SNIKE
Junior Member
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Posts: 27
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Hi Carl,
This is a third order effect. I have the following setup. Ideal Opamp with Unity gain bandwidth of 10GHz. Also have "parasitic" capacitance at the input node. [50fF].
I have 3 switches controlled by phase1, phase1early, phase2. Phase1early opens up first [bottom plate]. Phase1early switch is the feedback switch. Phase1 opens up after some nonoverlap time. [Phase1 is the top plate sampling switch] Phase 2 closes after additional nonoverlap time. [Phase2 is flip around switch].
when the phase1early is closed ,input parasitic is at the virtual ground.
during the nonoverlaptime [phase1early is opened , phase1 is still closed]. In this mode you have input connected to sampling capacitance, and the second node of capacitor connected to input of opamp. This forms a capacitive division of input signal at the input of opamp[ only during this non overlap time].
Since the opamp has bandwidth faster than nonoverlap time, the opamp amplifies the divided voltage in openloop. since the opamp has large openloop gain, output is a gained up version of input. Now because of parasitic CDS of phase1early switch, there is a small coupling of output to input node. Hence the charge conservation is lost. Also this error is highly proportional to input causing non linearity.
The solution was easy, I reduced the non overlap time and reduced the opamp bandwidth to realistic value of 100MHz.
Let me know if this is not clear. I can try drawing some figures.
Thanks
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