CMOS, Delta-Sigma pH-to-Digital Converter as New Integrated Device for Potentiometric Biosensors Applications

Abstract

The explosive growth in the healthcare and lifestyle application has fueled recent research efforts to increase both integration and adaptability in biosensor readout design. Potentiometric sensor is emerging as important sensing devices in the areas of biomedical applications since the pioneer work on pH sensor has been introduced (Bergveld, 1970). It has appreciable advantages over traditional glass-electrode sensors on the basis of its small size, robustness, simplicity in fabrication and low cost (Yin et al., 1999). Moreover, many research works have exploited different circuit architectures of readout circuits, which usually play the role in translating the values to voltage domain presentation, with the goal to obtain good sensitivity as well as linearity (Chan et al., 2007). However, the conventional readout circuitry of pH sensor comprises multiple chips and discrete components which are fabricated in different technologies. The goal of this research is to propose a novel pH sensor readout circuitry onto a single chip in standard CMOS technology. Such fully integrated solution results in pH sensor readout devices with lower cost and higher performance. New architectures and circuit techniques need to be explored in the design of fully integrated potentiometric sensor readout circuit. The wide dynamic range and low power are necessitated by the elimination of discrete high Q filters in traditional potentiometric sensor readout channel. In addition, readout circuits need to adapt to the different dynamic range, linearity, and signal bandwidth requirements of multiple potentiometric sensors. This programmability can be easily achieved by performing signal processing in the digital domain. Moreover, the reliability of signal processing in digital form can readily permit sensor systems to combine with other instruments. In moving the signal response of pH sensor to the digital domain, a high dynamic range, low power analog-to-digital converter (ADC) is needed to quantize a small desired signal. Delta-sigma modulators are uniquely suited to this application because the high pass shaped quantization noise falls into the same band as the environment noise (Kuo et al., 2001). This implies that a single programmable digital decimation filter following the deltasigma analog-to-digital modulator can attenuate both the quantization and environment noise.