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I'm currently attempting to design a small circuit that takes in a differential signal at < 4mv. I want to amplify this by around 1000 times.

I was reading up on Instrumentation Amplifiers, and they seem to do what I want to do, but I would also like to get the common mode signal as an output.

Are there any ICs out there that act as instrumentation amplifiers, but also have a pin outputting the common mode signal?

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  • \$\begingroup\$ What specifically in the common mode do you want to preserve? \$\endgroup\$ – Matt Young Sep 6 '12 at 22:05
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I haven't seen an InAmp chip with a dedicated common mode output.

I've used the circuit below to gleam the common mode voltage. Notice that gain-setting resistor RG is split into 2 equal parts R5 and R29.

I haven't thoroughly tested the accuracy of this common mode output. I only needed an approximate common mode voltage for a driven guard.

enter image description here

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One of the important things a instrumentation amp does for you is to specifically reject the common mode signal. The common mode rejection spec is higher in better inamps. So no, I don't know of any that extract the common mode signal.

However, you can do this yourself easily. The common mode signal is basically the average of the two, which is easy to get from just passive components. Take two equal resistors, one to each signal, then connect the other ends together. That common point will now be at the average of the two signals, which is the common mode component. Now amplify that with a normal single ended amplifier. Pick resistors that are high enough to not load the signals, like 10x their impedance should be good enough. Then make sure the single ended amp has high enough input impedance to not load the result.

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