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Operational amplifier and instrumentation amplifier ICs can provide excellent performance, but some basic application mistakes can degrade the results
Second of two parts
3) Driving in-amp reference inputs
A common application problem occurs when designers try to connect high-impedance sources to the reference pins of in amps. This often introduces serious errors, especially in in-amp circuits that comprise three op amps and their monolithic IC equivalents. Figure 8 makes this problem easier to understand.
This example uses the standard three-op-amp/in-amp circuit; most IC in amps also follow this same basic architecture. The in amp has its reference pin tied directly to a simple voltage divider. The voltage divider ties directly to resistor R4 in the in amp's subtracter section; therefore, it becomes part of the R3/R4 subtracter network. Note that any added resistance between R4 and ground increases the total resistance of R4 and unbalances the subtracter section.
This, in turn, reduces both the in amp's commonmode rejection and its gain accuracy. Attempts to use small resistor values in the voltage divider will increase power supply current consumption, may overheat the resistors, and certainly is not a good design approach.
Figure 9 shows a much better solution: Here, an op amp buffer amplifier is inserted between the voltage divider and the in amp's reference pin.
However, note that even though the reference is now being driven by a very low-impedance source (typically less than 1 ohm), the addition of the op amp buffer alone still leaves a potential problem: lack of adequate power supply rejection.
4) Preserving PSRR...