The Designer's Guide Community
Forum
Welcome, Guest. Please Login or Register. Please follow the Forum guidelines.
Mar 28th, 2024, 3:03pm
Pages: 1
Send Topic Print
How to compensate the parasitic cap of the top plate of DAC in SARADC (Read 2051 times)
Jacki
Senior Member
****
Offline



Posts: 237

How to compensate the parasitic cap of the top plate of DAC in SARADC
Mar 14th, 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?
Back to top
 
 
View Profile   IP Logged
Jacki
Senior Member
****
Offline



Posts: 237

Re: How to compensate the parasitic cap of the top plate of DAC in SARADC
Reply #1 - Apr 17th, 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.
Back to top
 
 
View Profile   IP Logged
DanielLam
Community Member
***
Offline



Posts: 76

Re: How to compensate the parasitic cap of the top plate of DAC in SARADC
Reply #2 - Apr 17th, 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.
Back to top
 
 
View Profile   IP Logged
Jacki
Senior Member
****
Offline



Posts: 237

Re: How to compensate the parasitic cap of the top plate of DAC in SARADC
Reply #3 - Jun 13th, 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.
Back to top
 
 
View Profile   IP Logged
Pages: 1
Send Topic Print
Copyright 2002-2024 Designer’s Guide Consulting, Inc. Designer’s Guide® is a registered trademark of Designer’s Guide Consulting, Inc. All rights reserved. Send comments or questions to editor@designers-guide.org. Consider submitting a paper or model.