opamp with different common modes

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santhoshv78

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I need to design a telescopic diffrential output opamp for different input common mode and output common mode voltages.
say, the input common mode is 1.25V and the output common mode is 1.65V. When the opamp is connected in feedback. do I need a level shifter to bring down the 1.65V to 1.25V at the opamp input pairs.
Since the opamp gain is high, a small mismatch in the level shifting values to the differential inputs may lead to the opamp saturation.
How can I solve this problem. Any body else have a better architecture.
Any help will be appreciated.
 

Can't you just attenuate the output using a resistor chain and then feed it back to the input? If your opamp cannot drive resistive loads, then you shoukd choose large resisitors - which can be a problem if you plan to put all this inside a chip.
 

You can use switched capacitor opamp.
 

Does it mean that if the opamp is used in switched capacitor applications,
level shifting is not required??!!
I want to use this opamp in the MDAC of pipelined ADC.
 

oh, For the use of MDAC's buffer, I think you can add a levelshift . But I have another question. In pipeline adc with digital correction, the reference voltage is not critical. Is it necessary to use a telescope opamp?
 

The input signal is referred with a voltage of 1.65V dc. I feel, this has to be constant for all the succeeding stages. Digital error correction relaxes the comparator reference voltage accuracy.
Telescopic is prefered for the low power supply consumption and high gain and high BW.
 

If your pipelined ADC is implemented in switched-capacitor, you don't need a levelshift, I think.

Stan
 

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