a question about ADC- why we use Cp to hold the voltage

[fmgay]

a question about ADC

As the operation of SAR-ADC, In sample mode, the capacitor Cp is connected to AC gnd "com" by close the switch; in redistribution mode, the switches is opened, the voltage of plus port of comparator is hold by Cp.

My question is why we use Cp to hold the voltage of plus port of comparator, but not to connect the plus port of comparator to "com" derictly? the "com" is also AC gnd.

Thank you very much!!!

 

[fmgay]

a question about ADC

Anyone can help me?

 

[flyingsky]

a question about ADC

hi
the "com" is connected to AC gnd and DCged,
but the Cp is just connected to AC gnd.

 

[fmgay]

a question about ADC

Hi flyingsky:
At first, thank you for applying.
But I still puzzled.
What mean's DCged? Is it DCgnd?

If I connect "inp" to "com" derictly, what shortcoming would be come? after all, If that is feasible, the '"Cp" would do not needed again.

 

[fmgay]

a question about ADC

Anyone can help me?
Thanks a lot!

 

[Usman Hai]

Re: a question about ADC

Can you please tell from where you get this schematics? I am not an expert in SAR ADC but i never see open loop open loop in SAR...perhaps I am wrong.