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Common mode voltage range

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Jester

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The circuit shown below is used for measuring MAINS voltage, why are R3 and R6 used in this circuit?

My guess is to ensure the common mode range of U1 is not exceeded during power up?

diff circuit.png
 

FvM

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They form a low-pass filter in combination with C1 and C2.
 

erikl

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They form a low-pass filter in combination with C1 and C2.

Do not R2 & R5 form the low-pass filter in combination with C1 and C2?

I think R3 & R6 together with R7 & R8 fix the gain of the opAmp U1:A for the low passband frequencies.
 

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The low-pass time constant is C1*(R5||R6) which is effectively C1*R6, that's why I said R6 rather than R5 makes the low-pass. But all components play their part. You could also emphasize that R6 is required to achieve stability when C1 is present.
 

Borber

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If the corner frequencies of both low-pass filters should be the same then differential amplifier must be designed different or C1 and C2 must be calculated regarding the input impedances of both inputs of subtractor.
R3 and R6 are part of subtractor and must not be omitted. BTW without R6 input impedance of that input will be 0 ohm and circuit will oscillate as already said.
 
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