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AD7745/AD7746 Cables and parasitic capacitance to ground

Question asked by Jack.B on Mar 18, 2016
Latest reply on Aug 19, 2016 by Wa$iM

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?

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