AnsweredAssumed Answered

ad7746 measurement calculation

Question asked by OleHansen on Jan 31, 2013
Latest reply on Mar 16, 2017 by raush

Hi all


I am working with the ad7746 capacitance to digital converter. And I need your help.


Can anyone please explain how to calculate the measured capacitance. I am having an ad7746 eval board and the measured values a different from what I end up with in my own project (using the same sensor).


From the data sheet 0x000000 represent -4.096 and that 0xFFFFFF represent 4.096. So an increase or a decrees of one in the CAP data register must be ( 4.096 * 2 / 0xFFFFFF). Then multiplied by the CAP data register, would be the actual measured value. (but this I not the correct value compared to the eval board) 


The ad7746 are having an internal offset calibration register. I tried to set the converter into calibration mode and the values changed from the default 0x8000. Should the register be subtracted or added to the calculation or what should be done with this values.


The converter are having 2 CAP DAC registers to programmable balance the measurement. What should be done whit these registers in my calculation. When I am running with the eval board I have tried compare the values without CAP DAC enabled to simplify the equation.


Best regards

Ole Hansen