|2800μm x 2600μm
A capacitive humidity sensor is easily integrated on silicon substrate. The sensor consists od a metal-metal capacitor with a dielectric that changes its value according to the ambient humidity. The structures of these sensors have been developed at IQE.
The intention o fthis chip design was to integrate the sensor capacitor together with theinterface of a microprocessor on one chip. A first part of the circuit converts the capacitance to an electrical signal. The second part is the analog to digital conversion of this signal. The third part is the actual interface to the processor.
The approach chosen in this design is o use a sigma-delta AD converter. The sensor is part of the sigma-delta AD converter itself. The output of the second order sigma-delta converter is ifltered digitally using a ROM as look-up table. The 8-bit output can be read serially by a microprocessor through a Serial Periphery Interface (SPI).
The sensor itself is a metal-metal interdigit capacitor. Two such structures are placed on the left side of the chip. The upper capacitor is used as a reference and the lower as the actual sensor. This cancels out the absolute value of capacitors from measurement.
This chip has won the "Elektronik Design Award" in the category, "Best ASIC Design" of the German "Elektronik" magazine in year 1994, the first such award that the magazine awarded.