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AD7746 Calibration

I am working on a circuit design with AD7746. Now I am facing a problem is about the offset calibration. I write the register into calibration mode and when the calibration is done, the offset registers always shows zeros, like having never been calibrated. I also has an EVL-board, its calibration works well. Is that wrong to do calibration with connecting the capacitor? Or there is something special design in EVL-board?

  • Hi Ye,

    Are you sure you are communicating with the part correctly on your board?  Can you write to other registers correctly?  The full range of the offset calibration register is +/-1pF, note the CAPDAC is used to offset larger values.  The offset calibration compensates for parasitic capacitance to "zero" the part, so connecting your capacitance is correct.  Do you know the size of your capacitor?  Have you tried to do the calibration without the cap? Note, the offset calibration register is reloaded by the default value at power-on or after reset, so you should check that this is not happening on your board.  The default value should be 0x8000 (register 13 and 14, see page 14 of datasheet).



  • Hi, Eric

    Thank you for your replying. I am certainly sure about the offset calibration function works, and I could write the register correctly. But the problem is whatever I did with the circuit (open loop, short, with cap, etc), the calibration returns the same 0x8000 as you said, which means zeros. I have tried to use LCR measure the parasitic cap on the board, but it can't give out a value from CIN and EXC. It caused by circuit inside the chip. I am going to try CAPDAC by your advice.



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