IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 49, NO. 1, JANUARY 2014 271 A 0.5 V 4 W CMOS Light-to-Digital Converter Based on a Nonuniform Quantizer for a Photoplethysmographic Heart-Rate Sensor Mohammad Alhawari, Nadya A. Albelooshi, and Michael H. Perrott, Senior Member, IEEE Abstract—A 0.5 V CMOS light-to-digital converter (LDC) based on a nonuniform quantizer and off-chip photodiode enables a pho- todiode bias current range spanning 4 nA to 3.5 A while consuming less than 4 W of power. Using an off-chip LED as a modulated light source, measurements with a photodiode cur- rent signal having modulation frequency of 1.2 Hz (72 beats per minute) and 0.5% peak-to-peak amplitude relative to per- formed at the low and high end of the range confirm over 30 dB of SNR for an integration bandwidth spanning 0.5 to 5 Hz. Using off-chip digital signal processing of the LDC output, instan- taneous period jitter (a proxy for instantaneous heart rate) is mea- sured to be less than 0.45% (rms) of the period, and the high sen- sitivity of the LDC allows detection of the heart-rate signal from a finger pressed against the off-chip photodiode using only ambient light. Key circuit components of the LDC include a wide range logarithmic digital-to-resistance converter (DRC) utilizing digital multibit modulation to achieve fine resolution and a nonuni- form quantizer based on a laddered inverter quantizer (LIQAF) which also acts as a low-noise front-end amplifier and filter. Index Terms—Analog-to-digital converter (ADC), Delta-Sigma, digital-to-resistance converter (DRC), heart-rate sensor, laddered inverter, light sensor, laddered inverter quantizer/amplifier/filter (LIQAF), low-noise amplifier, nonuniform quantizer, photode- tector, photoplethysmographic, resistor DAC, oximetry. I. INTRODUCTION H EART rate is a key vital sign to assess the health of an individual and is routinely monitored within clinical set- tings using pulse oximetry or EKG instruments. Unfortunately, for most individuals, the clinical setting is the only place where their heart rate is observed, which leads to long time durations in which this important vital sign is largely ignored. This issue has been addressed within the context of athletic activities with the advent of sport watches which measure heart rate using chest straps, contact electrodes, and photoplethysmographic methods as used in pulse oximetry devices. However, thus far there has Manuscript received April 17, 2013; revised August 24, 2013; accepted Au- gust 27, 2013. Date of publication October 21, 2013; date of current version December 20, 2013. This paper was approved by Guest Editor Michiel Pertijs. M. Alhawari was with the Masdar Institute of Science and Technology, Abu Dhabi, UAE. He is now with Khalifa University, Abu Dhabi, UAE. N. A. Albelooshi was with the Masdar Institute of Science and Technology, Abu Dhabi, UAE. She is now with Abu Dhabi Company for Onshore Oil Op- erations (ADCO), Abu Dhabi, UAE. M. H. Perrott was with the Masdar Institute of Science and Technology, Abu Dhabi, UAE. He is now with Silicon Laboratories, Nashua, NH 03062 USA. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSSC.2013.2284349 Fig. 1. Overview of heart-rate sensor based on measurement of light fluctua- tions due to pulsed blood flow through tissue such as a finger. been very limited impact of such heart-rate monitor devices on the general public due to issues such as their relatively high cost and inconvenient form factor. In this paper, we present a light-to-digital converter (LDC) for photoplethysmographic-based heart-rate monitoring which achieves high sensitivity and wide dynamic range while con- suming very low power and operating with a low supply voltage. As illustrated in Fig. 1, the LDC circuit, which utilizes an off-chip photodiode, acts as the front-end of an overall system which includes an off-chip LED, an LED driver op- erating from a 0.5-V supply voltage [1], and digital signal processing to estimate heart rate. The heart-rate signal is sensed as fluctuations in light intensity as the light passes through tissue such as a finger, with the photodiode generating a current proportional to the light intensity. The peak-to-peak amplitude in photodiode current is typically in the range of 0.5% to 2% of its bias current [2]. In general, it is highly desirable to achieve high sensitivity by being able to sense the signal at low values of , and to support a wide dynamic range (i.e., a large ratio of maximum to minimum ). Within the overall system in Fig. 1, the LDC is the key circuit for achieving high sensitivity and wide dynamic range. Low- power operation is critical in order to achieve a small form factor by minimizing the size of the energy storage device. A low supply voltage of 0.5 V potentially enables a simple en- ergy-harvesting approach consisting of a direct connection to a solar cell that operates at such voltage levels. Using an LED as the light source leads to the LED driver becoming the dom- inant power consumer in the system, but improved sensitivity in the LDC reduces the light intensity required from the LED (since lower photodetector current becomes acceptable) and, therefore, lowers the power of the LED driver. The preferred 0018-9200 © 2013 IEEE