Document created by analog-archivist on Feb 23, 2016
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### Q

I am you sing your AD8231 as a precision current sensing monitor. How can I
evaluate the CMRR of the AD8231? Is it enough to just measure the Vout at the
fix Vcm and gain in one point or two? (Equation - AN-539; page2) Is the CMRR
stabile for a gain (DC measurements)?

### A

The same technique does work for the AD8231 - join the inputs and apply a
signal to both,
but in this case it might be a DC signal or an AC signal. It would be
interesting if you did two
measurements - join pins 2 & 3 of the AD8231 for one, and join the signal sides
of R3_1 and
R3_2 for the other. You may see small differences in your results arising from
the mechanisms
I discuss below.

The CMRR of the AD8231 depends on the gain, as is normal with instrumentation
amplifiers,
and ranges from 80 dB at G=1 to 110 dB at G≥32. I do not know exactly what you
mean by
"Is the CMRR stabile for a gain (DC measurements)? " - the CMRR of a particular
device will
change if the gain is changed but will remain constant if the gain is not

I assume that you are working at DC or very low frequencies, since you have a
Low Pass
Filter (LPF) at the AD8231 input. Do note, however, that at 15 Hz that filter
has a CMR of 113 dB
if the capacitors C3_1 and C3_3 are mismatched by 5% high and 5% low
respectively - so if
these capacitors are at the limits of their tolerance and G>32 the filter,
limits CMRR above about 20 Hz.

The worst case offset current is 0.5 nA over temperature. This current flows in
one of the resistors
R3_1 and R3_2 giving rise to an offset of about 25 uV.As you can see from
figures 10 & 11 in the
data sheet the bias current varies with common-mode voltage, and we must expect
the offset
current to do the same. Therefore the CMRR will be affected by resistors in
series with the input.
The effect is small, but not completely negligible, and you might expect in the
worst case a value of
the order of 100 dB with 50 kΩ input resistors, although most AD8231 devices
will be much better
than this.