EVAL-AD7745/46EBZ External Capacitance

Dear Community,

I'm facing some issues with the EVAL-AD7745/46EBZ capacitance to digital converter.

I'm using the I2C interface of the AD7745/46 to write and read its registers.
First I tried to set up the on-board "capacitance sensor length demo" and it works perfectly fine with my code.

However I don't get any meaningful values if I connect an external capacitance to the SMB connectors.
Of course I changed the register values so that the AD7746 should measure an external capacitance but it doesn't.
I always get a value close to zero even though I tried to connect capacitors ranging from 1pF to 4pF.

Of course I can explain the problem more detailed if there's somebody willing to help.

  • 0
    •  Analog Employees 
    on Sep 11, 2020 6:23 AM 7 months ago

    On board capacitance is connected to CIN2 and EXCA. 

    To which CIN and EXC channels is the external capacitance connected to? How have you connected the external capacitance?

    Also, what register configurations have you done?

    Please share the above details.

  • Thank you very much for your response.

    When executing my code all the register values are set to default via the reset command 0xBF.
    Then I set the Cap Setup register (address 0x07) to a value of 0x80.
    EXC Setup (0x09) to 0x23
    and the Configuration register (0x0A) to 0x3D for offset calibration (without having the external capacitance connected to the board.)

    Then the Configuration register is set to 0x3A to enable single conversion mode (with the external capacitance connected before).
    After reading the three 8-bit Cap Data registers, the Configuration register again is set to 0x3A to perform another single conversion and the Cap Data is read again.

    Every time I set the Configuration register value there is a delay of 111ms according to a conversion time of 109.6ms.

    In summary, I try to get the Single Ended Capacitive Input mode to work as described on page 21 of the AD7745/AD7746 data-sheet.

    The external capacitance (1pF capacitor) is directly connected to EXCB and CIN1+ using the SMB plugs and a coaxial cable.

    The problem is that I always get a value close to zero (respectively close to 0x800000).

  • +1
    •  Analog Employees 
    on Sep 16, 2020 5:52 AM 7 months ago in reply to TKG

    Are you using your own code instead of the Eval software? 

    Please read back all registers and let me know the details. I do not see any issue with the command sequence that you have executed so far. You need to check whether the 1pF is properly connected to the EXCB and CIN1+.

    What happens if you do not do the Offset Calibration?

  • Thank you very much for your help.

    I checked the connection to the capacitor as you suggested and it turned out that one of the cables didn't work for whatever reason.
    I replaced it and now get a meaningful value.