# Math for CMRR

Discussion in 'Math' started by jmoore313, Jul 16, 2014.

1. ### jmoore313 Thread Starter New Member

Jan 16, 2014
2
0
Greetings,
I'm working on a problem with decibel calculations for CMRR on an instrumentation amplifier. The formula is straightforward:
CMRR = 20 log (Av(d) / Av(cm)). I've been given a CMRR of 130dB and I have a measured Av(d) (differential gain) of 46.15 and I'm trying to solve for Av(cm) -(common mode gain). I've been told that the ratio for a CMRR of 130dB should be 3.16 * 10 (6).
Just having a little trouble figuring out how to make my TI-83 do this. Any help would be appreciated.
Thanks.

2. ### djsfantasi AAC Fanatic!

Apr 11, 2010
3,486
1,245
Do some manual work on the equation to isolate the variable you are looking for. Then, the calculator operations will become obvious.

Sol'n

CMRR=20 log( Av(d) / Av(cm) )
130/20=log( 46.15/Av(cm) )
10^6.5=46.15/Av(cm)
Av(cm)=46.15/10^6.5

3. ### jmoore313 Thread Starter New Member

Jan 16, 2014
2
0
Thank you so much for your help.