True Capacitance Conversion of the AD7142

In addition, since I don’t have the value of the capacitor connected to the CIN pin to ground, does that mean I can’t calculate the actual capacitance?

In addition, since I don’t have the value of the capacitor connected to the CIN pin to ground, does that mean I can’t calculate the actual capacitance?

Hi HuaWuque ,

Refering to this post, it is possible to get a rough estimation of the capacitance value based on the Offset value and the CDC result.

Following that logic and considering the values mentioned for the initial condition and the results after the capacitance changes, the estimation of the measured capacitance would equate to (0-0)*0.32 pF + (45968-31276)*0.244 fF = 3.58 pF. The AD7142 is sensing 7.63 pF in the initial condition and 11.22 pF after the capacitance changes. 

Regards,

Tiago

Does this mean I can only obtain the change in capacitance, rather than determine the absolute capacitance value?

From the datasheet, "The CDC is capable of sensing changes in the capacitance of the external sensors and uses this information to register a sensor activation.". So it is meant to primarily detect a change in capacitance.

It is possible to get a measurement of the initial condition where the capacitor is connected from the CIN pin to ground. However, this measurement will include more than just the capacitance value that is connected from the CIN pin to ground (there are different parasitics that are included in this measurement).

So, as mentioned in the previous post, it is possible to get a rough estimation of the absolute capacitance value based on the Offset value and CDC result.

If the goal is to get higher accuracy in the capacitance value of the capacitor connected between the CIN pin and ground, perhaps it is better to use a Digital Multimeter (DMM) or a LCR meter.

Regards,

Tiago