INAMP_CMRR in single power supply and dual power supplies

Document created by analog-archivist Employee on Feb 23, 2016
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I wanted to know whether the CMRR will be influenced by this different
conditions?
Take AD8221 for example.
Case 1: supply the +5V single supply to it, the input signal will be Voff=2.5V,
and the AC signal (under the frequency of 10Hz) to AD8221 (gain = 10). The CMRR
will be CMRR1?
Case 2: supply the +/-5V dual supplies to it, the input signal will be Voff=0V,
and the AC signal (under the frequency of 10Hz) to AD8221 (gain = 10). The CMRR
will be CMRR2?
Case 3: supply the +5V single supply to it, the input signal will be
Voff=1.25V, and the AC signal (under the frequency of 10Hz) to AD8221 (gain =
10). The CMRR will be CMRR3?
I think CMRR2 will be the best, then maybe CMRR1 will be close to CMRR3, but
maybe CMRR1 will be better than CMRR3.

 

CMRR for the three op amp in-amps are essentially dictated by the resistor
matching of the subtractor. I spoke to one of our designers and he said that
CMRR is not really affected by changing the supply rail. The problem you would
have is reduced headroom, but that is not related to CMRR.  CMRR for in-amps
that do not use matched resistors, may be different. Some of our in-amps use a
flying capacitor. Those are in a different category. The AD8230, which uses a
flying capacitor, should not be affected either because the capacitor only
takes the differential signal and ignores the common-mode signal.
So CMRR should be the same to a first order for all the cases you listed below
for a three op amp in-amp which is what the AD620, AD621, AD622, AD623, AD8220,
AD8221 and AD8225 use.

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