A current digital to analog network appliance converter (IDAC) charges the variable capacitor

This voltage is measured using anCOMe Module and the digital output of the ADC is used by the microcontroller. The microcontroller is programmed with an algorithm that uses the known relationship between the variation in resistance of the sensor and the physical parameter being measured to calculate the physical parameter.

Second, in passing a known current, a known current is passed through a transducer whose resistance varies with the changes in the parameter being measured. As perembedded system law: The voltage across the transducer will vary depending on the parameter being measured. This COMe Module voltage can be measured using an ADC. The ADC reading can then be used to calculate this parameter.

Capacitance
There are sensors that are capacitive in nature. In these network appliance sensors, the effective capacitance of the sensor is measured to find out the value of parameter of interest. There are various ways in which a capacitor can be measured.

A current digital to analog network appliance converter (IDAC) charges the variable capacitor and, when the voltage at the capacitor crosses the voltage at the inverting terminal, the switch is turned on. This discharges the capacitor and the cycle repeats. During every charging and discharging cycle, the value of the counter registers are COMe Module captured and processed in the firmware to calculate the capacitance value by applying the relevant mathematical equations.

refer to:
http://embedded-computing.com/articles/analog-front-ends-for-gas-sensors/