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.
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.