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Category: Hardware
Product Number: Analog Devices AD629 Difference Amplifier
Software Version: NA

The AD629 Difference Amplifier has a 380kohm feedback resistor (lets call it R2) connected between the output and the inverting pin of the amplifier.

The AD629 also has a 20kohm resistor (lets call it R4) connected between the non inverting pin of the amplifier and GND.

Difference Amplifiers normally have R2=R4.

Why is this not the case with the AD629?

## Top Replies

• I agree to what Achim said wherein the resistors are still match, it is just that the 380k and 20k ohm resistors are in parallel in the -IN. Resistor matching is done for the rejection…

• Hello Bluto,

if you look at a typical circuit with the AD629 you can see, that there is not only R2 with 380kOhm as feedback resistor. There is also R5 with 21.1kOhm going to the inverted input. R5 and R2 together (i.e. parallel) also give a total resistance of 20kOhm - just like R4.

The reason for this special setup is, that the AD629 is intendend to handle a huge input common-mode voltage range. (+-270V). Standard difference amplifiers will often handle just a small fraction of that. The AD629 uses the  voltage divider consisting of R3 and R4 to divide this potentially huge input voltage by a factor of 20, so that the input of the opamp itself can handle it. The network of R1 and R2 and R5 serves the same purpose and gives at the same time a gain of 1 for the difference voltage.

best regards

Achim

• I agree to what Achim said wherein the resistors are still match, it is just that the 380k and 20k ohm resistors are in parallel in the -IN. Resistor matching is done for the rejection of the common-mode signals while amplifying the differential input signals.

Thank you!

Best regards,

Mae

• Hi,

I believe that the question has already been answered so I am closing this thread. Thank you!

Regards,
Paul