As shown in Figure 5-6, this is the ADC value of photodiode signal after filtered and amplify. You can define the highest point in a period as onset and the lowest point as peak.

You need to calculate AC and DC values for the RED/IR signal, and then use the below formula to calculate the blood oxygen level.

• For AC value, find the onset and peak point.
• For the value of A, B, C in this formula, it is a medical calibration constant that requires a lot of test data to calculate.

For the heart rate, it can be calculated by the time of two onset/peak points.