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Design >> Analog Design >> CMFB loop Stability OR design issue?
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Message started by ssuchitav on Sep 8^th, 2011, 8:32am

Title: CMFB loop Stability OR design issue?
Post by ssuchitav on Sep 8^th, 2011, 8:32am

Hi,

I am trying to simulate the circuit shown in the attachment. An ideal OPAMP model (dc gain=100dB UGB=1G) is used for CMFB. The input is of 100MHz.

The circuit is showing unexpected rail to rail swing instead of the expected across 1.25V. I have cheked the CMFB loop stabilty and it looks fine.

Please help me in debugging this behaviour.
Thanks in advance for your suggestions.

Title: Re: CMFB loop Stability OR design issue?
Post by raja.cedt on Sep 8^th, 2011, 9:34am

hello,
what is the circuit functionality? is it an amplifer or h-bridge driver for serial comunication? Any how what kind of input you applied? Have you verified DC operating point of output and is it settling to 1.25. Show some inputs and outputs, otherwise very tough to suggest.

One thing i can tell you may be you have applied high frequency signal and may be due to CMFB low BW it is reaching rail to rail.

Thanks.

Title: Re: CMFB loop Stability OR design issue?
Post by ssuchitav on Sep 8^th, 2011, 9:59am

Hi,

please see the loop ac response. It's H bridge driver for serial communication.

Title: Re: CMFB loop Stability OR design issue?
Post by raja.cedt on Sep 8^th, 2011, 10:42am

hello,
this looks ok. But whats the input data rate you are appling, try to apply very slow datarate, then if you are getting no problem it might be the problem with slow CMFB.
Could you please show input and out data.

Thanks.
raj.

Title: Re: CMFB loop Stability OR design issue?
Post by raja.cedt on Sep 8^th, 2011, 10:44am

hello i forgot to ask, you have used opamp with UGB 1e9, but still your maginitude plot is showing LBW is 1e9 how come is no bosy in the loop is not offering any poles?

Thanks.

Title: Re: CMFB loop Stability OR design issue?
Post by rfcooltools.com on Sep 8^th, 2011, 11:29am

ssuchitav,

In regards to your bode plot. it appears that your system has three poles before unity gain which would normally be a bad thing for stability.

Also, Could you provide more detail about your setup?

The phase is initially 0 degrees.

This could be that the excitation source polarity relative to the measured is the reason

Or that you have positive feedback at DC to around the point where your gain is below unity.

http://rfcooltools.com