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https://designers-guide.org/forum/YaBB.pl Design >> Mixed-Signal Design >> Input common mode of Switched Capacitor Integrator https://designers-guide.org/forum/YaBB.pl?num=1357944578 Message started by luoshuer on Jan 11th, 2013, 2:49pm |
Title: Input common mode of Switched Capacitor Integrator Post by luoshuer on Jan 11th, 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 |
Title: Re: Input common mode of Switched Capacitor Integrator Post by sheldon on Jan 12th, 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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