AD8429 offset voltage

Hello,

in my circuit I have a problem, with the offset voltage. I dont how is the best way to solve it.

Below you can see my circuit.

I have measurement the volteges Up, Un, Uref and Uout with different values of Rg. The relay are reed relay. My aim was it, to make a offset compensation  with the Uref voltage. But if the gain (small Rg) is to high, than I get an offset voltage, which is bigger I have calculated. Thats the reason I measurment the voltages to find out, where the big offset come from. For this measurement the Uref voltage is constant.

v Rg Up Un Uref Uout
1 10M 123,5mV 126mV -4,1mV -6,2mV
10 666,6 123,4mV 125,8mV -4,09mV -29mV
100 60,6 123mV 175mV -4,09mV -250mV
circa 1000 6 123mV 139mV -4,09mV -2,3V
circa 2000 3 124,7mV 140mV -4,1mV -3V

1.The output voltage is: (Up - Un) * v + Uref. For the first and the second line thats ok. But the other lines it is funny.

2. I understand the voltage Up, the bias current is constant and flow throw the 1Meg resistance and create the offset voltage of circa 123mV.

3. But I dont understand, why the voltage Un is change from v=100. Is this normal for the AD8429 ?

4. Have you any idea, where problem should be? Is it a measuremnt error?

Thank you in advance.

Best regards

Dirk

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