[Arny Krueger]

"Ian Bell" wrote in message

Actually, I just noticed you said 'differential
amplifiers' rather than operational amplifiers and to
that extent I agree with you when you use selected
descrete components. However, when it comes to op amps,
they are not the prime determinant of achieved CMRR. It's
all the other components around it, and by the time you
have included the dc blocking caps and zener diodes,
series inductors etc to prevent RFI and damage from
phantom power and so on necessary for a mic preamp, on a
PCB then actual measured CMMR across the audio band will
be little better than a good transformer with phantom
power fed through a primary centre tap.

Good points.

Another hidden gotcha with specifying CMRR is dynamic range.

What is the CMRR of an op amp when the common mode voltage is 5 Kv?

120 volts?

Peak common mode voltage VCC which might only be 5 volts.

Some or all of these voltages are probable in the real world. Of course they
are inceasingly probable as the voltage goes down.

The answer can be very close to 0 dB. :-(