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I wonder whether the discrete 3-opamp instrumentation amplifier requires any compensation capacitors:

schematic

simulate this circuit – Schematic created using CircuitLab

Is it in general recommended to add compensation capacitor(s)? For example, in parallel with the RD connecting OA3 output to OA3 inverting input. What about one in parallel with RF1 and one in parallel with RF2?

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  • \$\begingroup\$ Shopping questions are off-topic on this site. Per given op-amp or instrumentation amp the datasheet and addendums normally cover such issues in detail, including PCB layout for high performance op-amps. You need to set rational goals, then go shopping via Google. \$\endgroup\$
    – user105652
    Mar 9, 2019 at 1:45
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    \$\begingroup\$ No compensation capacitor is added without a well defined reason. They either roll off spurious HF noise or act as a phase shift due to internal phase shifts, so they cancel out. \$\endgroup\$
    – user105652
    Mar 9, 2019 at 2:32
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    \$\begingroup\$ However, external compensation will reduce CMRR. \$\endgroup\$
    – LvW
    Mar 9, 2019 at 10:09

2 Answers 2

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Depends on the step-input response, the settling-time, you must have.

Also, if OA3 is not unity-gain-stable, that is a problem.

And if you use Meg Ohm resistors, then with typical opamp and PCB capacitances you can easily have 10KHz f3db points. The added phaseshifts may convert that 10 usec/channel telemetry system into an oscillator

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No need for Caps using stable Op Amps BUT Rf needs to be 0.1% for only 60dB CMRR.

If you need >100 dB CMRR, use an INA.

The source RsCs=T1 which you not defined, can be matched to the same product RfCf=T1 for gain matching.

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  • \$\begingroup\$ I am indeed assigning 0.1% resistors for Rf's and also for the RD's. Extra-high CMRR is not critical for my application; mostly, I need a differential amplifier with bandwidth in the tens of MHz and very high input impedance (including very low input capacitance) on each of the inputs. \$\endgroup\$
    – Cal-linux
    Mar 9, 2019 at 13:03

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