I've been looking at the AD7745 for measuring a capacitive sensor of about 2pF. Since the sensor is in a hostile enviroment, it must be connected to the chip by coaxial cables. To prevent interference with other sensitive measurements the shields of the coaxes must be grounded. This introduces a parasitic capacitance to ground as per figure 36 of the datasheet. The capacitance will be more than the 60pF the chip is rated for, probably around 300pF.
1) Is there a better chip for this measurement? I've had a look, but I may have missed something.
2) What are the consequences of the parasitic capacitance to ground? I've seen figures 9-11 in the datasheet, which suggest a few fF offset. Will that offset be stable? Can I calibrate for it, subtract it and ignore it?
4) Figures 9-11 are "typical". Any guidelines on how it will vary from device to chip to chip?
3) Is there anything I can do to reduce the effect? Such as dc biasing the coax shields relative to ground?
