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Opamp saturation Nucleo-U575ZI

Skfir
Associate III

Hello friends!

I have been messing around with the onboard opamp and here is the problem. When I configure it as the follower, things are okay, but as soon as I configure it as an AMP with PGA on and set any gain, I see quite dramatic DC shift on the output. I have the positive input connected to a voltage divider (no any signal), that gives 1.6v bias. Yet, when the gain is 2, the output shows about 2.3V. Gain >2 already results in saturation. The inverted input isn't externally connected

Doesn't anyone know, what the problem might be?

1 ACCEPTED SOLUTION

Accepted Solutions

right, ref to gnd. if you want AC amp, need connect external to inv + non inv input.

like this:

0693W00000Y7T7fQAF.png

If you feel a post has answered your question, please click "Accept as Solution".

View solution in original post

6 REPLIES 6

What's your VDDA?

1.6V x2 = 3.2V so 2.3V is already way lower than expected.

JW

Skfir
Associate III

It is 3.3V. I don't know how the opamp is connected internally, it would be great to see a schematic. Because I need to connect an audio AC signal to it, so 1/2 supply biasing is required.

AScha.3
Principal III

reading in rm -> Operational amplifier (OPAMP) could help 🙂

0693W00000Y7SlZQAV.png

If you feel a post has answered your question, please click "Accept as Solution".

Thank you AScha. Am I correct in my presumption that even when we connect the negative input to GPIO, if we want to use PGA, then the bottom part of the voltage divider will still be connected to ground? In such case, is there a way to use PGA and yet have constant bias at 1/2 VDDA in order to amplify audio signal from a mike? Sorry if I am asking something silly, the thing is I am a pianist, not an engineer.

right, ref to gnd. if you want AC amp, need connect external to inv + non inv input.

like this:

0693W00000Y7T7fQAF.png

If you feel a post has answered your question, please click "Accept as Solution".
Skfir
Associate III

Thank you for the explanations. It is a pity that in such case I will lose the ability to control the gain through software.