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AD7745/AD7746 Cables and parasitic capacitance to ground

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?

  • Hi Jack

    Apologies for the Late reply, the range can be extended, please see the link here

    CN0346 Circuit Note | Analog Devices 

    my answers to your Queries

    Ans 1>> the 7745 is 24 bit device and a Better chip With regards to measurement, i need to understand your application for suggesting maybe a better alternate.

    Ans 2>> Parasitic capacitance will add to the values and as long as the setup is stable it would remain constant. you can calibrate it in SW or using the CAPDAC register

    Ans 3>> Shielding will help and should be good enough.

    Let me know if you have any more queries


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