IM3 simulation spectre cadence

[Suresh R]

CMRR

Friends,
i require some assistance in understanding the CMRR value in the datasheet to make efficient use of it.

for LM358 : CMRR = 85dB for the entire range(0 - (V+ - 1.5)) of common mode voltage.
For the above value, what can be my differential input to the OpAmp, to achieve better common mode rejection ratio(CMRR)?

In simple terms could i get some practical description to understand it.

Kindly revert to me for any clarifications.

regard's
Suresh.

 

[Davood Amerion]

CMRR

as i know CMRR is
CMRR = 20 log (difGain/CMgain)
difGain is maximum diffrencial gain, and CMGain is opamp gain when two input connect together .

 

[Suresh R]

Re: CMRR

as i know CMRR is
CMRR = 20 log (difGain/CMgain)
difGain is maximum diffrencial gain, and CMGain is opamp gain when two input connect together .

Let me consider an example:
I have shown the calculations for CMRR for the circuit below.

From those calculations, i could not infer what i need to. i.e., what does that 3.521dB mean with reference to CMRR of LM358 that is 85dB. What is that iam supposed to learn by doing such a calculation??

i want to know this because i need to understand how i should work on with the CMRR value given in the data sheet when designing a circuit.

regard's
Suresh.

 

[Davood Amerion]

CMRR

Suresh R!
Ad is diffrentioal OPEN-LOOP gain
and AC is common mode voltage gain
(V1=V2) connect to Vin

 

[DMan]

Re: CMRR

CMRR is a factor that tells us how good differential amplifier we have. Usually it's in the range of 80dB to 100dB.
It tells us how much is the unwanted common phase component of signal is atenuated (idealy infinite). If you apply superposition to the signals on amplifier input you get Vout:

Vout=AD*VD+AC*VC=AD*UD*(1+AC*UC/(AD*UD))
where AD is differential gain, VD differential voltage, AC common gain and VC common voltage

From a good differential amplifier we expect good differential gain and the atenuation of common phase signal to be as high as posible so the second factor should be as small as possible, that is AC/AD. Or to put it diferetly:
CMRR=20*log(AD/AC)
should be as big as it can be.

 

[Suresh R]

Re: CMRR

Thank you. i got the point that the common mode voltage at the inputs produces some offset voltage at the output and this offset could be very well decreased by choosing an opamp with high CMRR.

regard's
Suresh.