(A) low noise high speed mutual capacitive touch screen panel read-out IC using code-division multiple sensing technique

Abstract

A capacitive touch screen technology introduces new concept in user interface such as the multi touch, pinch zoom-in/out gestures, thus expanding smart phone market bigger and wider. However, the capacitive touch screen technology still is suffering from the performance degradation like low frame scan rate, poor accuracy, etc. One of the key performance factors is the immunity to the external noise which is intruding randomly into the touch screen system. HUM, display noise, SMPS can be such noise sources. Main electricity power gives HUM, one of the huge noise, which has 50 or 60Hz. Display noise is emitted when LCD or OLED is driven by the internal timing controller which generates the driving signal with tens of KHz range. The touch performance of On-Cell or In-Cell touch display is affected much seriously by this kind of noise because the distance between the display pixel layer and the capacitive touch screen panel is getting closer. SMPS is another noise source which ranges up to 300KHz. The charger for smart phone, USB port in the computer, tri-phosphor fluorescent light bulb can be examples of SMPS. A time-interleaving sensing method is adopted in conventional noise reduction techniques. The access time to measure Cm of each node is stretched longer to reduce the noise. As more sensing nodes with bigger display size, however, the time-interleaving sensing method cannot avoid the degradation of the frame scan rate. This work proposes code-division multiple sensing (CDMS) method to solve all of the above problems. By Parseval’s theorem, to get higher SNR for the touch screen system with the wide band noise distribution, the power spectral density of the signal should be higher, which necessitates longer access time inevitably. Thus there is trade-off between SNR and access time in the time-interleaving sensing method. This can be overcome, however by adopting CDMA technique, a well-established method in RF communication system. The proposed CDMS method can emit higher excitation signal power than the time-interleaving method does, because it allows driving multiple TX electrodes simultaneously in a given time. By this simultaneous multiple stimulus and sensing characteristic of CDMA, a high frame scan rate can be achieved with low noise. This work also proposes a double sampling switched-capacitor (DS-SC). This circuit is consisted of 2 integrators. One is conventional non-inverting switched-capacitor and the other is inverting switched-capacitor. By its characteristics, the circuit can remove external low frequency noise (such as HUM) by its double-sampling characteristics and also can remove internal low frequency noise (such as input-referred flicker noise of OPAMP) by additional chopping operation. The circuit is free from offset and mismatch of 2 integrators and higher yield rate can be achieved. The prototype for CDMS touch sensor read-out IC is fabricated in a 0.18-um EEPROM/MIM CMOS technology. By adopting newly proposed CDMS DS-SC for 30 $\times$ 24 touch screen panel, this work obtained 55dB SNR for finger, 35dB SNR for ø1mm round-tip metal pillar with 240Hz frame scan rate. This performance is 2 times faster and 10 times better SNR than the time-interleaving sensing method.