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Jan 27th, 2021, 2:35am
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 PSRR and CMRR for fully differential OTA using xf analysis (Read 2085 times)
 kabir_fakir Junior Member Offline Posts: 29 PSRR and CMRR for fully differential OTA using xf analysis Aug 22nd, 2017, 6:06am   Hello everyone, Before making this thread i went to following threadhttp://www.designers-guide.org/Forum/YaBB.pl?num=1073321544so by definition CMRR= Adm/Acm                        PSRR = Aps/AdmTo simulate FD OTA((with CMFB inside))  I followed the famous Ken Kundert paper to make test bench ( http://www.designers-guide.org/analysis/diff.pdf) f. The feedback network is capacitive  so I put resistance of 1G in feedback path to provide the DC bias.  To simulate the transfer function ( like Aps, Acm, Adm) with XF analysis, small ac signal is put in every input port. For example, common mode input signal is V4 is given 4.5 of dc signal along with 100uV of ac magnitude, same is done with power supply, V2 is given 9 V of Dc voltage along with 100uV of ac signal. After setting this test bench the XF SIMULATION is run along with DC simulation. But I have found some unexpected results which I am not able to understand. You can see the results in below comments. So Please explain me what is wrong(or I am analyzing it wrong) with the testbench/simulation. Kindly let me know if you need more information Back to top Always a learner and will be .   IP Logged
 kabir_fakir Junior Member Offline Posts: 29 Re: PSRR and CMRR for fully differential OTA using xf analysis Reply #1 - Aug 22nd, 2017, 6:11am   Here is the setting for the xf analysis (see most right img). With this setting I have plotted the plotted the Aps=Voutd/Vsupply and Adm= Voutd/VindVout= Differential o/p Vind= Differential i/p What i dnt understand is: Aps? Why the power supply gain is so erratic in behavior? Back to top Always a learner and will be .   IP Logged
 kabir_fakir Junior Member Offline Posts: 29 Re: PSRR and CMRR for fully differential OTA using xf analysis Reply #2 - Aug 22nd, 2017, 6:20am   To simulate the common mode gain Acm=Vcm_out/Vcmin setting of the XF is shown in the image. Here Vcmin =V4. So can is say that my CMRR is 78db ?(as Acm= 54dB , Adm= 24dB)is that right?Also is that make sense to define CMRR in close loop or should it be defined for open loop? Back to top Always a learner and will be .   IP Logged
 Ken Kundert Global Moderator Offline Posts: 2296 Silicon Valley Re: PSRR and CMRR for fully differential OTA using xf analysis Reply #3 - Aug 23rd, 2017, 11:18am   Two things.1. With XF analysis, you need not specify an input magnitude on any input source. The input magnitude is assumed to be 1 on every source in XF, though you can adjust it using xfmag if needed.2. The reason why Aps is erratic is because it is zero. You circuit is perfectly symmetric, and as such any variation on the supply results is no differential variation on the output. All you are seeing is roundoff error 260dB down (fempto volts).-Ken Back to top IP Logged
 kabir_fakir Junior Member Offline Posts: 29 Re: PSRR and CMRR for fully differential OTA using xf analysis Reply #4 - Aug 24th, 2017, 10:03am   Ken Kundert wrote on Aug 23rd, 2017, 11:18am:Two things.1. With XF analysis, you need not specify an input magnitude on any input source. The input magnitude is assumed to be 1 on every source in XF, though you can adjust it using xfmag if needed.2. The reason why Aps is erratic is because it is zero. You circuit is perfectly symmetric, and as such any variation on the supply results is no differential variation on the output. All you are seeing is roundoff error 260dB down (fempto volts).-Ken thanks a lot of replying. Regarding first point1. You are right. I did the simulation without any input. . Actually in your book(The Designer Guide to spice and spectre) you have written "Transfer functions for nodes are computed assuming that a unit magnitude current source is connected from the gnd..." and so on. Thanks for clarifying this. Second point (PSRR)2. I got the point. It means i will see some reasonable value of PSRR when i introduce some mismatch or offsetCould you please comment on my value of CMRR(78dB)? Is that right ? Back to top Always a learner and will be .   IP Logged
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