Op Amp Diagnostics

[samovar]

I have an Op Amp OPA350 by Texas Instruments. It is powered by 3.3V positive and OV negative.

I measured 2.27mV on "+", and I expected the same on "-", but it is 17.43mV.

This is close to 0V, but the spec sheet says values up to -0.5V are ok.

The "-" is connected through a 10K resistor to a point which is -2.77mV, and through a 20K resistor to the output. As expected from these values, the output is 57mV.

If there was 2.27mV on "-", the output would be 12.35mV. According to the spec sheet, the op amp is capable of going within 10mV of the rail.

IMHO, this mean the Op Amp is bad and I need to re-solder a new one. Is this the correct diagnostics?

 

[ChrisP58]

Please post a schematic of your circuit. It's difficult to tell what voltages should be on the op amp pins without knowing what they are connected to.

 

[samovar]

Figured this out. There was a considerable ripple. When voltages go down, the output is limited by the rail. When they go up, they get amplified. When I measure average voltage I get it higher than it should be. Need to feed the op amp with less than 0V to fix that.