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Design >> Analog Design >> Clock phases for switched capacitor filter
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Message started by rbucket on Nov 8^th, 2005, 6:44pm

Title: Clock phases for switched capacitor filter
Post by rbucket on Nov 8^th, 2005, 6:44pm

Distinguished Veterans,

I have at hand two fully differential switched-cap implementations of a 5th order Chebyshev and a 3rd order Butterworth low pass filter (derived from ladder RLC prototypes). I am simulating the responses using a PAC analysis. The phases for the clocks are such that at phi1, all nodes are discharged, and at phi2, the circuit evaluates. The 3rd order butterworth shows the correct magnitude and phase response from PAC. The 5th order Chebyshev, however, shows a premature droop in the corner frequency and its phase response drops down close to -420 degrees but rises a little and then falls back down. I would expect it to fall monotonically down to -5*90 = -450 degrees.

Switching the clock phases for the 5th order chebyshev produces the correct magnitude and phase response. So, I am perplexed at this odd behavior, and my efforts to find literature on correct phasing of switched-cap filters is futile. Any pointers and/or enlightenment would be tremendously appreciated. Thank you all.

Title: Re: Clock phases for switched capacitor filter
Post by Ken Kundert on Nov 8^th, 2005, 8:13pm

Each phase of a SCF generally has a similar but slightly different tranfer function from the others. It is important to get the right one. How are you sampling the output. Is it possible that you are sampling the wrong phase? If you are not sampling the output, then SpectreRF is computing the average of both, which is generally not what you want. This issue is discussed in section 3.2 of http://www.designers-guide.org/Analysis/sc-filters.pdf.

-Ken