AD7747 - Liquid Level Measurement

Question

Hi Everyone, 
 I am using AD7747 for liquid level measurement, I can able to read all the registers as well as capacitance properly. But the issue is the capacitance increasing 1fF(femto farad) for every few minutes. Presently i am using Cin = (((code-0X800000)/0X800000)*Cref) this equation for measuring capacitance. Could anyone tell me whether this equation is correct or not as well as how to measure the Cref. End application is completed, once liquid level sensor is done then we will move production level, if anyone give a appropriate answer that is very much help full for us. 
 Thanks & Regards 
 Suresh Kumar.S

EricC · Accepted Answer

Hi Kumar, 
 Your equation for Cin looks inline with that used in the evaluation software, it should be ok. Cref is the internal reference capacitance. I have attached an application note that has some detail on this parameter (page 2) which should be true for the AD7147 except for instead for 4.096 you should use 8.192pF. 
 Alternative, to my knowledge another way to calculate Cin is to convert the hex code to decimal, divide by 2^23 and then multiply by 8.192pF. 
 Have you ruled out temperature and humidity to account for the 1fF change? It is a very small change and could be as a result of small environmental changes. 
 Regards, 
 Eric.

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AD7747 - Liquid Level Measurement

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