AnsweredAssumed Answered

AD8429 offset voltage

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

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.

AD8429_Offsetproblem.png

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.

 

vRgUpUnUrefUout
110M123,5mV126mV-4,1mV-6,2mV
10666,6123,4mV125,8mV-4,09mV-29mV
10060,6123mV175mV-4,09mV-250mV
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

 

Dirk

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