Capacitive sensing

A sensor has a transmitter and receiver block. The transmitter supplies the load (e.g.: a capacitor consisting of a conducting plate and human hand) with an alternating voltage signal. The sensor captures the reactive load current.

The current response is converted into a voltage and then I/Q-demodulated. After filtering and conversion, the outcome is the load impedance  (capacitive and resistive information). This means that the closer the human hand gets to the conduction plate, the more capacity is measured.

This allows the detection, of not only ideal situations, but also for cases where environmental changes (e.g. a wet hand, gloves, …) have added a parallel resistive component along with a change in the capacitance.

Operating Principle

• Transmitter block forces a sine wave voltage across the load
• Receiver block detects the current response of the load
• Current response is converted to a voltage and demodulated into in- phase (I) and quadrature (Q) components
• I/Q-signals were filtered and offset compensated
• single 10bit ADC is shared between I- and Q-path and digitizes the measurements automatically
• 14bit output data is available via SPI

Facts

• Measurement of the sensor impedance is realized with a transceiver architecture operating between 45 – 125kHz
• Up to 16 samples can be accumulated inside the chip
• Measuring Z and derive C helps to cancel out external influences (leather gloves, moisture etc.)