# Worst CMRR

#### Alphas

Joined Sep 14, 2007
2  An op amp differential amplifier as shown is built using four
identical resistors, each having a tolerance of ± 5 %. Calculate the worst possible
CMRR.  What values of the resistor tolerance is chosen?

#### Papabravo

Joined Feb 24, 2006
13,940
It's given at 5%. For each of four resistors you choose either +5% or -5% and you see which combinations produce the worst result. It seems intuitive that all four resistors at exactly the nominal value produces the best result.

#### techroomt

Joined May 19, 2004
198
won't cmrr be the least (worst) when the differential gain is less, and common mode gain is most. simply apply the appropraite min/max resistance values to the dc and ac analysis formulaes.

#### Alphas

Joined Sep 14, 2007
2
can anyone substitute the value, Rx=(1±0.05)R

my teacher say its a positive 10,R4’ = 1.05R , R3’ = 0.95R, R3 = 1.05R, and R4 = 0.95R

but I could get negative 10, and this should be the answer as the CMRR drops 10 units when R4’ = 0.95R , R3’ = 1.05R, R3 =0.95R, and R4 = 1.05R

so who is right? 