OPAMP and low level voltages

[StillTrying]

Dana, you don't need to know the value of the 2.5V, only that its impedance is zero the same as 0V. So all you've got is 10pA flowing into 90k.   (900K || 100K) * 10pA = 0.9uV   is a bit shorter/quicker!

I use a keithley 2700, it suppose to have an input impedance of more than 10Gohm.
I measured the offset as you proposed and I get the same values.

 
In Reply#6 you've got a TLV2369 on +-2.5V supply. Over your small CM test range of 25mV to 250mV you should only see a change in the input offset of < 3uV (which is the yellow square the graph) but you measured a change of +106uV to -70uA, I don't know why you getting such a big change in the TLV2369's offset.

[David Hess]

In Reply#6 you've got a TLV2369 on +-2.5V supply. Over your small CM test range of 25mV to 250mV you should only see a change in the input offset of < 3uV (which is the yellow square the graph) but you measured a change of +106uV to -70uA, I don't know why you getting such a big change in the TLV2369's offset.

The worst case CMRR of the TLV2369 could contribute 25uV of error.  My guess would have been that being a very low power part, the TLV2369 was having difficulty driving the meter so the extra error is a measurement problem but the OPA177 did not work either.

[alltheway]

Hi,

I don't know what's the problem here.
Maybe because I build the schematic on breadboard, but why is working fine with TC7650.
I ordered a few pcb's and I will continue the tests.
At the moment the topic is on hold.

I have a feeling that the issues are related to the breadboard.

Thank you,

[danadak]

StillTrying, yes for sure I took the long way home on calculations. No "shortcuts,
simplifications" were used.


For all here is a pretty good summary of imperfect OpAmp considerations -


http://www.ti.com/lit/sg/slyt701/slyt701.pdf


Regards, Dana.

[alltheway]

Hello,

I'm back with news 
It's been some time but finally I had the time to work on this topic.
This time I have a proper pcb board for this project and the results look really good.
It works how it supposed to work with a very small error on the output (basically the opamp offset error).

I still don't believe that the problem can be the breadboard.

Thank you,