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Design >> Analog Design >> Question about DC CM
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Message started by CherryQ on Jan 22^nd, 2008, 3:37am

Title: Question about DC CM
Post by CherryQ on Jan 22^nd, 2008, 3:37am

Hi, I got a problem when designing a differential Gm with CMFB. When the input ac signal is small, the output CM is Vdd/2. However, when ac signal is increased, the output CM drops hundreds of mv in the waveform of trans simulation, even though the dc operation point displayed on the schematic is still Vdd/2 in dc simulation.

May I know which value is the real CM voltage and how to avoid this? Thanks.

Title: Re: Question about DC CM
Post by thechopper on Jan 22^nd, 2008, 11:54am

Hi,

One thing that might be going on is that your CMFB circuit might not have enough input dynamic range to accommodate your large input signal swing. As a consequence the CM level will change since its gain will drop. The valid CM level is the dc operating point one. However you might want to check the CMFB dynamic range to make sure it tolerates the desired input signal amplitude.

Hope this helps
Tosei

Title: Re: Question about DC CM
Post by vivkr on Jan 23^rd, 2008, 5:48am

Hi Cherry,

What do you mean by the CM level dropping? Does it drop and then slowly return to the DC level or does it just stay at the wrong level
showing no sign of returning to the correct value? If this is the case, then probably your CMFB has too small an input operating range
as Tosei mentioned.

If this is not the case and the CM level just is undergoing transients, then you have either/or : (a)Too little current drive in some
branch of your CMFB, (b) Some amount of differential signal coupling into the CMFB.

The problem may be induced by your choice of architecture.

Regards
Vivek

Title: Re: Question about DC CM
Post by buddypoor on Jan 23^rd, 2008, 8:13am

Hi CherryQ,

I could imagine that the output you get during TRAN analysis is not pure sinusoidal, which means that its mean value is NOT zero and, therefore, adds to the DC level.
Lutz

Title: Re: Question about DC CM
Post by CherryQ on Jan 28^th, 2008, 2:35am

Hi thechopper,

As you said, "the valid CM level is the dc operating point one". So what am I supposed to see the waveform of positive and negative transient output? Both of them have a CM level of Vdd/2 but opposite phase, or each of them have a different CM level (one is bigger than Vdd/2 and the other is smaller than Vdd/2) but their average is Vdd/2 ?

Hi vivkr,

I didn’t understand your comment about the CMFB current and differential signal coupling.

I only designed CMFB to have enough loop gain and tuned the Gm output dc operating point to be Vdd/2. Is there any other issue I should take note? Please kindly advise. I would appreciate if you could introduce some reference.

Title: Re: Question about DC CM
Post by vivkr on Jan 28^th, 2008, 5:30am

Hi CherryQ,

I just meant that you ought to ensure that the common-mode sensing and feedback circuitry is linear enough that large differential
signals at the output of your amplifier do not lead the CMFB going out of its linear range, or cause some differential mode
signal from leaking into the common mode path.

To put it on a concrete footing, if you have outputs Vp and Vn, then your common-mode
sensing circuitry should always detect Vcm = 0.5*(Vp + Vn).

Due to nonidealities or poor choice of architecture, you might pass the signal Vcm = 0.5*(Vp+Vn) + Admcm(Vp - Vn) into
your common-mode path. However, your problem could be more due to the common-mode sensing circuitry having a limited
range of input swing (smaller than swing actually seen in amp).

Vivek

Title: Re: Question about DC CM
Post by CherryQ on Jan 31^st, 2008, 1:39am

Hi Vivek,

Thanks. As I have an Opamp stage after the Gm stage, the output swing of Gm is very small, which shouldn’t exceed the CMFB input range. The problem is that the Gm output has DC offset, even when using an ideal Gm model. I am wondering if the load cap is too much or the Gm is not big enough.

Besides, I want my design to be dominated by linear settling. Any issue to be noticed? Thanks.

Cherry.