Rev. 0.2 / 09 /2015
4
AN1111-2
1 Driving circuitry for HCT01
HCT01 is a capacitive, thin-film sensor for relative humidity (RH) in a robust SMD enclosure. The
enclosure can accommodate an optional high precision temperature (T) sensor – a prerequisite for
precisely calculating dew point.
The enclosure provides optimal mechanical sensor protection and can be used with a standard reflow
process. The E+E proprietary coating protects the active surface of the RH sensor against dust, salt
and other corrosive or low impedance deposits.
The choice of HCT01 sensor type
HCT01 is available as pre-adjusted
( HCT01-02 and HCT01-03)
or unadjusted
HCT01-00
RH sensor,
each of these with or without integrated T sensor. The choice is based on accuracy requirements, on
the availability of humidity calibration benches, on the available or intended driving electronics and on
cost/performance considerations and it is illustrated in
Figure 1
. Additionally,
Figure 1
gives an
overview on the accuracy which can be achieved by various combinations of HCT01 circuitry.
•
Using an unadjusted RH sensor (
HCT01-00)
implies humidity calibration for the device built
with it. The accuracy achieved depends mainly on the overall uncertainty of the calibration
process.
•
A pre-adjusted RH sensor (
HCT01-02/03)
eliminates the need for humidity calibration. The
accuracy of the final device rests solely on the spread of the initial accurate electronics
employed.
The choice of driving circuitry
The driving circuitry for HCT01 can be realized with an oscillator as “capacity to frequency converter”
or with a microcontroller.
Features of the capacity to frequency converter:
Cost efficient solution – material costs approx. 0,25 € (using an existing oscillator)
Accuracy ±0,8 % rH
Extremely short measuring time possible to save power and prevent self heating
Frequency (period) output
Features of the microcontroller based circuitry:
Cost efficient solution – material costs approx. 0,05 € (using an existing microcontroller)
Electronic accuracy of ±1 % rH with reference adjustment
Customer specific, digital output depending on the microcontroller
