This work presents a high-performance Capacitive sensing circuit for mobile handset``s touch panel applications. Among the technical challenges that must be solved, low-data rate and low resolution due to external noises in harsh environment are the most critical problems.
To obtain a noise rejection of over 50dB and scanning frequency of 200Hz for hundreds of channels, time-interleaved architecture with embedded moving average filtering is proposed. Conventional capacitive readout ICs solve this problem with complicated digital circuitry, causing high power consumption. Also, it is required to compensate the aging effect of ITO patterns and process, voltage, and temperature (PVT) variations, affecting the resolution of sensing circuitry.
With the proposed simple, yet effective readout circuit as well as time interleaved system architecture, noise rejection ratio, power consumption and data-rate can be greatly enhanced. To verify its noise rejection performance, a prototype sensor circuit is implemented using single-poly six-metal 0.18 CMOS technology. To test its operation, capacitive sensing circuit with time-interleaved method is simulated and fabricated by FPGA board simultaneously. Simulation result shows that noise rejection of 50dB at the frequency of 100kHz is attained, satisfying the required specifications.