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Design >> Mixed-Signal Design >> comparaor in SAR ADC
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Message started by duncandu on Dec 19^th, 2009, 9:14pm

Title: comparaor in SAR ADC
Post by duncandu on Dec 19^th, 2009, 9:14pm

Hello,

I'm currently working on a SAR-ADC based on charge-distribution scheme, and I would like to know if the input capacitor of the comparator will affect the INL and DNL of the ADC?
For example, if my LSB cap is ~30fF, it seems that the input capacitor of comparator should be much less than 30fF. However, in order to increase the gain/resolution of comparator, we usually choose large input transistors. How to solve this problem? Or, in fact, do I misunderstand the effects of the input capacitor?

Thanks very much.

~Ken

Title: Re: comparaor in SAR ADC
Post by vivkr on Jan 4^th, 2010, 2:05am

Typically, the input cap should only induce an attenuation. Since the operation of a classical SAR requires the comparator to merely detect the sign of the input, this should not affect INL/DNL. If your comparator is not well-designed, then you may have troubles due to kickback coupling through to the input though.

Vivek