Monitoring health parameters via wearables is rapidly gaining popularity. As part of wearable technology, optical Photoplethysmogram (PPG) has attracted considerable consumer interest. While PPG is traditionally used on the finger or on the wrist, thanks to advancements in miniaturization, improved power consumption and dynamic range, it uses more and more at other locations like on the chest and in the ear. Thus, rather than monitoring heart rate and blood saturation level, other biomedical signals such as the respiration rate and the glucose level can be extracted. These parameters might be helpful to monitor the health status or diagnose covid patients during the new normal. This paper reviews two state-of-the-art PPG sensor interfaces. The future development trends are provided as well.