Use Analog Techniques To Measure Capacitance In Capacitive Sensors

Posted on Feb 6, 2014

Capacitive sensors are found in a wide range of equipment, from consumer electronics to industrial/process control. Touch buttons are increasingly found in lamps and dimmers. Motion detectors can detect miniscule changes in deflection. Hygrometers indicate humidity changes. Pressure sensors and accelerometers are moving toward capacitive sensing.

Use Analog Techniques To Measure Capacitance In Capacitive Sensors
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And, capacitive displacement sensors are even found in disk drives. These sensors produce an output: capacitance. Measuring this capacitance is an inherently analog problem that can be very tricky. Often, these sensors provide the most trouble for engineers to interface to microcontrollers. While some microcontrollers offer built-in simple routines for capacitive touch buttons, their measurements are relatively crude and designed for measuring a change in capacitance ”useful for detecting a finger placed on a touch sensor, but not for industrial and displacement sensing requiring absolute accuracy. As a specific example of a capacitive sensor, consider a capacitive relative humidity (RH) sensor. In this type of sensor, the dielectric material is designed to absorb water vapor from the external environment when exposed. The electrical capacitance increases as the dielectric absorbs water, since the dielectric coefficient increases with increased moisture ”a direct indication of the relative humidity. The net dielectric coefficient is also sensitive to temperature variations, so the humidity calculation involves both a measurement of capacitance and temperature. Humidity sensors exhibit a relatively small change in capacitance over their output range. A capacitance change of 40 to 50 pF over 0% to 100% RH with a 0% RH capacitance (C0RH) of 100 to 200 pF is not uncommon. A typical sensor offering 3% absolute accuracy (with 1%...

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