[SOLVED] Op amp testing question about feedback loop

[chandlerbing65nm]

Hi folks,

I am just curious about the op amp test circuit of analog devices, why is the feed back loop connected to the positive input of the DUT, knowing that positive feedbacks are unstable and saturates the ouput.

Kindly refer to this article by analog devices.
https://www.analog.com/en/analog-dialogue/articles/simple-op-amp-measurements.html#

 

[dick_freebird]

Because in the classical servo measurement loop the
integrator (auxiliary amplifier) provides the inversion.

 

[chandlerbing65nm]

But what is the purpose of connecting the feedback at the positive input, rather than the negative input.

 

[sutapanaki]

If you connect the feedback to the negative input you will end up having a positive feedback. That's the purpose.

 

[Easy peasy]

you need to look at the WHOLE circuit - not just the op-amp bit ...

 

[Ata_sa16]

The way to test this is easy

Imagine TP1 rises

1) V+ of DUT will increase
2) output of DUT will increase
3) V- of Aux opamp will increase
4) TP1 will drop

Therefore the loop is negetive feedback and stable

Now assume you connect to negetive input


1) V- of DUT will increase
2) output of DUT will decrease
3) V- of Aux opamp will decrease
4) TP1 will increase further ...

positive feedback !