  Photoplethysmography (PPG) is an attractive method to acquire vital signs
such as heart rate and blood oxygenation and is frequently used in clinical and
at-home settings. Continuous operation of health monitoring devices demands a
low power sensor that does not restrict the device battery life. Silicon
photodiodes (PD) and LEDs are commonly used as the interface devices in PPG
sensors; however, using of flexible organic devices can enhance the sensor
conformality and reduce the cost of fabrication. In most PPG sensors, most of
system power consumption is concentrated in powering LEDs, traditionally
consuming mWs. Using organic devices further increases this power demand since
these devices exhibit larger parasitic capacitances and typically need higher
drive voltages. This work presents a sensor IC for continuous SpO$_2$ and HR
monitoring that features an on-chip reconstruction-free sparse sampling
algorithm to reduce the overall system power consumption by $\sim$70\% while
maintaining the accuracy of the output information. The designed frontend is
compatible with a wide range of devices from silicon PDs to organic PDs with
parasitic capacitances up to 10 nF. Implemented in a 40 nm HV CMOS process, the
chip occupies 2.43 mm$^2$ and consumes 49.7 $\mu$W and 15.2 $\mu$W of power in
continuous and sparse sampling modes respectively. The performance of the
sensor IC has been verified \textit{in vivo} with both types of devices and the
results are compared against a clinical grade reference. Less than 1 bpm and
1\% mean absolute errors were achieved in both continuous and sparse modes of
operation.
