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Design >> Analog Design >> Continous-time CMFB question
https://designers-guide.org/forum/YaBB.pl?num=1204276339

Message started by skythunder on Feb 29^th, 2008, 1:12am

Title: Continous-time CMFB question
Post by skythunder on Feb 29^th, 2008, 1:12am

Hi, everyone,

I designed a folded-cascode fully differential opamp, and the structure of the CMFB is two differential pairs, which is shown in the picture.
After simulation , I found that this OPAMP works well in the continous-time configuration, like a Multiple-feedback Lowpass filter. However, oscillating occurs when this filter was connected aftert a switched-capacitor filter. Then I just made a simple switched-capacitor amplifier using this fully differential opamp, the same thing happened. I guess this opamp is not suitable for SC circuit, but I want to know what the reason is .
Can anyone tell me the possible reason please ? thanks !

Title: Re: Continous-time CMFB question
Post by Frank Wiedmann on Feb 29^th, 2008, 2:01am

Have you checked the phase margin of your common-mode regulation? It might be very low so that there is a lot of ringing when the common mode changes quickly as it probably happens in your SC circuit. In your continuous-time circuit, the common mode is probably not changing so that you don't see the problem there.

Title: Re: Continous-time CMFB question
Post by thechopper on Feb 29^th, 2008, 7:17am

Frank,

I guess he is talking about oscillation on the output differential signal. Although the CM feedback might be marginal, if the opamp is not unbalanced you should not see oscillation on the output diff signal, but only on the output CM signal.

Skythunder: is it the output diff signal that oscillates?

Tosei

Title: Re: Continous-time CMFB question
Post by skythunder on Mar 2^nd, 2008, 10:08pm

I also doubt the problem is caused by the instability of my CMFB loop.
But I really don't know how to check the CMFB loop' phase margin of my OP schematic .
I tried to break up the connection between CMFB and the main OP, and give a AC signal input to the broken place of main OP , and then watch the output from CMFB. However, the differential pairs of CMFB worked in the linear region.
so i cannot get the right infromation from it. Can anyone explain how to check the stability of my CMFB loop in detail?

by the way, here is a figure of the output of a simple SC_amplifier using the OP. It is the differential output signals of the OP that oscillated.

Title: Re: Continous-time CMFB question
Post by Frank Wiedmann on Mar 2^nd, 2008, 11:30pm

You can use Spectre's stb analysis to simulate the loop gain of the common-mode loop (and also of the differential-mode loop). This was discussed in this forum a few days ago at http://www.designers-guide.org/Forum/YaBB.pl?num=1204029625.

Title: Re: Continous-time CMFB question
Post by thechopper on Mar 3^rd, 2008, 4:46pm

May be it is simply more load capacitance (degrading phase or gain margins) when you use the opamp as a building block for the SC filter as opposed to the continuous time approach?

Tosei

Title: Re: Continous-time CMFB question
Post by Frank Wiedmann on Mar 3^rd, 2008, 11:35pm

Well, his circuit is really an Operational Transconductance Amplifier (OTA) with a current source at the output. With such an amplifier, you usually have the dominant pole at the output of the amplifier, which means that the circuit will become more stable with an increased capacitance at the output. However, I agree that somewhere in his circuit, there is probably a loop with insufficient phase margin (might be a common-mode or differential-mode loop, inside the amplifier itself or together with the surrounding circuitry).

Title: Re: Continous-time CMFB question
Post by thechopper on Mar 4^th, 2008, 2:58am

Hi Frank,

I certainly was not clear enough, but you're absolutely right and actually that is what I assumed by suggesting a higher load cap might be causing stability problems.
There might be an "almost dominant" pole in the amplifier topology whose stability easily degrades as soon as you connect a certain load capacitance. However I insist on suggesting that in case the amplifier is not unbalanced the stability problems should be associated to the differential loop.

Tosei

Title: Re: Continous-time CMFB question
Post by loose-electron on Mar 14^th, 2008, 10:42pm

in a switched cap configuration an OTA (thats what the schematic is) is being briefly disconnected from its loading environment.

Last I checked, an OTA is dependent on the output load primarily for establishing dominant pole placement.

So, how about asking the question - for all states of operation (all switch situations)
what is the phase gain margin relationship of both the differential system, and the common mode system????