The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Simulators >> Circuit Simulators >> fully-differential opamp: how to simulate its FFT spectrum to check the SNDR
https://designers-guide.org/forum/YaBB.pl?num=1496938477

Message started by A.D.J on Jun 8^th, 2017, 9:14am

Title: fully-differential opamp: how to simulate its FFT spectrum to check the SNDR
Post by A.D.J on Jun 8^th, 2017, 9:14am

Hi, everyone.
The fully differential opamp is used in MDAC of a pipelined ADC.
I want to plot the FFT spectrum to check its SNDR.

For single-ended output, connect the output to negative input to build up the unity gain buffer. And then provide the sinewave to the positive input. Then we can plot the FFT of the output.

But for the differential output, how to plot FFT?

Thank you.

Title: Re: fully-differential opamp: how to simulate its FFT spectrum to check the SNDR
Post by davidshw on Jun 8^th, 2017, 5:39pm

maybe you can try this configuration:

Vin=VICM+Asin(wt)

Title: Re: fully-differential opamp: how to simulate its FFT spectrum to check the SNDR
Post by A.D.J on Jun 9^th, 2017, 11:38am

Thank you so much.
Leaving the negative node of the output floating, it doesn't influence the circuit?

Title: Re: fully-differential opamp: how to simulate its FFT spectrum to check the SNDR
Post by davidshw on Jun 11^th, 2017, 7:09pm

I think the CMFB will cause the Von to be 2*VOCM-Vop. Maybe the output loading should be added both at Vop and Von.