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AD7746: step change in conversion result

Question asked by Ayo on Nov 14, 2014
Latest reply on Nov 20, 2014 by jcolao

Hi ADI Experts,

 

I use AD7746 in a capacitive sensor-based liquid level transmitter. The design is based on the ADI circuit note CN-0129.

I'm currently experiencing a strange behaviour on some units (7 units out of a lot of 50units) where in their unloaded state (i.e. no change in input capacitance to the AD7746) the raw conversion results for the AD7746 changes by as much as 3% of the span.

 

A graph of the observed step change in the AD7746 conversion result:

 

 

As you will see from the graph there is a big change in the AD7746 conversion result even when the temperature is fairly constant (the temperature shown in the graph is as measured by the AD7746 chip).

I'm aware that the converter has a drift of -26ppm/C and the on-chip CapDAC has a thermal drift of 26ppm/C but the changes encountered are in the order of 3%.

 

What are your thoughts on this issue?

 

Thanks for your support.

 

Ayo

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