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how to simualte CMRR for diffrential amp?

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surianova

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cmrr formula

Hi all,

I have a question here. How to simulate CMRR for a diffrential amplifier and also common gain in Hpice? Thanks in advance...

suria
 

how to measure cmrr

I attached a very good tutorial about how to measure a op-amps. Also it deals with cadence tools, many ideas and skills can be transfered to other spice programms. Hope it will help you.

Characterizing Diff. Op-amps - Cadence from jonathan David
 

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  • 0pamp_wb_au.pdf
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how to simulate cmrr

First of all, consider that CMRR mostly appears due to mismatch between the two posible signal paths from inpue to output. So, CMRR simulation only means "something" if you make a Monte Carlo simulation.

By simulating your amplifier with a common mode inpue you get the common mode gain (Acm). To obtain the CMRR you must divide the Acm by the differential gain with your amplifer having "exactly the same" monte carlo parameters present in the circuit with wihch you obtained the Acm. This is not an easy task and depends on you simulator, as each time that a parameter (same parameter name) is called the simulator assigns to it a different value if the parameter was defined as DEV (device).
 

simulating cmrr

when used sigle power supply,how I can modify the simulational configuration provided by allen's book?
 

cmrr simulation

when the open loop gain is too large ,just 80dB.how can I simulate it?
 
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    ysrevol

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    Alice Lee

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cadence cmrr

At simulation of CMRR it is necessary to do mismatch of elements of the circuit.
If it to not take into account, there will be large mistakes!
 

simulate cmrr

gevy said:
At simulation of CMRR it is necessary to do mismatch of elements of the circuit.
If it to not take into account, there will be large mistakes!
Click to expand...
How to include mismatch in simulation? Thanks
 

how to measure cmrr

cmosbjt said:
gevy said:
At simulation of CMRR it is necessary to do mismatch of elements of the circuit.
If it to not take into account, there will be large mistakes!
Click to expand...
How to include mismatch in simulation? Thanks
Click to expand...

i think it maybe use monte carlo analysis.
 

cmrr+cadence

usually i use 2 opamps........to calculate the differential and common mode gain

Added after 41 seconds:

or much more simple,
use 2 vcvs......
 

monte carlo cmrr

First you have to correctly bias your OPAMP, i.e. set it in its closed loop configuration in DC. That is done by closing the loops, both positive and negative loops, in DC with an AC Killer, i.e. a big self, or a big RC Filter.

Then set in series with the AC Killer an AC current source of same direction on each input, which will provide a Common Mode AC signal.

Then, you simulate the circuit in Monte-Carlo Analysis (because, the mismatch is the most important source of CM gain for Differential to Differential circuits). And you obtain the common mode gain ACM.

You just have to divide the differential mode gain by the common mode gain to obtain the CMRR = ADM/ACM
 

cmrr cadence simulation

I think using 2 op is very good.
 

formula for cmrr

how to do the Adm/Acm
 

how to find cmrr

The document is very good, thanks eda4you!
 

test cmrr for differential amplifier

If use Ad/Ac to caculate CMRR, the CMRR is very high. But in silicon, it can not reach this high level.
Such as in jonathan David's tutorial paper, Ad=60dB, CMRR=129dB???

Who can explan this question?
 

simulating common mode rejection ratio

CMRR=Adm/Acm, Adm-differntial gain
Acm-common mode gain.

for common mode gain calculation ,short the i/p and perform ac=1V at the i/p.o/p will be Acm in log domain.
 

formula de cmrr

you can simulate by tying the diff-pair to vcom and inject small ac signal, and measure the gain at output.
 

formula to find cmrr

Could someone share some reference that tells how to simulate and determine performances of a fully differential OPA, for example plot the topology and save as a *.bmp file~
 

how to meaure cmrr

yaxazaa write:
you can simulate by tying the diff-pair to vcom and inject small ac signal, and measure the gain at output.

But use the manner which you have mentioned to simulate Acm, and then caculate CMRR. A very high CMRR date will be exported. Such as an example Ad=60dB, CMRR=129dB. In fact, CMRR=129dB is unreasonable.
 

example calculating cmrr with ad

hello
I can t download the tutorial.
 

simulation to calculate cmrr

i want to download the attach file.
thanks
 

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