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SAR ADC converter analog problem

yefj

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Hello , In the SAR ADC converter shown bellow, there is a basic logical problem.
Because we are comparing DC voltage to DC voltage in the proccess shown Bellow.
Thae comperator (which is implemented as OPAMP) has GND connected to it.All the calculations we assume a virtual ground.
But there is a contadiction because if we compare DC voltages then we have BIAS voltages on the input of the OPAMP.
a virtual ground exists if we have AC signal which is not the case in SAR ADC.
Where did i go wrong?
Thanks.

8.JPG
 
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FvM

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"virtual ground" is a descriptive term explaining the operation of an OP in inverted configuration where negative feedback zeros the input voltage. It's inappropriate for a comparator circuit that doesn't involve negative feedback.
 

    yefj

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sutapanaki

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The SAR ADC is also a feedback circuit and thus it tries to make a virtual short between the two inputs of the comparator. It doesn't really matter that in the diagram you've connected the inverting input to ground. You may as well consider it some fixed voltage, say VDD/2 or Vrefm if your comparator is supplied by by VDD and 0. Remember that not only is the comparator input connected to that voltage, but also is the upper side of the CapDAC connected to it during sampling, so you sample the input with respect to that voltage. Of course, you will need to connect the switches to that Vrefm or VDD/2 as well, so everything is consistent.
 

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Yes, the SAR logic is proving negative feedback, but discontinous and only during the approximation phase. There's a similarity with OP feedback operation, but it's not the same.
 

sutapanaki

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Well, it is not an opamp operation but it is a feedback system and as such it drives the loop to bring the error at the inouts of the comparator to 0. That's why at the end of the conversion the CDAC is within 1/2 LSB of the grounded input of the comparator in this specific configuration.
 

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