For a current project I am trying to measure the capacitance of a strain gauge sensor. The sensor has a base capacitance of about 150 pF and changes approximately +/- 25 pF. A bit of googling led me to the circuit note CN-0129 http://www.analog.com/en/circuits-from-the-lab/CN0129/vc.htm which describes a solution to a very similar project using an AD7746 CDC. I build the circuit exactly as described in the circuit note and attached a fixed capacitance of 150 pF for testing purposes.
After setting the following registers:
0x07 CAP SET-UP -> 0x80
0x09 EXC SET-UP -> 0x2B
0x0B CAPDAC A -> 0x56
I perform a capacitance system offset calibration (Reg 0x0A -> 0x55) and start a single conversion (Reg 0x0A -> 0xA2).
Afterwards I am able to read the following data values from the corresponding registers:
0x7FEC5F converts to a decimal value of 8383583. My question is: How do I calculate the exact measured capacitance from this decimal value?
Looking forward to your help.