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AD7746 measurement problem

Hi , I'm trying to use the AD7746 to measure a capacitance about 30pF, I used the extension circuit, F is about  7.6


but the AD7746 is feeding me with [00 00 00] (0x01, 0x02, 0x03) when the CAPDAC+ is enabled, and I tried different numbers, same issue. when I get the CAPDAC down to about 0, the CAP DATA REGISTER start providing numbers less than [80 00 00], which leads to negative capacitance reading.


and I also tried to use the single-ended circuit in Figure 30 on AD7746 spec, still I got numbers less than [80 00 00].

I'm wondering if it's possible my chip is damaged? Can anybody help me with this? Thanks.

I got some pictures attached

  • Hi Guan,

    Have you tried simplifying your setup.  Try removing the extension circuit and testing a known capacitance values.  Note, the capacitance range for this part is +/-4pF and you would need to set the zero scale value to calibrate for parasitic offsets in the application.  A zero scale offset within +/-1pF can be removed by using the capacitance offset calibration register, see page 23 in the datasheet.



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