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

Hello Martin,

The CAPDAC is used to offset the bulk capacitance from the dynamic capacitance, similar to common mode voltage. Therefore, the capacitance value will be the CAP Data + CAPDAC.

From the AD7746 datasheet:

The 0x000000 code represents negative full scale (–4.096 pF), the 0x800000 code represents zero scale (0 pF), and the 0xFFFFFF code represents positive full scale (+4.096 pF)

In your case, the range has been extended by your range extension factor (assuming 12.111 from CN0129); i.e. ±49.606pF.

CAP Data = [(0x7FEC5F + 0x800000)/0xFFFFFF * 49.606pF] - 49.606pF = -14.85fF

Therefore, the measured capacitance is -14.85fF + 150pF = 149.985pF.

150pF for the CAPDAC value is an assumption and you will have to calculate this from the formulae given in CN0129. This value will depend on CDACEFF, which is dependent on the gain calibration factor specific to your part.

Regards,

Sean