AnsweredAssumed Answered

AD8429 offset voltage

Question asked by dirkson on Mar 5, 2015
Latest reply on Mar 25, 2015 by dirkson



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.


circa 10006123mV139mV-4,09mV-2,3V
circa 20003124,7mV140mV-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