Development of Signal Processing Circuits for a Resistive Humidity Sensor

Abstract

The resistance or impedance of a resistive humidity sensor element changes widely a factor of up-to10⁴. To process the wide range resistance/impedance change without reducing any SN ratio, this paper proposes newly developed stable signal processing circuits for a resistive humidity sensor. The humidity signal is converted in order of humidity(h)-resistance(R)-frequency(f)-time(t)-voltage(V). The functions corresponding to the conversions are a h-R, a R-f and a newly developed time-constant controlled f-V converter respectively. A h-R converter is a humidity sensor element itself. The time signal (t) is treated in the f-V converter. As the frequency and time signals are able to have a wide dynamic range, the large dynamic range of R is held through the R-f and f-V conversions. So, the reduction factor of SN ratio is minimized. The f-V converter plays roles of a voltage amplitude compressor and a linearizer. As the resultant characteristics, it has been shown that the developed circuits are accurate (±2%RH, at room temperature), stable for very long term (over 40, 000 hours) and stable against supply voltage fluctuations. In addition, controlling the function of the time-constant controlled f-V converter, an absolute humidity sensor has been obtained.