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AD7747 - Liquid Level Measurement

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

  • 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.



  • Hi Eric,

                  Thanks for your reply.

                  1) I had one doubt, the capacitance data register is 24-bit so i want to divide by 2^23 or 2^24. I tried with 5.6pF fixed capacitance. The 5.6pF is connected to Cin(+) and Gnd. If i divide by 2^23 then capacitance value is 13.9618pF and If i divide by 2^24 then the capacitance value is 6.9809pF.   

                  2) How to use EXC SETUP REGISTER(0X09), Which value i want to set for good accuracy.

                  3) After power ON the AD7747 module it start with 13.9618pF and it keeps on increasing now it reaches to 15.2471pF(nearly one day)(Temprature b/w 28.3 to 29.6 - AD7747 Internal temprature sensor). For us every femto Farad is very much important. How to stabilize the capacitance variation.

    Thanks & Regards

    Suresh Kumar.S

  • Hi Everyone,

    I have worked with AD7745 (not AD7747) for a while.

    I guess that you use single-ended configuration for your liquid level measurement. If so, fluctuations of capacitance readings are observable. Capacitance (and, of cource, impedance) can be altered by temperature and humidity fluctuations around your electrodes and wires etc. Try to use differential configuration of AD7747.

    For my application differential configuration helped a lot.

    best regards

  • Hi Tomas,

              Thanks for your reply, but still i am struggling with the capacitance variation.

    Thanks & Regards

    Suresh Kumar.S