On-Chip Touch Sensor Readout Circuit Using Passive Sigma-Delta Modulator Capacitance-to-Digital Converter

This paper describes a capacitive type touch sensor readout circuit using passive sigma-delta modulator-based capacitance-to-digital converter. With the proposed approach, the panel condition (touched or untouched) can be effectively converted into the digital level using the panel capacitance as the summing element of the first-order passive sigma-delta modulator. In addition, the proposed touch sensor readout circuit does not require an analog-to-digital converter, since the digital format corresponding to the touch panel condition can be simply obtained by counting the number of modulator output pulses within the detection period. This along with the passive architecture leads to a compact and low-power on-chip touch sensor readout circuitry. Furthermore, due to the noise shaping property of the sigma-delta modulator, a comparable SNR and detection time with respect to other touch sensor readout circuits can be achieved. The proposed touch sensor readout circuit is implemented using CMOS 0.35-μm technology with core area of 0.1 mm². The operation of the sensor readout circuit is verified with a 10.4" projective capacitance type touch panel, where the measured SNR is 31.4 dB with power consumption of 65 μW.