A combined low power SAR capacitance-to-digital analog-to-digital converter for multisensory system
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A combined low power SAR capacitance-to-digital analog-todigital converter for multisensory system Hui Jiang • Ziqiang Wang • Chun Zhang Hanjun Jiang • Zhihua Wang
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Received: 18 November 2012 / Revised: 7 February 2013 / Accepted: 14 February 2013 / Published online: 28 February 2013 Ó Springer Science+Business Media New York 2013
Abstract A combined successive approximation (SAR) capacitance-to-digital converter (CDC)/analog-to-digital converter (ADC) for biomedical multisensory system is presented in this paper. The two converters have same circuit blocks and can be exchanged by four switches. Capacitance or voltage from different sensing elements can be measured and converted to digital output directly. This single chip takes place of separated CDC and ADC so that the power consumption of the multisensory system is reduced. The asynchronous SAR circuit has low power and small area. A dynamic comparator with zero-static power is adopted. Switches are carefully designed to reduce the nonidealities of the converter. Several techniques, such as bootstrapped switches, bottom-plate sampling, dummy switches are used to improve the performance of the circuit. The CDC/ADC is fabricated in 0.18 lm CMOS process. Measurement results show that the ENOB of this 11 bits converter is 10.15 bits and its FOM is 45 fJ/conversion-step under 200 kHz sampling. The power consumption is 9.4 lW with 1.4 V power supply voltage and the core area is 0.1764 mm2. Keywords ADC CDC SAR Asynchronous Low power
1 Introduction In a biomedical system, different sensors are used to monitor variable physical, chemical and biological signals. H. Jiang Z. Wang (&) C. Zhang H. Jiang Z. Wang Institute of Microelectronics, Tsinghua University, Beijing, China e-mail: [email protected]
These signals are usually converted to electric quantities such as voltage, current, capacitance or inductance, by sensing elements. Then they are further transferred to digital signals so as to be processed in digital circuits. However different electric quantities usually need different conversion circuits. For example, ADC is used to convert analog voltage to digital and CDC is designed to convert capacitance to digital output. In some case, the two convertors can be combined into a single one in a multisensory system and the power consumption of converters is largely reduced [1]. The architecture of a wireless health monitoring system is drawn in Fig. 1. It is comprised of two parts. One is the senor detection and transmission system which is placed in human body. The other is the remote end control system that is located outside. The internal system detects kinds of physiological parameters by multi-sensor heads in a timedomain duplexing (TDD) fashion. Different signals, such as temperature, PH-value and pressure are selected by the sensor signal selector and then the selected one is processed by the sensor signal converter. The temperature sensor and the PH-value sensor output voltage signals while the pressure sensor provi
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