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Analog Devices CN0346

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.

Benefits

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.

Parts used

  • AD7745 24-bit, 1 Channel Capacitance to Digital Converter: The architecture features inherent high resolution (24-bit no missing codes, up to 21-bit effective resolution), high linearity (±0.01%), and high accuracy (±4 fF factory calibrated).
  • AD8615 Precision 20 MHz CMOS Single RRIO Operational Amplifier featuring very low offset voltage, wide signal bandwidth, and low input voltage and current noise.

Block Diagram

RELATED LINKS
  • CN0346 Relative Humidity Measurement System
  • AD7745 24-bit, 1 Channel Capacitance to Digital Converter 
  • AD8615 Precision 20 MHz CMOS Single RRIO Operational Amplifier