INSTRUMENTATION
Advisory Panel Jonathan W. Amy Glenn L. Booman Robert L. Bowman
Jack W . Frazer G. Phillip Hicks Donald R. Johnson
Howard V. Malmstadt Marvin Margoshes William F. Ulrich
A Modular Approach to Chemical Instrumentation Richard G. McKee McKee-Pedersen Instruments (MPI), P.O. B o x 3 2 2 , Danville, Calif. 94526
Analog and digital circuit modules offering a wide variety of electronic functions are now available. These powerful and versatile tools are reliable and easy to use. By combining appropriate modules with a few accessories, the analytical chemist can design new instruments HE ANALYTICAL CHEMIST o f t e n m u s t
T develop a specialized or new instru ment for his work. In the past, this required a significant knowledge of elec tronics or the assistance of an electronic engineer. The advent of modern tran sistorized and integrated circuits has changed this. Now complex electronic functions are easily obtained using in tegrated circuit or discrete component operational amplifiers and digital inte grated circuits as modular building blocks. A detailed knowledge of elec tronics is not required to use these modules so that instrumentation design is simplified. The recent article by Springer (/) is a good reference on the use of inte grated circuits, both linear and digital. This article will not repeat the material he covered. Instead, it attempts to go one step further and show you how to go about designing an analytical instru ment. The principles of operational amplifiers (op amps) will not be cov ered as they are familiar to most read ers. [If you are an exception, the book by Morgenthaler (2) is an excellent primer.] However, the limits of op amps and the other factors which should be considered when designing an instrument will be discussed. The modular approach to instrument design involves a simple analysis of re quirements. First you must put down your desired specifications. Then you analyze your instrument, in terms of the functions the different parts must per form. Finally you select electronic cir cuits and modules to perform the vari ous functions. In this step, the selection depends on the original specifications.
Some of the important considerations in selecting modules are covered below.
and reduce the difference to zero. An example is the pION-stat discussed at the end of this article.
What Is an Instrument Input Transducers
To best understand the functional ap proach to instrument design, first re view what an instrument is. Simply stated, all instruments have three parts: input transducer, signal transformation modules, and output transducer. The input transducer, or detector, responds predictably to the physical property to be measured. The signal transforma tion modules perform necessary and/or desirable operations on the electrical output from the detector. The output transducer converts the final electrical signals back to physical ones we can read and interpret. With the addition of a reference signal and a feedback loop, an instrument becomes a control system (Figure 1). The signal from the detector is compared with the ref erence. The modifier output is then used to operate some device which will make a change in the physical system
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Essentially every effect which can oc cur in an electrical circuit has been used in some type of input transducer. Table I gives a few familiar examples. Note that only those devices which pro duce a voltage may be used without an external power supply. Most input transducers are inher ently analog devices, as they are mea suring continuous physical properties. The detectors respond steadily to their environment and give continuous elec trical outputs. Where events occur in a digital fashion, the detector is de signed to give pulse outputs. For ex ample, a lithium-drifted germanium radiation detector gives a current pulse whenever struck by a gamma ray. The performance of an instrument is usually limited by the quality and capa bilities of the input transducer. De tector design is difficult. Fortunately, a large variety is available, and you can probably avoid designing your own. But, you cannot always avoid the de sign of the associated equipment—e.g., such items as sample holders, detector housings, shielding, light sources, and optics. Some of these items are sold as individual modules by various manu facturers. However, many will be spe cific for your application and will have to be built to order. When the detector requires a power supply, you should obtain one that is well regulated. Otherwise, your instru-
ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970
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Instrumentation
ment may not give reproducible results. For example, if resistance is the dependent variable, it might be measured by applying a constant current and observing the resulting voltage drop (Ohm's Law: R = E/I). If the current is not constant, there will be a varying output even when the detector resistance is constant. An excellent way to provide constant currents and voltages for detectors is by means of operational amplifier circuits (3-6). Two examples are shown in Figure 2. A stable reference voltage is required for both. The resistors used should be metal film ± 0.1% tolerance or better with a low temperature coefficient. Many detectors require voltages or currents that are beyond the capability of operational amplifiers. If you are using one of these, a special power supply will have to be obtained. You should make certain that the supply may be operated floating—i.e., the common is not connected to the chassis. Then the common may be tied to the same ground point used by your other instrument building blocks. This helps to eliminate ground loops which can cause serious electronic noise. An op-amp circuit can often be used when the detector requires excitation with some waveform. Very accurate and stable waves (e.g., sine, square, triangular, sawtooth, ramp) are easily generated (7, 8). An advantage of the op amp circuits is their ability to generate very slow waves. The disadvantage of many general-purpose op amps is their inability to produce waveforms at frequencies much above 5-10 kHz. For higher frequencies, you can go either to high-frequency op amps such as the Fairchild type 715 or Analog Devices types 148 and 149, or to a commercial waveform generator. Again, you should make certain that it has a floating output. β
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proper power supplies and grounding, you should get good results without a lot of effort. Once you have determined the operations you wish to perform on signals, you have the problem of physically implementing the design. Much assistance is available from the applications literature noted before. Most of the basic circuits have been worked out. Only rarely will you have to design a totally new signal transformation module. To assemble your circuit, you can make use of the op amp manifolds offered by a number of manufacturers (Philbrick, Burr Brown, Analog Devices, MPI). These include the necessary power supplies. Also available are
Some Examples of Input Transducers
Detector Specific ion electrode/ reference electrode Thermocouple Photomultiplier tube Polarograpnic cell Thermistor Photodiode Hall effect device
Independent Variable Activity of some ion in solution Temperature Light level Concentration of electroactive specie Temperature Light level Magnetic field
Dependent Variable Voltage Voltage Current Current Resistance Resistance Resistance
Table il. Some Signal Transformation Operations
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The signal transformation modules operate on the signal representing the detector output. Depending on the type of detector, some of the operations may be necessary rather than optional. With high-impedance voltage sources, for example, it is impossible to draw any current without changing the voltage because of the resulting ir drop. The first job that must be done is isolation of the source with a circuit that draws a negligible current. Voltage amplification may or may not be required. With a current source, the objective is to measure the short-circuit current. Here a very low impedance device is needed. Since the signal current may be very small, the device must again draw a negligible current. The commonly used circuit is called a current-to-voltage converter (Figure 3). Table II lists some of the many operations performed by signal transformation modules in instruments and control systems. These are extremely diverse. If you tried to buy separate modules to do all these tasks, you would wind up with a high price and probably have serious ground loop and compatibility problems. Fortunately, most of these operations can be carried out by op amps and digital logic circuits. For background on the use of these devices, sec references (1-5, 9— 11). With a little care in providing
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