First of all, no motion.
Current PPG based SpO2 solution cannot support testing SpO2 in motion as the IR and Red channel PPG signals are too weak and unstable in motion.
Secondly, PI should be reach the threshold.
Algorithm calcuates SpO2 from IR and red channel PI ratio. If PI is too low, the ratio will be inaccurate. Thus the SpO2 accuracy cannot be garrenteed.
Lastly, the singal quality should be OK.
If the PPG signal is too noisy, the algorithm will consider the PPG signals as unqualified signals and output nothing waiting for good PPG signals.