Capacitive level sensor

Question

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

ADIApproved · Accepted Answer

Good afternoon Endre, 
 The only CDC device that we have that allows for capacitance to ground measurement is the AD7747. The device has a CAPDAC allowing a guaranteed minimum common mode capacitance compensation of 17pF and the full dynamic range of the device is +- 8pF (max=25pF). 
 It is theoretically possible to add a capacitance in series to bring your sensor into the required range of the part, but we do not have any experience using the part in such a configuration. 
 From your specifications below, I calculate a series capacitance of ~20pF would achieve the desired range. Using common values for capacitors, I suggest using a 22pF capacitor, which will give you a maximum reading of 19.8pF and a minimum of 14.9pF, for approximately 5pF range. Using a larger capacitor; 27pF, will give you more range, but will bring the maximum to 22.8pF which is close to the 25pF max. 
 Regards, 
 Sean)