PPG AFEs measure the PPG signal via the intensity of the light reflected into photodiodes, and as the blood volume in the arteries increases, following the contraction during the systolic phase, so does the absorption of the light being shined into the tissue. Since there is more absorption, the photodiodes will receive less reflected light and the output of the AFE will decrease. During the diastolic phase, while there is less blood in the arteries there will be less absorption and therefore more reflected light to the photodiodes. Hence the output of the PPG AFEs is just an inversion of the PPG plot that you find in medical texts.