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. Author manuscript; available in PMC: 2019 Jul 1.
Published in final edited form as: Conf Proc IEEE Eng Med Biol Soc. 2018 Jul;2018:3821–3824. doi: 10.1109/EMBC.2018.8513356

Fig. 1.

Fig. 1.

Schematic of the PPG sensing circuit. The signal from the transimpedance amplifier is sent into two sets of cascaded active filters tuned for heart rate sensing and respiration monitoring respectively. The outputs of the amplifier stages are sensed by two channels of an on-board 10-bit analog-to-digital converter on a Bluetooth-enabled microcontroller. The signals are processed by the on-board microcontroller. R3, R14, and R15 (highlighted with a dashed box) are digitally controlled potentiometers enabling automatic gain control. Using these potentiometers, the microcontroller can modulate system gain in order to adjust for differences in the optical reflective properties of skin across difference subjects.