OP AMP CMRR Measurement?

[mriechert]

Hello all,

I am on a quest to understand the specifications for op-amps more clearly.

On this quest I am trying to measure the common mode rejection ration for a 741 op amp.

But I am at a bit of a loss how to measure considering the measurement needs to be very precise.

I have tried one method where I shorted the inverting and non-inverting inputs and then inserted a signal but this lead me no where definate.

Any enlightenment would be appreciated

Michael

 

[CliffMichael]

Michael,

You may not have gone down deep enough with your ACVM on the output. When you feed a signal, say 1kHz at 1 volt RMS, into the shorted pos & neg inputs, the output will be very low, say around millivolts or less. A finely tuned CMR could be tens microvolts. At this point you'll be competing with noise, too.

Make sure the opamp is properly configured and working correctly before the CMR test. An oscillascope is very useful to confirm proper operation, too. The 741 is an old workhorse and lots of fun to play with.

Cliff Michael

 

[raghuramkarthik]

Hi There,

CMRR = Differential Gain/Common Mode Gain(of course!).

You are on the rigth track.Just ensure proper grounding of teh supply,use by passes for blocking stray currents,and then apply a 10Vp-p 1Khz Signal in Common Mode and Differential Mode.Just measure the gains and find the ratio.