TI's OPA2237 differential signal acquisition, the zero point does not return to zero.

dlb123

TI's OPA2237 differential signal acquisition, the zero point does not return to zero. [Copy link]

A differential signal acquisition was designed using TI's OPA2237EA/2K5 op amp, and
the differential signal was raised with 1.25V.

Under the premise that no signal is applied, the current test results are:
the initial value of the op amp is not zero (output 0.089V) and
the "virtual short" of the op amp does not exist (the - terminal voltage is 0.049V higher than the + terminal voltage).
It is preliminarily judged that the op amp is in the non-linear region, but according to the op amp manual, 1.25V is within the allowable range of the modified op amp Vcm. It feels strange. I have measured for a long time and have not found the crux. I would like to ask the experts for guidance.

The resistance accuracy is 0.1% (excluding impedance)
and the test condition is room temperature (excluding temperature drift)

呜呼哀哉

Consider the equivalent resistance of the load of the op amp. It seems from the manual that it can reach the lower rail, but it will not work when the load reaches 10K. Disconnect resistor R56 and test the output again.

dlb123

Alas, published on 2023-3-21 12:41 Consider the load equivalent resistance of the op amp. It seems that it can reach the lower rail according to the manual, but it will not work when the load reaches 10K. Disconnect the resistor R56 and measure again...

Yes, I have considered this situation and have removed the 10k resistor, but the zero drift is still the same, with no improvement. I have also replaced the op amp, but there is still no improvement.

呜呼哀哉

dlb123 posted on 2023-3-21 13:02 Yes, I have considered this situation and have removed the 10k resistor, but the zero drift is still the same, no improvement. I have also replaced the new op amp, but there is no improvement. ...

If conditions permit, try using a battery to power it. Turn off other powered devices that can be turned off. I have encountered similar problems before, but my op amp has a reverse input. Even if it is grounded, the output is still tens of mV. My op amp has RF rectification characteristics, and the resistance of the inverting input terminal is in the M ohm level. I wonder if the explanation for your small resistance is the same.

dlb123

Woohoo! Posted on 2023-3-21 13:44 If conditions permit, try using batteries for power supply. Turn off other powered devices that can be turned off. I have encountered similar problems before, but my op amp is...

I haven't found the problem yet. I think it's the op amp itself that has a problem. I just need to replace it with another model.

呜呼哀哉

It's good to be solved. There are these inexplicable things when single power supply is used.