Analog Devices CN0346 Relative Humidity Measurement System
The circuit is ideal for applications where accurate, temperature controlled, noncontact humidity measurements are critical, such as HVAC, telecommunication cabinets, infant incubators, and other industrial or medical applications.
Moisture causes the dielectric constant of a capacitive sensor to change with respect to the relative humidity of the surrounding environment. For example, the Innovative Sensor Technology P14-W capacitive sensor used in this circuit is comprised of a top electrode, polyimide layer and a bottom electrode, and has a sensitivity of (0.25 pF/% RH) and a linearity of 1.5% RH.
The humidity sensor output is digitized by the AD7745, 24-bit, Σ-Δ capacitance-to-digital converter (CDC). A 2-wire, I2C-compatible interface allows access to the internal configuration registers as well as to the data conversions.
The AD8615 rail-to-rail amplifier with very low offset voltage (65 μV) and wide signal bandwidth (>20 MHz) acts as a unity-gain buffer and provides the appropriate drive signal to the sensor.