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Design >> Mixed-Signal Design >> Role of Dummy capacitors in SAR ADC
https://designers-guide.org/forum/YaBB.pl?num=1404751069

Message started by eternity on Jul 7^th, 2014, 9:37am

Title: Role of Dummy capacitors in SAR ADC
Post by eternity on Jul 7^th, 2014, 9:37am

Dear all,
I just want to get clarified about the role of dummy capacitors in SAR ADCs. The dummy capacitor in the DAC array is used only for realising Vref quantities in terms of base-2 radix. or is it also used to realise some other functions too?

Thanks to all

Eternity

Title: Re: Role of Dummy capacitors in SAR ADC
Post by loose-electron on Jul 7^th, 2014, 5:32pm

Dummy devices are often used to serve as a similar perimeter device so all active devices exist in an identical environment. This often helps with component matching

Title: Re: Role of Dummy capacitors in SAR ADC
Post by eternity on Jul 8^th, 2014, 2:51am

Hi there
thanks for the reply.
I guess you interepreted my question in terms of layout

I meant in terms of schematic. There is usually an extra unit cap placed in the DAC of SAR ADC to get binary weighed Vref levels(Vref/2, Vref/4..etc.,). I recently spotted in a journal seeing that extra cap connected to input voltage rather than to vref. So I want to get cleared on this.

Thanks
Eternity

Title: Re: Role of Dummy capacitors in SAR ADC
Post by loose-electron on Jul 8^th, 2014, 12:03pm

If it is on a reference voltage it could be for high frequency stability or reduction of coupling noise.

Title: Re: Role of Dummy capacitors in SAR ADC
Post by RobG on Jul 10^th, 2014, 5:30pm

eternity wrote on Jul 8^th, 2014, 2:51am:

Interesting... which article had that?