Uncompromising optical integration

Photoplethysmography (PPG) is a common technique for measuring blood oxygen saturation (SPO2) levels. A light transmitter is used to emit light into the body, and a light receiver is used to measure the amount of light that is reflected or not absorbed. Based on the ratio of the two wavelengths, the amount of oxygenated hemoglobin can be measured. Similar techniques are also used to measure heart rate (in conjunction with optical techniques) or heart rate variability.

All of these systems require the use of one or more optical transmitters (which need to be controlled), and a photodetector to measure the amount of light received. This received signal ultimately needs to be amplified, conditioned, and digitized. This optical system may sound simple enough; however, without any knowledge of optics, it is easy to retrieve an optical signal that has nothing to do with the signal the user is looking for.

To help companies achieve their optical goals, we have introduced a new fully integrated optical module. This module has been tested and compared to a mature discrete optical system with impressive results. We will detail the results and methodology used in this testing.

PPG Measurement Theory and Introduction

With the increased focus on family health, wellness and prevention, a new market has formed around smart devices that track multiple vital sign parameters. First there were chest straps that used biopotential technology to monitor heart rate, but in the last 5 to 8 years the market has generally moved to optical systems that use photoplethysmography (PPG). One of the great advantages of this technology is that we can take a single point on the body to measure, whereas biopotential systems require a minimum of two electrodes to measure the heart. This is not very convenient for the user, and as a result, interest in optical heart rate monitoring (HRM) and heart rate variability (HRV) monitoring has increased dramatically.

Before designing such a system, several questions need to be answered. What is the end application? Where in the human body do you want to measure? How much time do you have to develop the system? Depending on the answers to these questions, the designer may take different design paths.

There are two different principles for measuring PPG. You can either pass light through a body part, such as a finger or earlobe, and measure the amount of light received or not absorbed on the opposite side; or you can send light on the same side of the body and measure the amount of light reflected. Measuring the amount of light that passes through the body gives about 40 dB to 60 dB more signal than a reflective system; however, with a reflective system you can freely choose where to place the sensor.

Since most users place more emphasis on sensor comfort than performance, the reflectance measurement method is more popular. Therefore, this article only introduces the reflectance measurement technology.

During a heartbeat, blood flow in the cardiac system changes, causing the received reflected light to scatter. The wavelength of the light source used to measure optical HRM/HRV depends not only on the measurement point on the body, but also on the relative perfusion level, as well as the temperature and color tone of the tissue. Typically, for wrist-worn devices, the artery is not located at the top of the wrist, and you need to detect the pulsating component from the veins and capillaries below the surface of the skin. In this case, green light gives the best results. In locations with sufficient blood flow, such as the upper arm, temple, or ear canal, it may be more effective to use red light or infrared light, which can penetrate deeper into the tissue and give a stronger received signal.

Figure 1. Typical block diagram of an optical HRM/HRV system

ADPD188 Is the game changing?

When weighing trade-offs, such as sensor location and LED wavelength, you need to choose the most appropriate optical solution. There are many choices for analog front ends, either discrete or fully integrated, and a large selection of photodetectors and LEDs. The key is to place the transmitter and receiver in a way that maximizes the amount of received signal per milliamp of transmitted current. This is called the current transfer ratio and is usually expressed in nA/mA. In optical systems, the modulation index is also important, which is the amount of AC signal relative to the optical DC bias. When increasing the distance between the light sensor and the LED, the modulation index increases. There is a sweet spot between the photodetector and the LED, which also depends on the LED wavelength. In a poorly designed mechanical system, the LED light can reach the light sensor directly without penetrating the human tissue. This causes a DC bias that adversely affects the modulation index. It manifests as optical crosstalk, also known as internal light pollution (ILP).

Figure 2 ADPD188GG optical subsystem

To minimize the design effort and reduce time to market, especially for companies with little optical knowledge, Analog Devices has built a fully integrated optical subsystem for reflectance measurements. The ADPD188GG contains all the components needed to perform optical measurements. Figure 2 shows a photo of this module.

The ADPD188GG is a completely new design of optical module with different dimensions compared to the previous generation modules. It has an almost square shape with dimensions of 3.98 mm x 5.0 mm and an overall thickness of 0.9 mm. The most modified part is the photodetector, which is rotated 90° compared to the previous generation. This position of the sensor relative to the LED allows for higher sensitivity. The light sensor itself is available in 0.4 mm2 and 0.8 mm2. This provides flexibility to increase the overall photodiode surface for higher sensitivity or to use a smaller detector to prevent the sensor from saturating. The photodiode is placed on top of the analog front end. ADI is using the standalone ADPD1080 AFE. It has 4 input channels, each designed around a transimpedance amplifier with selectable gain (25k, 50k, 100k, and 200k), an ambient light rejection block, and a 14-bit SAR ADC. The ambient light rejection is done in the analog domain and offers better performance than other solutions available on the market. Finally, the two green LEDs are controlled by an integrated current source capable of driving up to 370 mA with narrow 1 μs pulses to reduce the overall average current. The package design makes it difficult for the emitted LED light to reach the light sensor without penetrating human tissue. This prevents optical crosstalk and provides the user with the best modulation index, even if the sensor is placed under a glass or plastic window. This feature is very useful when designing optical reflective systems. For applications where emission measurements are more appropriate, the ADPD188GG can bypass the internal LEDs and work with externally connected LEDs.

Comparison with mature solutions

Before starting a new optical design, it is important to determine the target market and the specifications required for the final product. Generally speaking, optical systems with medical-grade performance will have higher specifications than those for devices used in the sports and healthcare markets.

The ADPD107 is an analog optical front end for discrete optical systems. It is considered a benchmark product among the optical front ends on the market and is widely used in many medical products due to its excellent performance. DataSenseLabs Ltd. has extensive experience with the ADPD107. However, since fully integrated optical modules have certain advantages in certain use cases, DataSenseLabs Ltd. started to study these modules and conduct a comparative analysis to compare the performance between the ADPD107 and the ADPD188GG integrated optical module. Next, we will describe the test setup, configuration, and test results in detail.

Test Setup and Data Collection

For optical comparison, raw PPG readings from the ADPD188GG and ADPD107 were recorded simultaneously over a 2-minute period. The ADPD188GG was set up using a standard evaluation board, while the ADPD107 was part of an optical system inside a wearable demonstration platform (EVAL-HCRWATCH). Both systems were controlled by the ADI user interface application wavetool software.

For testing purposes, the configuration settings were optimized to achieve the highest signal quality. The AFE configuration was retained, including keeping the LED pulses, timing, and transimpedance gain within a specific range to keep the power consumption of both systems the same for a fair comparison (see Table 1).

Table 1 Optical module comparison between ADPD188GG and model product ADPD107

Table 1 shows the ADPD188GG LED current, which is up to 2 times the LED current in the ADPD107 setup. The reason is that the photodiode surface of the integrated solution is smaller than that of the discrete solution, and this must be compensated. Using two LEDs powered by a 3 V supply increases the overall power consumption by 156 μW, which is almost negligible compared to the overall power consumption. We sampled the ADC at 100 Hz, which is very common in wearable systems. In addition, we measured at a sampling rate of 500 Hz, which is common in systems with clinical performance.