# CMRR; output changing depending on change in common-mode input

Document created by analog-archivist on Feb 23, 2016
Version 1Show Document

### Q

We are using AD8220 for current measurement in our application (In normal
measurement conditions a 100R shunt is placed between Vin+ and Vin-). In order
to calibrate the offset of the AD8220, we are shorting Vin+ and Vin- of AD8220
and check the Vout. We have following issue:
Vout depends of Vin voltage whereas Vin+=Vin- (shorted). In other words, when
Vin+ = Vin- = 0V then Vout = 0V. But when Vin+ = Vin- = +12V then Vout = 400µV
(Vin+ & Vin- still shorted together!) Is it normal? If so, could you explain
why?

### A

The difference in the offset voltage depending on the common-mode input voltage
can be attributed to the common-mode rejection ratio (CMRR) of the AD8220.

The definition of CMRR is: CMRR (dB) = 20*log10(A_d/A_cm) where A_d is the
input to output differential gain, and A_cm is the input to output common-mode
gain.

For a change in the output voltage by 250µV in response to a change of 5V in
common-mode voltage, or A_cm = 50 µV/V, the CMRR amounts to

CMRR (dB) = 20*log10(2 / 50µV/V) = 92 dB for a gain of 2 (A_d = 2).

This is well within the A grade specification and equal to the B grade
specification of the AD8220. Kindly compare with the Specifications section of