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MDAC for a Pipelined-SAR ADC

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Majid_Vatan_Parast

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Hello everybody
I want to design a MADC for a Pipeliled-SAR ADC , but I have problems . I have designed a 2-satge Op-Amp that first stage is a Cascode . Op-Amp needs 0.78 V common mode in input to work correctly . when I put it in a capacitive feedback Op-Amp dose not work because capacitors remove common mode voltage of inputs and so Op-Amp dose not have appropriate DC voltage on its input transistors. Can you please tell me how can I implement needed common mode voltage (0.78 v) on Op-Apm's input transistors ??
The picture below shows architecture of my Op-Amp and feedback circuit .
It is urgent :-(
Thanks photo_2018-05-22_10-31-13.jpg
 
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Did you try to connect a 0.78V voltage source through 2 resistors to the inputs?
 

Hi,

I assume you need any resistive fedback to stabilize operating point.
With purely capacitive feedback the output will drift away.

Klaus
 

Level shifting front end is the usual approach to getting
an input common mode range that includes ground on a
single-supply op amp or comparator.

Capacitive feedback can work to DC if you have the
right autozero switch scheme. But you do need to do a
DC reset if you want DC correction.
 

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