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Design >> Analog Design >> OTA-C (Gm-C) Filter Design
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Message started by aaron_do on Aug 16^th, 2006, 5:03am

Title: OTA-C (Gm-C) Filter Design
Post by aaron_do on Aug 16^th, 2006, 5:03am

Hi all,

i'm looking into differential OTA-C filter design in CMOS for channel selection, and so i designed a simple OTA and created a biquad bandpass filter then cascaded four of them to get a 8th order BPF. While individually the OTAs seemed normal in terms of input referred noise, when i cascaded them i noticed a large spike in the IRN. The center frequency is 2 MHz and the bandwidth is 1 MHz, and the spike appeared at 1.5 MHz. While the IRN at 2 MHz was around 200 nV/rtHz, the IRN at 1.5 MHz was about 1.3 uV/rtHz. I also noticed that this large noise spike is common mode noise since when I simulated the IRN with the output noise as a differential voltage the spike disappeared. It isn't flicker noise.

Does anybody know where this noise came from? Also since it is common mode noise can i neglect it?

thanks,
Aaron

Title: Re: OTA-C (Gm-C) Filter Design
Post by ACWWong on Aug 16^th, 2006, 9:43am

Generally, one is interested in the differential signal to differential noise ratio, so common-mode noise is rejected somewhat. But in real circuits, especially in mismatched conditions when CM rejection is poor, common-mode noise can then become differentail and problematic. A monte-carlo sim will show you.

In you case, I would definately be looking at the Common-mode feedback scheme you are using in your OTA. I think you might find it improves if instead of using differential capacitance to get your TF, you use the capaciators in common-mode (at the cost of of 4* cap area)... this will should give you a noticable change in common-mode response.... and usually CMFB stability is the scurge of your type of design... i've seen a few high Q gmC filters oscillate nicely in common-mode...

Anyway in cascaded biquad design, there is always an optimum cascade order for either noise or linearity/signal handling. Usually one choses the choice for best dynamic ratio (ie signal handling to noise ratio). So changing the order of the biquads will change the noise.
Also remember if the bandpass is real and created by bilinear transform of low-pass transfer function, then it will be symetrical in log(frq) rather than linear frq, so the noise always grows faster on the low side of the characteristic than the high side (ignoring effects like flicker noise etc.). This is not the case in your problem as you are quote noise at the -3dB point.... rather than just of of band... but its something to bare in mind.

cheers

aw

Title: Re: OTA-C (Gm-C) Filter Design
Post by loose-electron on Aug 16^th, 2006, 2:56pm

You can't just ignore common mode noise. CMRR is very real and needs to be profiled over frequency and under mismatch conditions.

Ergo, I am in agreement with AW... ;)

What you describe there sounds to me like you have the CMFB circuits interacting with each other. Change the BW of the CMFB circuits and see how it affects the spectral placement of the CM noise.

Jerry

Title: Re: OTA-C (Gm-C) Filter Design
Post by aaron_do on Aug 17^th, 2006, 12:25am

thanks,

yeah it was noise from the common mode detector. I was hoping i could get away with using a very simple common mode biasing scheme but it looks like i'm gonna have to do the whole CMFB loop.

Aaron

Title: Re: OTA-C (Gm-C) Filter Design
Post by loose-electron on Aug 17^th, 2006, 11:08am

Been there, done that.

I did a gm-C filter last year for an UWB receiver, and the CMFB was a pain in the #ss...

The modulation and the CMFB BW were interacting and it took a while to straighten out.
Good luck with it!

Jerry