Differential Coded Multiple Signaling Method with Fully Differential Receiver for Mutual Capacitive Fingerprint TSP

Abstract

This paper presents a fingerprint mutual capacitive touchscreen panel (TSP) readout IC, which uses a differential coded multiple signaling (DCMS) method. A readout IC with high SNR and fast frame rate is required for fingerprint recognition. However, achieving high SNR is challenging owing to the limited capacitance difference originating from the small depth variations between the ridges and valleys of the fingerprint. In addition, scanning the entire fingerprint TSP with multiple electrodes is time-consuming. A fully differential receiver with DCMS is proposed to detect the low capacitance difference in a fingerprint TSP. The internal noise is minimized by the low-noise amplifier, and external noise is eliminated by a lock-in sensing architecture. In addition, DCMS reduces the offset and enhances the SNR while achieving faster frame rate in multiple channels. The proposed architecture can detect capacitance of 50 aF, which is the capacitance difference resulting from the ridges and valleys of a finger under a 0.3-mm-thick (T) cover glass. The readout IC achieves 15.1-dB peak-to-peak SNR and 23-Hz frame rate with a transparent mutual capacitive fingerprint TSP under 0.3T glass. The power consumption is below 21 mW at 3.3-V supply voltage. The IC was fabricated using a 0.18- $ {\mu }\text{m}$ standard CMOS process.