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Design >> Analog Design >> comparator delay and impedance calibration
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Message started by Tofu on Apr 15^th, 2009, 2:07pm

Title: comparator delay and impedance calibration
Post by Tofu on Apr 15^th, 2009, 2:07pm

Hello:

I am designing a continuous time comparator based on single-end folded cascode amplifier. The comparator could not reach the delay spec. Here are my questions:

- Apart from slewing, is the output impedance of the folded cascode the only factor that contributes to the delay (output cap is very small)? are there any other factors that can potentially make it slower?

- I tried to calibrate the output impedance of the folded cascode (biased in common mode) by connecting its output to a vsource with ac mag=1 and check the low freq current. This calibrated low freq impedance (actually Rout) is several times lower than the one obtained via standard textbook calculation (based on Gds etc). Did I do something wrong here?

any help is highly appreciated!

Title: Re: comparator delay and impedance calibration
Post by boe on Apr 16^th, 2009, 9:14am

Tofu,

Tofu wrote on Apr 15^th, 2009, 2:07pm:

- Apart from slewing, is the output impedance of the folded cascode the only factor that contributes to the delay (output cap is very small)?

I'd say it's bandwidth, not impedance. Also: (very) small diff. input signal = (very) small diff. current = slow, even for small load cap.

Quote:

are there any other factors that can potentially make it slower?

A few Farad of cap should do it.... (Sorry, coudn't resist. [Added:] Usually, the more details you add to your question the better the answers.)

Quote:

- I tried to calibrate the output impedance of the folded cascode (biased in common mode) by connecting its output to a vsource with ac mag=1 and check the low freq current. This calibrated low freq impedance (actually Rout) is several times lower than the one obtained via standard textbook calculation (based on Gds etc). Did I do something wrong here?

Did you check the DC operating point of your voltage source (idc should be 0)? Why not use a current source?
B.O.E.

Title: Re: comparator delay and impedance calibration
Post by Tofu on Apr 30^th, 2009, 2:09pm

Dear Boe:

Thanks a lot for the hint. I will take a look on my settings.