handbook of modern sensors

10.6 Capacitive Sensors 349
(A)
(B)
Fig. 10.9. Temperature characteristics of a piezoresistive pressure sensor: (A) transfer function
at three different temperatures; (B) full-scale errors for three values of compensating resistors.
diaphragms and bellows. In either case, silicone oil, such as Dow Corning DS200,
can be used to fill the air cavity so that system frequency response is not degraded.
All silicon-based sensors are characterized by temperature dependence. The temperature
coefficient of sensitivity b T as defined by Eq. (10.15) is usually negative,
and for the accurate pressure sensing, it must be compensated for. Typical methods of
temperature compensation of bridge circuits are covered in Section 5.7.3 of Chapter
5. Without the compensation, the sensor’s output voltage may look like the one shown
in Fig. 10.9A for three different temperatures.
In many applications, a simple yet efficient temperature compensation can be
accomplished by adding to the sensor either a series or parallel temperature stable
resistor. By selecting an appropriate value of the resistor, the sensor’s output can be
tailored to the desirable operating range (Fig. 10.9B). Whenever a better temperature
correction over a broad range is required, more complex compensation circuits with
temperature detectors can be employed. A viable alternative is a software compensation
where the temperature of the pressure transducer is measured by an imbedded
temperature sensor. Both data from the pressure and temperature sensors are relayed
to the processing circuit where numerical compensation is digitally performed.
10.6 Capacitive Sensors
A silicon diaphragm can be used with another pressure-to-electric output conversion
process: in a capacitive sensor. Here, the diaphragm displacement modulates capacitance
with respect to the reference plate (backplate). This conversion is especially
effective for the low-pressure sensors. An entire sensor can be fabricated from a
solid piece of silicon, thus maximizing its operational stability. The diaphragm can
be designed to produce up to 25% capacitance change over the full range which