"Samuel Groner" wrote in message
om...

IMO perfectly matched resistors/caps do _not_ guarantee best CMRR -
they result in opamp-limited CMRR. If we allow slight unbalances, we
even get better than the opamp-CMRR. I had the pleasure to measure an
input with 120 dB CMRR from 20 Hz to 40 kHz - it used a 5534, an OP275
and 2 trimmers.

Or is this just BS what I'm writing here? I do not claim that we need
120 dB, but it won't hurt...

It won't hurt on its own merits, but if the technique that gets you there
causes stability problems or other issues, then you have to decide if the
benefit is worth the trade-off. Not to mention the extra costs in money and
board real estate. Remember the old saying:

"An optimist thinks the glass is half full. A pessimist thinks the glass is
half empty. And an engineer thinks the glass is twice as big as it needs to
be in order to hold the specified amount of water."

Peace,
Paul