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How to choose the opamp

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how to choose an opamp

I need to design a input-buf for a SAR ADC.The architecture was showed in pic.
The loading of first opamp is resistors and the second is cap.So the first opamp
I want to use telescope casode architecture without output buffer for first opamp.
And the second with class AB output buffer to drive the cap loading.
Is that right?Any problem?
 

telescopic can not be used as buffer, so you can not use it as first opamp. and you should also consider the opamp gain as its load is resistor, resistor load will reduce the gain. Maybe folded-cascode is ok.

for the second, maybe OTA is possible, I think.
 

    K

    knights

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iamxo said:
telescopic can not be used as buffer, so you can not use it as first opamp. and you should also consider the opamp gain as its load is resistor, resistor load will reduce the gain. Maybe folded-cascode is ok.

for the second, maybe OTA is possible, I think.
Click to expand...


Thanks.I made a mistake.The first opamp should be a folded casode rail to rail input opamp.

Added after 1 minutes:

lijianheng said:
How about the frequency of the ADC working?
Click to expand...

Low frequency SAR ADC.
 

iamxo said:
telescopic can not be used as buffer, so you can not use it as first opamp. and you should also consider the opamp gain as its load is resistor, resistor load will reduce the gain. Maybe folded-cascode is ok.

for the second, maybe OTA is possible, I think.
Click to expand...

For first opamp,amplifier with output buffer stage may be need because of resistors loading?
For second,OTA means no output buffer stage amplifier,so the simple folded cascode is ok.
 

1. for 1st opamp, gain error is a concern if no output buffer.
2. for 2nd opamp, class AB is better for application need fast slewing
 

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