Offset voltage at lower voltage output.

Hi Umberto,

It may come from different factors. Offset error, error due to Ibias, but one thing that may have possibly caused this is the mismatch of the resistors that you used-- this causes the CMRR of the part to degrade and have some significant effect on the output. 

Could you share what resistor are you using [including the tolerance] and how did you match it? I would recommend reading this article for more information regarding the effects of resistor mismatch: MT-042: Op Amp Common-Mode Rejection Ratio (CMRR) 

Thanks,

Gio

Hi Gio, thanks for the fast answer! we are using ERJ-6ENF3831V and ERJ-6ENF2431V resistors with 1% of tolerance. You might be right, we are going to change them to 0.1%.

Thanks in advance!

Umberto

Hi Umberto,

That's a very big tolerance and would really limit your CMRR. I found another article regarding CMRR, with an example of using 1% resistor tolerance and the CMRR was limited to 34dB due to the resistors, there are also calculations on the last part of the article: Precision Matched Resistors Automatically Improve Differential Amplifier CMRR – Here’s How . If you can get a resistor network with a low tolerance and the gain that you want then that would be better.

Best regards,

Gio

At the end the resistors were not the problem, it was the layout (long leads), we change the amp to another with way less bandwidth and footprint compatible and everything works! Thanks!