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Design >> Mixed-Signal Design >> SH configuration for pipeline ADC
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Message started by sachinagg on Jul 4^th, 2006, 6:41pm

Title: SH configuration for pipeline ADC
Post by sachinagg on Jul 4^th, 2006, 6:41pm

I would like to clarify a doubt about the single-capacitor flip-around Sample & Hold configuration for pipeline ADC. I am aware of the relative advantges of this configuration as compared to the other configurations [like two capacitor configuration etc] but I find that this configuration is hardly used by any of the designers.

Can someone please let me know the disadvantages of this circuit?

Thank You

Title: Re: SH configuration for pipeline ADC
Post by vivkr on Jul 5^th, 2006, 8:23am

Hi,

The flipover S&H is actually quite popular. I think you have not looked at enough designs. It does have one limitation that I know of - The input common-mode range of the opamp needs to be wider, as the input CM will take a step when you switch from sample phase to hold phase.

You typically need to use a folded cascode, and make sure that the CM level is not so large as to cutoff the amp.

Regards
Vivek

Title: Re: SH configuration for pipeline ADC
Post by sachinagg on Jul 5^th, 2006, 3:08pm

Thank You Vivek. You were informative. Most of the designs I have come across allowed both the AC coupling [Internal reference will set the common mode] and DC coupling [with wide common mode range allowed]. As per your explanation, I now understand that this DC coupling requirement would make the use of single capacitor flip around configuration quite unsuitable [due to wide input CM range requirement for the op-amp].

With Regards
Sachin

Title: Re: SH configuration for pipeline ADC
Post by sheldon on Jul 6^th, 2006, 5:56am

Sachin,

Using the flip-around capacitor means that the S/H needs more
common-mode rejection compared to the standard back-plate
sampling. In back-plate sampling the common-mode level is reset
to the common-mode level with every sample. However, with a
flip-around sampler, the common-mode level is not reset, so the SH
needs to reject the variations in common-mode level.

Best Regards,

Sheldon

Title: Re: SH configuration for pipeline ADC
Post by sachinagg on Jul 6^th, 2006, 4:19pm

Sheldon, Thank you for the response.

With Regards
Sachin