An 102dB DR Current Mode Readout Frontend with Level-Crossing Based Ambient Light Monitoring and DC Cancellation

Photoplethysmography (PPG) has been widely used in consumer and medical devices for assessing heart rate blood oxygen and blood pressure levels, together with electrocardiography (ECG). One of the major challenges is recording PPG with fast changing ambient light and motion artefacts that generating a changing baseline of the signal. Light to digital converters (LDCs) proposed in recent years show excellent resolution and power efficiency, however it may get saturated during fast ambient and motion events. Existing chopping ambient light removal and hysteresis DC removal techniques cannot track fast-changing components. Therefore, this paper proposes a current domain level crossing-based technique to assist a dual slope LDC, including a dynamic biased low-power continuous time comparator. The PPG readout frontend is implemented in a 55nm standard CMOS process and consumes 12.4μW-56.6μW depending on the ambient light intensity. It achieves a dynamic range of 102dB and an SNDR of 75dB at AC signal path.