"Consumers need more accurate fitness and heart health data from smart watches, health wristbands and other battery powered mobile devices. In order to meet these expectations, developers have to deal with complex and expensive multi-component solutions. Although these solutions can eventually provide high accuracy, they pay the price of higher power consumption, larger occupied space and longer development time. We need simpler and more sophisticated solutions.
This article will introduce the way to build such a solution based on the highly integrated module of Maxim integrated. Firstly, this paper will briefly discuss the difficulties related to accurate cardiac function monitoring. Then, this article will show how developers can use this module to perform FDA certified heart rate monitoring during activities and electrocardiogram (ECG) measurements at rest.
Cardiac function measurement
Medical institutions usually rely on ECG (also known as EKG) to provide the most detailed data about heart health, and there is no invasive treatment in this process. ECG equipment can capture the waveforms generated by myocardial depolarization and repolarization during the cardiac cycle (Fig. 1). This process requires placing 10 electrodes in important positions around the human body. They are then combined to form 12 pairs or leads designed to align with the different waveform axes generated on the volume of cardiac tissue.
Figure 1: Although the electrocardiogram (ECG or EKG) provides more details, a simpler photoplethysmogram (PPG) can also provide useful information, such as the occurrence of ventricular premature contraction (PVC) shown here.
For example, an electrode placed in a patient's leg can be paired with another electrode to provide a lead that can collect details of the decline of ventricular depolarization waveform through heart tissue. Medical grade 12 lead ECG equipment uses this method to measure the waveform along the optimal axis associated with each phase of the cardiac cycle in combination with data from different electrode pairs.
On the contrary, ECG measurements performed by consumer fitness equipment usually use only one electrode pair, so the ECG of such equipment is called single lead ECG. Although single lead ECG may lack the detailed information required by cardiologists for diagnosis, it provides sufficient information about cardiac function to remind medical institutions of pathology that may need to use 12 lead ECG for accurate diagnosis.
In practice, because the measurement is easily damaged by the user's violent exercise, the single lead ECG measurement in fitness equipment may be particularly problematic. Any muscle movement will produce the corresponding electric wave shape of muscle fiber depolarization, which propagates through conductive tissue blocks. The movement of large muscle groups may produce biological potentials, which can easily block signals from deeper buried signal sources (such as myocardium). Therefore, accurate ECG measurement requires patients to remain stationary, whether lying in the hospital or during physical exercise.
In fact, attempts to perform single lead ECG measurements on users who are playing sports are likely to fail. For this reason, personal fitness equipment that provides heart rate data during sports usually relies on the photoplethysmography (PPG) method.
The most basic working mode of PPG is to use optical sensors to change the volume of blood vessels with each blood pulse and measure the relative difference of light reflection (or absorption). The earliest consumer heart rate monitors used this basic method, but today's fitness products usually use a more advanced PPG to measure peripheral oxygen saturation (SpO2) levels. This allows users to have a deeper understanding of their physiological response to exercise.
SpO2 measurement makes full use of the different absorption spectra of oxygenated blood and hypoxic blood when irradiated by red light and infrared LED. The emission spectrum is centered on two corresponding hemoglobin states (see "adding heart rate monitoring function to fitness equipment"). Although SpO2 focuses on the ratio between the two states, the basic heart rate measurement data can be extracted from the same data by measuring the peak to peak cycle time of the optical signal. Consumer pulse oximeters use this method to provide more reliable heart rate measurement without being affected by sports, individual differences of users or other factors.
Although the method based on optical PPG has been used in fitness equipment for many years, single lead ECG technology has recently sprung up in consumer products such as apple watches. Driven by competitive pressure, manufacturers of fitness wristbands, smart watches and other personal electronic devices increasingly need to provide PPG and single lead ECG functions in their products.
But for developers, implementing only one of these functions brings a lot of difficulties. The dual LED PPG design requires us to best drive red and infrared LEDs, capture reflected or absorbed light, synchronize results, and finally calculate heart rate and SpO2. Single lead ECG design requires extensive expertise in constructing analog signal paths and can process noise signals related to the measurement of any active biopotential phenomenon.
Perhaps the most basic problem is the power requirements, design size and number of parts necessary to realize the two types of designs and synchronize their results. This is a very difficult design work for most battery powered mobile products. In order to solve these problems, Maxim integrated max86150 biosensor module provides an almost direct solution, which can add PPG and ECG functions to any power limited design.
Biosensor module
The max86150 module is specially designed for portable systems. It combines the subsystems of dual LED PPG and ECG in a single device with an overall dimension of 3.3 x 6.6 x 1.3 mm. For optical measurement, the max86150 combines the complete input / output optical signal path with red LED, infrared LED and photodiode, which are located behind the glass cover built in the package (Fig. 2).
Figure 2: the max86150 PPG subsystem integrates all necessary components, including the signal path of LED output and photodiode input, so as to provide fitness measurement based on optical technology. In addition, red LED, infrared LED and photodiode devices are located behind the glass cover.
In the aspect of PPG signal path, the module integrates ambient light cancellation (ALC) circuit and 19 bit continuous time oversampling triangular integral（ ΔΣ) Analog to digital converter (ADC) and discrete-time filter (further noise elimination). Inside the ALC, the digital to analog converter (DAC) helps improve the input dynamic range by eliminating ambient light. To help developers balance power consumption and performance, the device's integrated LED driver can be programmed to provide a current from 0 Ma to 100 Ma with a current pulse width of 50 microseconds（ μ s) To 400 microseconds.
In order to further save power, developers can realize the proximity sensing function to keep the device in a low-power state between measurements. In this state, the device drives the infrared LED at the lowest power consumption level programmed by the developer. When the photodiode detects a useful signal (indicating that it is close to the user's finger or other skin surface), an interrupt will be generated, and the device returns to the normal working state to continue sampling.
In order to measure ECG, max86150 integrates a complete differential signal path. It only needs two dry electrodes and several other components to realize single lead ECG (Fig. 3). As with any small signal application, any number of noise sources in the environment will continue to challenge the measurement accuracy. In fitness applications, the related cardiac waveform is not only affected by the biological potential related to muscle movement and other physiological processes, but also subject to the interference from external RF sources, line frequency and electrical noise.
The max86150 ECG subsystem processes the signal noise in ECG measurement through the complex signal chain used to suppress the common mode signal.
Figure 3: in addition to the PPG subsystem, the max86150 module also includes a complete single lead ECG subsystem. Only a pair of dry electrodes and a small number of other components are required to provide ECG measurement data for the microcontroller.
The integrated ECG analog front end of the device includes chopper amplifier, filter and programmable gain amplifier (PGA). Its design purpose is to maximize the signal-to-noise ratio of cardiac waveform. After the signal chain, there is an 18 bit ΔΣ ADC, which can convert each sample and push all results to the shared 32 sample FIFO of the device, so there is no need for continuous data polling by the host microcontroller.
In order to further reduce power consumption and limit data access requirements, developers can adjust the sampling rate of ECG and PPG subsystems. The sampling rate range of ECG is from 3200 samples per second (SPS) to 200 SPS. For PPG, the minimum sampling rate can reach 10 SPS. However, developers can also use this device in advanced applications that require simultaneous ECG and PPG / SpO2 sampling and synchronization results. If developers need to apply this method and use different minimum sampling rates of the two subsystems, the device only uses the latest PPG samples to load FIFO and provide new PPG data in the next sampling cycle of the subsystem.
Design and Implementation
As mentioned above, since the max86150 integrates the core functions required for ECG and PPG measurement, we only need a pair of dry electrodes and a few other components for decoupling and buffering to form a complete max86150 hardware interface. Therefore, developers can combine the microcontroller with max86150 and a few external components to realize an advanced biopotential measurement system (Fig. 4). Developers can even use Maxim integrated max86150evsys evaluation system to quickly start studying ECG / PPG applications, thus skipping the hardware design steps.
Figure 4: developers can combine Maxim integrated max86150 with host microcontroller and a few other components to realize advanced cardiac function measurement in mobile fitness products.
The max86150evsys evaluation system can be used as a real-time application platform and reference design at the same time. It includes max86150 board, max32630fthr board and 500 MAH lithium polymer battery (Figure 5). Like the max86150, the max86150 board provides two stainless steel dry electrodes and other components mentioned above.
The max32630fthr board is connected through a pin seat. It provides a complete system supporting Bluetooth. It is built based on the max32630 microcontroller of Maxim integrated. It can also charge and power manage the attached battery pack.
Figure 5: Maxim integrated max86150evsys evaluation system provides max86150 board with dry electrode (left), max32630fthr development board based on max32630 and battery pack, so that developers can quickly start evaluating cardiac measurement methods.
This out of the box evaluation system has max32630fthr board and pre installed firmware of the basic max86150 application, so that developers can immediately start exploring ECG and PPG measurements. Developers only need to connect the board group to the windows PC system through Bluetooth and start the maxim integrated windows based graphical user interface evaluation suite software for the max86150evsys suite. The GUI package displays ECG and PPG data from the max86150, allowing developers to easily modify device settings to detect the impact on performance (Figure 6).
Figure 6: the company's relevant software applications are connected to maxim integrated's max86150evsys evaluation system, allowing developers to easily detect ECG and PPG measurements performed by max86150.
For developers preparing to build custom applications, Maxim integrated's max86150 driver package provides the source code of core device functions. Among various functions, the driver software package shows how to reduce the time required for the host processor to maintain its active state by using the FIFO of the device, so as to minimize power consumption. The core of this method is that the software relies on a pair of interrupt handlers to respond to device events, and then take action when data samples are available.
This interrupt driven method starts with an initialization routine. This method registers a device interrupt request (IRQ) handler max86xxx_ irq_ handler()。 When an interrupt event occurs, this handler checks the available device data and calls a separate FIFO handler (max86xxx) if necessary_ fifo_ irq_ Handler() and performs important housekeeping functions, including checking the device chip temperature and VDD level.
When called by the IRQ handler, the FIFO handler performs the required low-level operations to recombine 86150 sensor readings stored in the FIFO buffer. Here, the handler traverses the available samples in the FIFO buffer and recombines three bytes, which are used to store data from the 18 Bit ADC of ECG channel and the 19 bit ADC of PPG channel.
summary
In addition to PPG based heart rate measurement, single lead ECG function has gradually become a growing demand for smart watches, fitness wristbands and other mobile devices. However, facts have proved that it is very difficult to realize practical, accurate and low-power PPG and ECG functions in such wearable devices.
Maxim integrated 86150 biopotential sensor module with integrated PPG and ECG subsystems can provide effective solutions. The combination of 86150 module and MCU enables developers to quickly realize mobile health and fitness products that can provide detailed data of heart function.
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