A TDC based ISFET readout for large-scale chemical sensing systems

This paper presents a novel ISFET based readout system by minimizing the analogue instrumentation. Quantization of sensed pH is realized by means of a novel ISFET pH-to-Time sensor integrated with a time to digital converter (TDC). By using this scheme, interference usually introduced in the analogue front end can be minimized, and time based signal processing can be used to simplify the sensor interface. This system is implemented in a typical 0.18um standard CMOS technology. It includes a 3×3 sensor array of pH-to-Time sensors, and a TDC dedicated to this sensing scheme. ISFET pixels are fabricated with chemical sensing area of 10μm × 10μm. The TDC is delay line based flash converter with resolution of 781.25ps, when a reference clock 160MHz is used for both sensors and TDC. The occupied silicon area is 0.036mm², and the total power consumption of this time-to-digital converter including the delay-locked loop is equal to 230μW.