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Design >> Analog Design >> How to compensate the parasitic cap of the top plate of DAC in SARADC
https://designers-guide.org/forum/YaBB.pl?num=1521015492

Message started by Jacki on Mar 14^th, 2018, 1:18am

Title: How to compensate the parasitic cap of the top plate of DAC in SARADC
Post by Jacki on Mar 14^th, 2018, 1:18am

Hi,

Normally people in SAR ADC design focus on the mismatch of DAC cap array, and always mention the parasitic capacitance of the top plate in the DAC array just attenuate the input signal, but not add any conversion error. I completely agree. However, this attenuation will reduce the dynamic range of the ADC if we want to measure exactly the input signal voltage. Some applications, i think we are ok with this attenuation, but in some other applications, i think it is important to cance the parasitic capacitance.
for example, we want to measure the supply voltage by a 10-bit SAR ADC, if we have 7% parasitic capacitance from the top plate of DAC cap array to the ground, we will reduce the dynamic range by 23dB (actually it is 6-7 bit), it is quite a lot. My question is how to compensate this parasitic capacitance within 0.1%, and make the SAR-ADC to be able to detect the supply votlage in the fully dynamic range? One way I consider is to trim the Vref with the same attenuation, but I don't think it is a good idea because it is hard to do the measurement.
another way is we an additional 7% cap of the DAC to VDD, during the bit cycling, we flip the voltage of DAC, and the additional cap will compensate the parasitic cap to the ground.
Any other comments?

Title: Re: How to compensate the parasitic cap of the top plate of DAC in SARADC
Post by Jacki on Apr 17^th, 2018, 1:34am

I think I can end my thread. Maybe just accept the gain drop, keep the resolution, and compensate the parasitics as much as possible by the custom designed MOM cap.

Title: Re: How to compensate the parasitic cap of the top plate of DAC in SARADC
Post by DanielLam on Apr 17^th, 2018, 2:44pm

1) Use a top-plate sampling SAR ADC.
2) Make your caps better so that you don't have that much top-plate parasitic (7%) is quite high.
3) Have an adjustable reference. You kind of need this in practice.

Title: Re: How to compensate the parasitic cap of the top plate of DAC in SARADC
Post by Jacki on Jun 13^th, 2018, 12:47am

Hi DanielLam,

Thank you for your comments. I agree with you. Just try to shield the top plate as much as possible by the bottom plate in the custom-designed MOM cap.