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AD8495 - input RC filter causes reading offset

Question asked by charlie80 on Oct 23, 2013
Latest reply on Oct 28, 2013 by charlie80

Dear all,

    I am trying to set up a type K thermocouple measurement system for my project. I have recreated the circuit shown here on page 3. However, what I see is that the output is correct only if I remove the two 10kOhm resistors on the input filter. If I insert the resistors, I get a reading of about 70°C instead of about 22°C.

 

I initially thought that the problem is that the two resistors must have exactly the same values, otherwise they cause two different voltage drops which offset the reading. However, I tried to replace them with two resistors showing both 9.99kOhm on a multimeter, but the problem is almost the same. I am using metal-film resistors, two 1nF multilayer ceramic capacitors and a 10nF polyester film capacitor.

 

Can anyone help me? Maybe I have to use some different kind of resistor/capacitors, or be even more precise on their value?

 

Thank you in advance!

 

Carlo

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