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RMS Value of AD8436

Question asked by intec on Dec 13, 2012
Latest reply on Dec 17, 2012 by intec

Hello everyone,

 

I have another question concerning the output of the TRMS device AD8436.

In September 2012 we tried to solve the problem with the output of the chip that was to low in comparsion to another calibrated DMM. We thought that we found the problem and could minimize the effect by adding additional capacitors in the circuit before AD8436 but now after testing and evaluating more PCB we could see the problem again.

We double checked things like replacing capacitors but without success. To be sure that there is no mistake in our setup we ordered the AD8436 Eval Board but unluckily it shows the same effect.

 

With the increase of the input amplitude the error of the output in comparsion with another DMM rises. The total error is about 1% -3% below the expected value.

Attached you'll find some data of our tests proving what we have seen.

 

To be honest we don't have any idea what is wrong here. I looked through the datasheet over and over but could not find any hint explaining the difference. It it is normal I would have expected any formular or schematic explaining that but anyhow: Is there a factor that we forget in our calucaltion ?

We use a 12V- single supply voltage to power the chip can that cause problems because qualification in the datasheet was done with +- power supply. I could recognize that the change of the midvoltage influences the output value althoug the ac- input did not change at all.

 

Any help concering that issue is very welcomed

 

Thanks

 

Christian

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