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OPA356: Unexpected output

Savva Morozov
Prodigy 60 points
Part Number: OPA356

Hi,

I've got a weird output when feeding an oscillating signal into the OPA-356, hoped somebody could clear it up for me.

Please see the schematic attached to this post.

I've got a 1KHz signal (Vmin = -0.31V, Vmax =  4.08V). I'm using a voltage divider to lower the altitude of the signal (to Vmin = -0.20V, Vmax =  2.43V), and then I'm feeding this signal into the op amp, OPA-356. I've set the gain to 1.22 (using 2.2k and 10k resistors), so theoretically the output should be between about 0V and about 3V. However, I'm getting the oscillations between 0V and 7V. I attached the oscilloscope readings to the post.

Can somebody please explain me why this is happening? I assume that the problem is with the fact that I'm feeding an oscillating signal into the op amp, cause the op amp is amplifying straight 0V or 2.4V signals perfectly. The purpose of all this is to get the output between about 0 and 3V. How can achieve that? And what am I doing wrong? 

Many thanks in advance.

  • 0
    TI__Expert 8000 points
    Hi Savva,

    Based on my simulations, you only have 13 degrees of phase margin left in your loop gain with the circuit configuration you are using. That is why you are seeing oscillations show up in your measurements. A quick check to verify this is to replace your 10k gain resistor with a 1k resistor and look for oscillations. You should see that the amplifier is stable in that configuration. If you are not able to add more gain in your application there are a few other ways we may be able to stabilize the circuit.

    Regards,