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Input common mode of Switched Capacitor Integrator (Read 3390 times)

luoshuer

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Input common mode of Switched Capacitor Integrator
Jan 11^th, 2013, 2:49pm

Hi Guys,

I am developing sigma-delta ADC and have a basic question about SC integrator like the Fig. shows below. We have sampling phase (CK1) and integration phase (CK2). Usually we can have common mode feedback circuit to determine the output common mode. But, how to define the INPUT common mode voltage to the opamp? (the differential input)??

Both sampling phase and integration phase do NOT define the input common mode of this differential amplifier?

Many thanks!

Suer

FDSCINT_001.jpg

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sheldon

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Re: Input common mode of Switched Capacitor Integrator
Reply #1 - Jan 12^th, 2013, 7:08am

Luoshuer,

Consider the schematic, when the CK1 is high and the switches
labeled CK1 are shorted. The sampling capacitor is connected from
vin/2 to ground. When the switch label CK2 are closed the capacitor
is connected from ground to the input, so the voltage across the
capacitor is inverted. On the other side the opposite process occurs.
So effectively as you have drawn it, ground defines the input common
mode level.

Best Regards,

Sheldon

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