Opamp saturation Nucleo-U575ZI

[Skfir]

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?

[waclawek.jan]

What's your VDDA?

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

JW

[Skfir]

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]

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

[Skfir]

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.

[AScha.3 · Accepted solution]

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

like this:

[Skfir]

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