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Instrumentation AMP Virtual short.

mrblueblue1234

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If I am using an instrumentation amplifier such as the AD8421 in what situations can I make an assumption of a virtual short across the input terminals of the amplifier? For example will the + terminal be ground?

https://www.analog.com/media/en/technical-documentation/data-sheets/AD8421.pdf


IN_AMP.png

I thought that the virtual short / ground had to do when there was feedback as part of the amplifier. Thanks for any clarification.
 

barry

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Yes, it's a virtual short. If you actually analyze the circuit, you'll see that as the IA open-loop gain goes to infinity, the voltage at the two inputs becomes the same.
 

FvM

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No, instrumentation amplifier works different than basic OP. No "virtual short" involved. The sketched schematic has no relation to instrumentation amplifiers.
 

RushPCBUSA

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One of these assumptions is called the virtual short circuit, or just virtual short. Actually, this is not one of the fundamental characteristics of an ideal op amp. Rather, the virtual short is a theoretical situation that arises from one of the fundamental characteristics of an ideal op amp, namely, infinite open-loop gain.

Let’s imagine that we have an op amp connected as an inverting amplifier. As is almost always the case with op amp circuits, the functionality is based on the use of negative feedback..
techart_opampVS_1.JPG
 

FvM

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Agree with your explanation. But it doesn't apply to instrumentation amplifier like AD8421. It has no external feedback.
 

asdf44

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Right, no virtual ground for an inst amp.

They act as a fixed differential in to differential out gain block with high input impedance.

This inst amp defaults to gain of 1 which can be increased by adding a single external R. The R to gain table is on sheet 20.

Learn a simulator: LTSpice is provided for free by Analog and includes a model and application circuit for AD8421.
 
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