Designing the instrumentation amplifier, with its few external components, appears to be a simple task, but to obtain a reliable design, you must avoid certain traps. You must correctly match the instrumentation amp and the transducer to obtain the optimum performance with reliable operation. High-output-impedance sensors, such as piezoelectric and pH electrodes, require high-input-impedance, low-bias-current instrumentation amps, and that requirement dictates JFET and CMOS IC processes. The instrumentation-amp input impedance must be higher than the sensor-output impedance, or excessive loading reduces the sensor-output signal. The instrumentation-amp bias current must be small, because when it flows through the large load resistor, it creates input-offset-voltage errors in some circuit configurations. Also, a rule of thumb is that very-low-noise instrumentation amps require low source impedance; hence, they are unusable with high-output-impedance sensors.

All instrumentation amps require an input-bias current regardless of how small that bias current may be. The standard circuit configuration that capacitively couples the input signal directly to the instrumentation amp requires adding external resistors (connected to the return or ground) to complete the bias-current path. You can complete the bias path through an input transformer or inductor.

All instrumentation amps have common-mode-voltage limitations, and power-supply voltage, gain, and reference-pin voltage all influence the common-mode-voltage range. Determining the instrumentation amp's common-mode-input-voltage operating range is difficult because you must make detailed calculations, including those for internal instrumentation-amp nodes, for the complete operating range. Normally, manufacturers provide minimal data describing the common-mode-voltage range for selected operating conditions, and this data is inadequate if you are operating the instrumentation amp outside the prescribed operating conditions. You can measure the common-mode-voltage range with an analog storage scope, or you can download some calculation programs from the Web and draw a graph of the safe common-mode-voltage range. Beware: Three-op-amp instrumentation amps invert the output signal under certain operating conditions, so it is mandatory that you control the common-mode voltage and gain when using these instrumentation amps.

RFI can get into the input signal and cause dc-offset voltages, offset-voltage drift, or arbitrary jumps in the output voltage. The errors originate in the instrumentation-amp input circuitry, which rectifies and filters the RF signal into a dc voltage. Filtering, or shielding the input, output, and power leads can eliminate these errors. A lowpass filter effectively eliminates the RFI before it gets into the internal instrumentation-amp circuits. Simply adding a lowpass filter in series with each instrumentation-amp input lead accomplishes some of the filtering, but this method is inadequate because it reduces the instrumentation amp's differential bandwidth too much. Connecting a capacitor (approximately 100 times the lowpass-filter-capacitor value) across the instrumentation-amp input leads somewhat relieves the differential-bandwidth problem, but a loss of differential gain still exists at high frequencies. Adding a lowpass filter works effectively when the input-signal bandwidth is low; the filter pole can be low under these conditions, and the differential-signal error is so far down the attenuation-roll-off curve of the instrumentation amp that it contributes little to the common-mode error.