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Design >> Analog Design >> how to simulate ICMR of fully differential amp?
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Message started by alicia8283 on Oct 17^th, 2007, 6:37pm

Title: how to simulate ICMR of fully differential amp?
Post by alicia8283 on Oct 17^th, 2007, 6:37pm

hi all,

i am new to analog design, i need to simulate and understand a fully differential op-amp. i tried to simulate a single op-amp before but i never simulate a differential output amp before. Can anyone tell me how to simulate the input common mode range of a fully differential op-amp? i tried to search document online but i couldnt get any as well.. thank you in advance :)

Title: Re: how to simulate ICMR of fully differential amp
Post by jimwest on Oct 19^th, 2007, 7:08pm

There are alot of ways.
You may simulate the open loop gain at low frequency by sweeping the common mode voltage. The range the gain remains the constant is the common mode input range.
Or use the verlog a to create a uni-direction unity gain block.
Another way is using the cadence test bench.

K/R,

Jim

Title: Re: how to simulate ICMR of fully differential amp
Post by sheldon on Oct 22^nd, 2007, 3:50am

Alicia,

Just to expand on Jim's comments a little. The methodology I have used
for the measuring the input common mode range of a single-ended op-amp
in ADE is,

1) Set up a test bench to measure open-loop gain
2) Setup a parametric sweep of the input common-mode voltage
3) Simulate
4) Plot the dc, low frequency, open loop gain versus input common-mode voltage
5) Find the max value of the open loop gain, Avo_max
6) Then use the cross function to identify the input common-mode levels for
Avo_max-3dB levels of the open loop gain

The big challenge is that there is no standard definition of input common-mode
range. My preference is for using the -3dB level to determine the range where
open loop gain is not degraded by the input common-mode level.

It might actually make a good topic for discussion, How should input
common-mode level be defined/measured?

One other suggestion is that the ideal_balun can be a useful block for creating
the input signal. Connect pin d to the differential input signal generator and pin c
to the common-mode input signal generator, connect pins p and n to your
circuit.

Best Regards,

Sheldon

Title: Re: how to simulate ICMR of fully differential amp
Post by alicia8283 on Oct 29^th, 2007, 10:40pm

hey.. thanks everyone :)
Btw, can you explain what's ideal_balun ?
Sheldon, the method you explain is to simulate single ended op-amp? How about differential output op-amp?
hi Jimwest, how do i get the cadence test bench that you mentioned?
thanks everyone once again ::)