 posted
16 years ago
I searched three detailed books that had hundreds of pages about
opamps and I can't find the solution to this problem. I think there's
a simple solution but I just can't work it out..
Here's the schematic of the opamp differential amplifier:
:
R2
/\/\/\/\/\
 
 
 
 \ 
R1  \ 
Vi_1 /\/\/\/\/\  \ 
 
V_o
R3  /
Vi_2 /\/\/\/\/\ + /
  /

 R4
\/\/\/\/\/\


Gnd
The first part is to give V_o in terms of the other variables, which is
easy enough and is in the books. The next part is:
Vi_1(t) = 10^(3) Sin(2pi x 10^3t) + Vn(t) Vi_2(t) = 10^(3)
Cos(2pi x 10^3t) + Vn(t)
R_1 = R_2 = 1kohm R_2 = R_4 =
30kohm
where Vn(t) denotes unwanted noise. If the Common Mode Rejection Ratio
(CMMR) of the opamp is 97 dB and both inputs are "contaminated" with
50 Hz noise of average amplitude 1V, calculate the ratio of output
signal amplitude to average output noise amplitude.
I know what the CMMR equation is but the frequencies are confusing
me...