed. The control bus contains data direction, timing, and special control signals. Reprinted from Reference 2, with permission


Figure 10. D a t a acquisition s y s t e m This hybrid data acquisition system (Datel HDAS-16) is also available in an eight-channel differential input system (HDAS-8). A HI - » LO transition at Strobe ini tiates acquisition and conversion. Total acquisition and conversion time is ~ 2 0 με, which gives a throughput rate of 50 kHz. Reprinted from Reference 2, with permission

Figure 1 1 . B l o c k d i a g r a m of a digital s t o r a g e o s c i l l o s c o p e The analog input signal is digitized and stored in a solid-state memory at a rate determined by the time base settings. The memory content is then read out in order to the y DAC for display on the CRT or (more slowly) for the external recorder output. The readout can be repeated indefinitely. The x-axis deflection signal is obtained by analog conversion of the memory address. Data acquisition in the digital oscilloscope is synchronized with the input signal waveform by using the trigger signal to control the memory address counter. If the counter is running continuously before the trigger and proceeds less than a full count after the trigger, the memory will contain a pretrigger portion of the waveform. This unique ability to trigger after the event can be very useful. Reprinted from Reference 2, with permission

remain relevant through m a n y genera t i o n s of e l e c t r o n i c d e v i c e s . T h e r e i s i n deed m u c h more to modern instru mentation t h a n electronics.

A Framework for the Future A s w e h a v e s e e n , t h e i n v e n t i o n of t h e transistor a n d its evolution into integrated circuits have h a d a pro

f o u n d effect on scientific i n s t r u m e n t s a n d t h u s on our work as scientists. As new techniques and capabilities are i n t r o d u c e d into a discipline, its frame work m u s t be expanded to accommo d a t e t h e m . T h i s e x p a n s i o n is o n l y s u c cessful u n t i l t h e old f r a m e w o r k b e c o m e s t o o c u m b e r s o m e a n d ineffi cient. T h e n a new structure, one t h a t

t a k e s a d v a n t a g e of u n i f y i n g c o n c e p t s o n a h i g h e r level, t a k e s its place. T h i s is t h e w a y s c i e n c e t r a d i t i o n a l l y r e sponds to the geometrically increasing g e n e r a t i o n of n e w i n f o r m a t i o n a n d t e c h n i q u e s . I t is o u r p r e m i s e t h a t f o r t h e a r e a of e l e c t r o n i c s i n s c i e n t i f i c i n s t r u m e n t a t i o n , t h e t i m e for a n e w framework has come—a framework (continued


on page

393

A)

that can accommodate not only the dramatic advances in the recent past but can also provide a structure for future developments. In the teaching/learning laboratory, we must plan for the continued rapid introduction of new and higher level ICs. There is no sign that the technology has reached a plateau in this area. Therefore the experimental equipment should be flexible. Experimenter's systems in which the hardware is committed to present-day devices (such as special-function circuit boards) encourage the use of these devices long after they are no longer the best choice. This is one of the reasons that all of the job boards in the experimenter's system described above are "soft-wired." When a better, cheaper, or easier-to-use IC becomes available for any function, the system can be easily altered to accommodate it. Thus the lab can remain current with no loss in hardware investment and with no dependency on a teaching equipment manufacturer. In the research laboratory, the adoption of a data-centered concept of the electronic parts of our instruments can help to restore or even enhance

C. G. Enke S. R. Crouch C. G. Enke is a professor of chemistry at Michigan State University, East Lansing, Mich. He obtained a BS degree from Principia College in 1955 and a PhD in chemistry from the University of Illinois in 1959. Enke's research interests include analytical instrumentation, mass spectroscopy, electroanalytical chemistry, applications of spectroscopic instrumentation, and mini- and microcomputers in chemical research. Stanley R. Crouch is a professor of chemistry at Michigan State University, East Lansing, Mich. He attended Stanford University from 1958 to 1963 and received BS and MS degrees in chemistry. He received his PhD in 1967 from the University of Illinois. Crouch's research interests are in the areas of flame and nonflame ΑΕΙ AA/AF spectrometry, analytical laser spectroscopy, emission spectrometry in sparks, arcs, and plasmas, molecu-

the rapport we once had with chemical systems under investigation. Many scientists now have a kind of lovehate relationship with their advanced instruments: They love their enhanced capabilities but are anxious about an increasing remoteness from the system being studied. As more data-processing and interpretation features are added to instruments, the loss of a "feeling" for the system and a personal involvement in the measurement processes could increase. Thus the very tools that provide increasingly powerful insights into nature are, for some scientists, becoming a barrier between them and their subjects. Trying to learn all the current electronic techniques and then trying to keep up with the flood of new devices is not an attractive alternative. The higherlevel nature of the data-centered approach can avoid this dilemma. The principles of data encoding, transformation, and processing will not be made obsolete by a new technology for their implementation. Indeed, the focus on data operations serves as a very effective way of categorizing and interpreting the functions of all new devices as well as a means of analyzing

H. Malmstadt

and explaining the latest instrumentation. All this can be done without getting into the detail level of electronic devices. From the higher level of data operations, technology is providing increasingly powerful and inexpensive tools to perform these functions. In this way, the focus stays appropriately on the science, but the science now includes the principles of the data manipulations used in modern measurement systems. References (1) Enke, C. G. Anal. Chem. 1971,43, 69-80 A. (2) Malmstadt, H. V.; Enke, C. G.; Crouch, S. R. "Electronics and Instrumentation for Scientists"; Benjamin/Cummings: Menlo Park, Calif., 1981. (3) Holler, J. F.; Avery, J. P.; Crouch, S. R.; Enke, C. G. "Experiments in Electronics, Instrumentation, and Microcomputers"; Benjamin/Cummings: Menlo Park, Calif. 1981. (4) Blakeslee, T. R. "Digital Design with Standard MSI and LSI," 2nd éd.; New York: John Wiley & Sons, 1979. (5) The experiment book, Reference 3 above, gives the construction details and parts needed. For information on the copyrighted printed circuit tab and frame boards, write Printed Circuits, P.O. Box 27315, Lansing, Mich. 48909.

F. J. Holler

lar absorption and fluorescence spec trometry, fast kinetics, reaction-rate methods of analysis, heteropolymolybdate chemistry, signal-to-noise ratio theory, theory of analytical measurements, theory and applica tions of photon counting in spectros copy, automation of chemical mea surements, and clinical analytical chemistry. Howard Malmstadt is professor emer itus of the University of Illinois and vice-president of academic affairs at Pacific and Asia Christian University in Hawaii. He received his PhD in chemistry from the University of Wisconsin in 1950, and for over 30 years has been involved in teaching and research in applied spectroscopy, analytical chemistry, and electronics and instrumentation for scientists. His recent research includes clinical/ analytical automation and chemical methodologies, and he is helping to

develop a new type of

J. P. Avery university.

F. James Holler is assistant professor of chemistry at the University of Kentucky. He received his PhD in an alytical chemistry from Michigan State University in 1977. His research interests include theory and practice of reaction-rate methods of analysis, applications of stopped-flow tech niques, conductometry, applications of computers to analytical chemistry, and chemical instrumentation. James P. Avery is an assistant profes sor of chemistry at the University of Illinois. He received his BS at Michi gan State University and his PhD from the University of Illinois. His re search interests include computerbased instrumental systems, especial ly for reaction-rate methods of analy sis, electrochemistry, spectrophotom etry, and the development of systems and languages for the automation of analytical measurements.


Electronics, Instrumentation, and Microcomputers - Analytical

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