Figure 2.

(A) Shows the circuit of the PPG analog front end. It contained an LED, adjustable R-C filter, TIA to enhance the signal from the PD and convert the signal to digital data by ADC. (B) The optical principle of the PPG sensor that reflecting the blood volume variation from PPG signal. The PPG sensor with the optical shielding design for decreasing noise and disturbance acquired reflective PPG signal including AC and DC components. AC was defined as the pulsatile component of the artery and DC was consisted of non-pulsatile components of artery blood, venous blood, and other tissues.