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Calculate capacitance with range extended AD7746

Question asked by martin.zens on Mar 10, 2013
Latest reply on Mar 13, 2013 by seanbrown

Hey everyone,

 

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:

 

0x01: 0x7F

0x02: 0xEC

0x03: 0x5F

 

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.

 

Martin

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