Instrumentation
a number of systems for breadboarding IC op amps and logic elements (Cor related Educ. Serv., DEC, EDTEC Assoc, EL Insts., Heath, M P I ) . The op-amp manifolds and IC bread board systems are only partially help ful, however, as many additional build ing blocks are usually needed to implement all of the desired signal transformations. Examples are voltage references, timers, relays, integration networks, switches, and precision re sistors. Also amplifier-voltage balance and input-current compensation cir cuits are essential and must usually be supplied by the user. When building a specialized device to be housed in a box and used as a finished instrument, you have no choice but to wire up the unit with individual components. You have to go to the trouble of locating the various parts and assembling them. For methods development and shortterm projects, one system is available which offers the amplifiers ready to use and also includes a wide variety of ac cessory units (MPI System 1000).
from the prime source
Output Transducers
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The instrument readout device may be as simple as a meter or complex as a high-speed teletypewriter. Some of the commonly used output transducers in clude: meter, strip chart recorder, X-Y recorder, oscilloscope, photo graphic film, potentiometer and null meter, digital printer, and alpha-nu meric display. Occasionally ear phones are used to detect a null point. Most readouts can be purchased as ready-to-use modules. Again, you should avoid ground loops when lowlevel signals are fed to the device. It is best to buy equipment which can be operated floating.
Power supplies can drift too. The modern units have temperature-com pensated zener diode references, which help considerably in reducing drift caused by temperature changes. Mer cury cells are still often used for refer ence voltage sources in instruments. These cells are reasonably stable when handled properly. Short-term drift can be as low as 0.005% of the halfscale range. Power supply variations can seriously affect the drift of some cheaper op amps. An important speci fication for amplifiers is the power sup ply rejection ratio. A reasonable value for a quality unit is 105 (100 db). Drift in solid state amplifiers is largely caused by temperature differen tials. Integrated circuit amplifiers can have quite low voltage and current drifts, since the devices are so small and all the circuit components are close together. In fact, they are on a single silicon chip. Typically, the offset volt age drift of IC op amps runs from 3-30 μΥ/° C. The input bias current drift is about 0.6-2 nA/° C. Well-de signed differential units built from dis crete components can do about as well. Amplifiers employing field-effect tran sistor (FET) input stages have voltage drifts from 3-50 μΥ/° C. The input current doubles for every 10° C tem perature rise. This may seem like a large drift. However, the bias current is so low that this change is usually unimportant. Chopper stabilization can be used to reduce drift to below 1 μΥ/° C. Some amplifiers are available with tempera ture drift as low as 0.1 μΥ/° C. Most chopper amplifiers have very low-in put current drift. Chopper amplifiers draw about 10-100 pA at 25° C. Some can be adjusted to draw less than 1 pA. Bias current drift varies from 0.03-100 pA/° C for these devices.
Practical Considerations
Practical considerations in the design of an instrument include drift, accuracy, stability, and noise. An understanding of the factors affecting each of these items is necessary, if you hope to de sign an instrument that will meet your desired specifications. These factors are discussed below. Drift can occur in each of the parts making up an instrument, but as often as not, the detector gives more trouble than the others. For example, we all are familiar with the glass electrode and its unstable performance if it is not properly soaked before use. Since each detector has its own peculiarities, it is best to carefully follow the manufac turer's instructions. The need for well-regulated power supplies, for those detectors which use them, has already been mentioned.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1 1 , SEPTEMBER 1970
I WW' Figure 4. network
General inverting amplifier
