Characterization of Fully-Differential OpAmp

[shuratox]

Hi,

I saw many threads talking about this subject, but any of them have a clearly explanation. The books of analog design that i have only specifies the measurement of a single-end OpAmp. The works that i read about this subject presents only graphs.

With this, i would like to know if anyone knows how i make the characterization of a Fully-Differential OpAmp in special CMRR and PSRR, and things related with this measurements like: equations and circuit configuration.

Thanks.

 

[prestonee]

cmrr is same as for single ended, in single ended you tie the 2 inputs together, put an ac source on them, run ac analysis, if vac=1 then your ratio is just 1/output * output of diff gain
psrr is also the same as for single ended. 'ground' the inputs to common mode, put ac source on your avdd or whatever you call it, run same ac sim, ratio is 1/output * output of diff gain

note: the results are not going to be the amplifier rejection alone but the amplifier rejection + cmfb rejection
also note to simplify this many times you see the sim ran in unity gain so the ratio can be simplified to being simply 1/(cm out) or 1/(ps out)
as far as how to do the feedback for unity gain with a generic ota, id recommend placing vcvs with gain of 1 feeding back around the amp, holding it in unity gain without loading it down.
if doing it in real life then you will be measuring the circuit itself not just the amp. and need to ratio the outputs as mentioned above (Output of diff signal / Output of either common mode sig or powersupply sig)

hope this helped and not confused
-Pb

- - - Updated - - -

to clarify when i say * output of diff gain , i am referring to running a normal ac sim with the 1v ac sig as an input and measuring its gain (to keep the signal as a diff signal i always run 2 ac sources on each input , each being .5v ac sig.

 

[micpro]

Hi,

I saw many threads talking about this subject, but any of them have a clearly explanation. The books of analog design that i have only specifies the measurement of a single-end OpAmp. The works that i read about this subject presents only graphs.

With this, i would like to know if anyone knows how i make the characterization of a Fully-Differential OpAmp in special CMRR and PSRR, and things related with this measurements like: equations and circuit configuration.

Thanks.

You may find this document helpful: https://www.designers-guide.org/analysis/diff.pdf by Ken Kundert.