Capacitive level sensor

Dear all,

     I need to design a capacitive continous level sensor for a fuel tank. The sensor it self is a rod placed inside tube having empty capacitance of 47pF and with the tank filled about 150pF. The usual method is to use the capacitance in an RC oscillator and than measure the frequency than convert it to volumetric leveling. The success is unfortunately temperature dependent and should be compensated. Is there any better way to build up this sensor? I would need at least 10bit over 100pF so a definition of 0.1pF is requested. One important remark is, that one of the measuring capacitors armature is grounded to the tank (tank is made out of regular steel sheets). I have looked some of the CDC (AD7151) chips but those seems to work only for feed-in capacintances from excitation to the CDC and the output is only treshold based.

Any hint or advice is welcome! 

Thank you , Endre

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  • UPDATE: I made a prototype with the AD7148 CAPtouch sensor a couple months ago, interfacing to a Zynq chip for readout It works great so far!

    The reading on all the channels did just what I suspected. I found I indeed need to ground the "finger" in proximity, otherwise the reading is very erratic with it floating. With grounding, the reading is very stable and cleanly increases the capacitance reading to a few pF as I approach closely (1-2 mils gap) with the pad.

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  • UPDATE: I made a prototype with the AD7148 CAPtouch sensor a couple months ago, interfacing to a Zynq chip for readout It works great so far!

    The reading on all the channels did just what I suspected. I found I indeed need to ground the "finger" in proximity, otherwise the reading is very erratic with it floating. With grounding, the reading is very stable and cleanly increases the capacitance reading to a few pF as I approach closely (1-2 mils gap) with the pad.

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