PPG Sensor: Advantages and Disadvantages

This page covers the advantages and disadvantages of a PPG (Photoplethysmography) sensor. It mentions the benefits or advantages and the drawbacks or disadvantages of PPG sensors.

What is a PPG (Photoplethysmography) Sensor?

PPG is the short form of photoplethysmogram. It converts an optical signal to an electrical signal to derive various health-related parameters.

As we know, most of the changes in blood flow occur in arteries and not in the veins. A PPG sensor detects changes in blood flow volume using the reflected optical signal from the tissue or its transmission through it.

It works on the principle that changes in light intensity are proportional to the small variation in blood perfusion. There are two modes of PPG as follows:

• Transmissive mode: In this mode, light is transmitted through the medium using an LED and detected by a photodiode attached opposite the LED. The measurement time is limited. It is placed at fingertips, toes, or earlobes.

• Reflective mode: In this mode, the photodiode detects light that is back-scattered or reflected from the tissues, bone, and blood vessels. It eliminates problems encountered in mounting the PPG sensor. It can be placed at the forehead or cheek, and a variety of measurement sites can be used.

The PPG waveform is represented by two components of current: direct current (DC) and alternating current (AC). DC detects signals from tissues, whereas AC detects signals from changes in blood volume.

Benefits or Advantages of PPG Sensor

Following are the benefits or advantages of PPG sensors:

• It is simple, reliable, and inexpensive.
• It can easily be integrated into wearable healthcare devices for various health-related measurements such as pulse rate (or heart rate), blood flow, Heart Rate Variability (HRV), etc.
• The use of PPG-based wearable devices does not require special training or guidance.

Drawbacks or Disadvantages of PPG Sensor

Following are the drawbacks or disadvantages of PPG sensors:

• It cannot measure blood pressure.
• It relies on the ECG signal as a reference for HR measurement.
• PPG sensors require a longer settling time than ECG sensors.
• It consumes more power (~30mW) than an ECG sensor (~2.5 mW) during operation.
• It requires an external crystal oscillator for accurate timing control.
• It occupies a large PCB footprint.
• Peak interval accuracy is limited by the usable sampling rate due to the high power consumption of LEDs.
• HRV (Heart Rate Variability) requires longer periods of measurement (usually > 5 minutes).
• It is only suitable for average HR measurement. ECG sensors measure heart rate accurately on a beat-by-beat basis.