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At 40:00, I can't see any advantage in CMRR at all, compared to using the same gain of 80 dB (10,000) with the single op amp difference amplifier design mentioned earlier. The improved CMRR just comes from using a gain of 10,000 instead of 100. It appears to be nothing to do with using a fully-differential-mode amplifier before the difference amplifier. The only benefit seems to be high input impedance (which is definitely a great feature) but that could be achieved with buffer amps. In fact, the three op amp design reduces the allowable Vcm common-mode voltage compared to the single op amp design (otherwise the output of the input amps will saturate due to gain in the first stage). The only advantage I can find is the ability to set/adjust the gain with a single resistor. Or am I missing something here? Thanks!
I agree with you about the CMRR part. But I think the reason why we usually not use a gain of 10,000 instead of 100 may be that such a larger feedback resistor is hard to implement on chip and the matching will get worse too.
@@chaowang772 I take your point about large resistors, but, for example, a gain of 10,000 can be achieved with 100k and 10k. I've seen amplifier and other chips with >100k resistors - far easier than 10M for example. Also, my understanding that matching resistors is easier on chip - for example by laser trimming - and they remain at a more balanced temperature, etc. Why would the matching get worse (given that we care about relative errors not absolute errors)?
It's because 1% is a high tolerance for this application. You would typically use matched resistor networks with relative tolerance below 0.025%, which bumps CMRR a few orders of magnitude.
