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Design >> Analog Design >> Gm-OpAmp-C integrator
https://designers-guide.org/forum/YaBB.pl?num=1178297821

Message started by webb0111 on May 4^th, 2007, 9:57am

Title: Gm-OpAmp-C integrator
Post by webb0111 on May 4^th, 2007, 9:57am

Hello,

I have designed a two-stage Miller fully-differential amplifier, with cmfb. It seems that all benchmarks that I need from opamp are met (BW, slew, gain). I am having a problem with the common mode output voltage in a Gm-Opamp-C integrator. I am using a -0.9V to 0.9V supply. At dc, both the OTA and the opamp have 0.3V CM when connected together and separately. The problem occurs as follows: when integrating the output CM slowly rises to around 0.75V for one end and 0.8V for the other end.

When using the opamp in an Active-RC integrator the circuit behaves as expected with CM at 0.3 for both ends. Also, a Gm-C integrator behaves as expected with correct CM. Simulations are done in Cadence.

Any reasoning as to why this is happening would be very helpful. If more information is needed, I would be happy to oblige.

thanks, Matt

Title: Re: Gm-OpAmp-C integrator
Post by SATurn on May 5^th, 2007, 2:56am

Could you please put a plot describing this problem?

SATurn

Title: Re: Gm-OpAmp-C integrator
Post by webb0111 on May 7^th, 2007, 9:49am

Here is the waveform image. If allowed to run longer, Vop and Von tend tend get closer to each other.

Title: Re: Gm-OpAmp-C integrator
Post by SATurn on May 7^th, 2007, 1:01pm

What I can not exactly understand is that why the CM value of the blue and red waveforms are not 0.3? I suppose that the green and violet waves are outputs, but for the rest it is difficult to guess ...

SATurn

Title: Re: Gm-OpAmp-C integrator
Post by webb0111 on May 7^th, 2007, 1:29pm

The blue wave form is the input, and the red waveform is the differential output (Vop-Von). So the output should have CM of 0, like the input, assuming that the CM of output end is the same.

Title: Re: Gm-OpAmp-C integrator
Post by SATurn on May 10^th, 2007, 2:11pm

As it is evident from this figure, there should be a "systematic" "common mode" mismatch in the circuit! For example, one side draws more current respect to the other side from the Opamp. Meanwhile, maybe the CMFB circuit is not properly designed. For example it saturates or it is not well matched to the circuit ...

SATurn