Output of AD8421 instrumentation amplifier: AC or DC?

Can anyone please let me know if the output of the AD8421 instrumentation amplifier is AC or DC?

by RohithB
November 06, 2021

What is the input? Problably DC if it amplifies current, if it's voltage then ac out problably.

by vdjke.vs.capandws
November 06, 2021

I am giving an AC input at 7Hz. It's a voltage amplifier.

by RohithB
November 06, 2021

Problably most likely ac, although you could go very close to the output, open the device up, and if there are diodes there then it outputs dc.

by vdjke.vs.capandws
November 06, 2021

Yes, output has two diodes connected to it. One p-n diode is connected to output where p side is connected to output and n side to +Vs. And an n-p diode is connected to output where n side is connected to output and p side to -Vs.

by RohithB
November 07, 2021

You can see AD8421 internal circuit in this link: https://www.analog.com/media/en/technical-documentation/data-sheets/AD8421-KGD.pdf

by RohithB
November 07, 2021

Yeah it outputs dc.

by vdjke.vs.capandws
November 07, 2021

Depends on your definition of AC and DC.

Clearly, the ADD8421 is proud, among other characteristic, of its bandwidth, so its output, dependent of its inputs, can vary very rapidly through time. Not really a DC, even if it could probably do DC too.

And its output is given by

VDIFF_OUT = V+OUT − V−OUT = Gain × (V+IN − V−IN)

which shows a possibility to be in the positive or in the negative regions (within limit of the voltage rails). Is it what you meant by being "AC" ?

I strongly suggest that you look at the datasheet for tons of "do" and "don't".

On the other hand, I may have not understood your question at all.

by vanderghast
November 09, 2021

My question is, whether the instrumentation amplifier AD8421 outputs AC or DC voltage for an input AC Voltage at 7Hz? Please look at page number 22 of the datasheet, you can find the internal circuitry of the amplifier. https://www.analog.com/media/en/technical-documentation/data-sheets/ad8421.pdf

by RohithB
November 09, 2021

Look at page 11 of AD8421. The pin REF allows to add an offset to the output, I haven't look in details, but it seems that the circuit you refer too, at page 28 in my reference, is about to shift the output in the positive, to be consumed.

by vanderghast
November 09, 2021

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