An interface circuit for grounded capacitive sensors has been designed. The circuit is designed using a simple relaxation-oscillator topology and has been implemented in a 0.7-(mu)m CMOS process. The output signal is a period-modulated square-wave signal, which can directly be processed with a microcontroller. The period time of the output signal is proportional to the measured capacitance. By using the "three-signal" autocalibration methodology, obvious disadvantages from simple relaxation oscillators, such as offset, long-term drift, etc., have been overcome. The paper shows that even with a very simple circuit a rather good performance for the measurement of grounded capacitances can be achieved. An active-guard amplifier has been applied to reduce the influence of parasitic capacitances. For the active-guard amplifier, an opamp as well as an operational transconductance amplifier (OTA) have been used. Their performances, regarding to their high-frequency stability and accuracy for large capacitive loads at the working frequency, have been compared. For the measurement range of 100 pF up to 2 (mu)F the nonlinearity is found to be better than 0.5percent and the standard deviation less than 20 ppm for a measurement time of 100 ms.
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