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This is the conceptual circuit diagram for a ideas detector. It's from The Brain Bulb - I've Got an Idea! by Jeri Ellsworth. She uses an instrumentation amplifier (AD620) with a feedback element which is highlighted below:-

schematic

This arrangement is consistent with that chip's datasheet example for an ECG monitor, as:-

ECG circuit

I found this question that has a similar circuit, but outputs the common voltage which I can understand if you want to measure it. By why would Jeri's circuit (and the ECG example) feed it back into the person? Isn't the point of an instrumentation amplifier to automatically remove the common signals? And why then do this for Jeri's circuit, and not a Wheatstone bridge? She just says that it's to reduce noise. Lorry weigh bridges have noise too.

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  • \$\begingroup\$ Without a low impedance Vcm reference, it can exceed INA input range. Right Leg drive (RLD) is std., practice for ECG . \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 4 '17 at 22:59
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To expand on Paul's answer, it looks like the common mode voltage is being inverted and fed back to remove the common mode voltage. This prevents Vcm from becoming too high for the input to the instrument amp, and reduces noise that would otherwise be introduced by the common-mode gain.

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  • \$\begingroup\$ There's no answer by "Paul". \$\endgroup\$ – try-catch-finally Apr 9 '17 at 10:22
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The general idea of common mode rejection is that you can subtract the unwanted noise from itself. The unwanted noise is thus the exact tool you want; because you can invert it and remove it (negative feedback) from your unfiltered signal containing the desired signal + the noise you're looking to remove.

You want to be mindful of the speed of that feedback loop so that your negative feedback isn't too far out of phase from where you are removing it from.

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  • \$\begingroup\$ What I can't understand is why inject the common noise back into Jeri, when an in amp automatically does that subtraction. Isn't the output of an in amp GAIN(V+ added to V-) anyway? You don't have this noise feedback circuit in a load cell or thermocouple sensor... \$\endgroup\$ – Paul Uszak Apr 5 '17 at 9:27

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