Op amp CMRR question?

I suspect you mean the feedback around an opamp is 4 identical resistors with 5% tolerance.

Reference has the equations for the gain.

Vo = ((R1+Rf)/R1) (Rg/(Rg+R2)) (V2) – (Rf/R1) (V1)

Set the two inputs equal, which should give you zero output. Set inputs at 1 volt for easier calculation.

Vo = ((R1+Rf)/R1) (Rg/(Rg+R2)) – (Rf/R1)

Now figure out which combination of out of tolerance resistors gives you the highest Vo. You may have to try several combinations and do the calculations for each. When you get the highest Vo, the CMRR = 20 log Vo

for example, my first guess would be to make Rf and R2 high, the others low. this would be the same if you made Rf and R2 10% high and the others nominal.

Vo = ((R1+Rf)/R1) (Rg/(Rg+R2)) – (Rf/R1)

Vo = ((1+1.1)/1) (1/(1+1.1)) – (1.1/1)

Vo = (2.1)(1/2.1) – 1.1 = 1 – 1.1 = 0.1

in this case, CMRR = 20 log 0.1 = –20 dB

Second guess would make R1 and Rg high

Vo = ((1.1+1)/1.1) (1.1/(1.1+1)) – (1/1.1)

Vo = 1– 0.91 = 0.09

CMRR = 20 log 0.09 = –21dB

looks like 21 dB is the answer

looks to me this must be uncomplicated to discover on your textual content cloth e book till you're interior the midst of an examination and posting from a crackberry. :) on the grounds that i've got not got my textual content cloth e book attainable and that is been so long on the grounds that college, i'm purely glade I remembered what CMRR became.

I won't do the math, but I can tell you the performance will be horrible.