Engineer’s selection guide for different heart rate detection technologies

Heart rate data can provide very valuable information about personal physical conditions. By detecting irregularities such as arrhythmia, heart rate data can be used for applications ranging from personal fitness monitoring to vital signs for hospital-level patient monitoring. Today’s wireless and wearable technologies can not only collect complex data in a non-invasive manner, but also perform real-time analysis and Display, and can be stored for future use.

Heart rate data can provide very valuable information about personal physical conditions. By detecting irregularities such as arrhythmia, heart rate data can be used for applications ranging from personal fitness monitoring to vital signs for hospital-level patient monitoring. Today’s wireless and wearable technologies can not only collect complex data in a non-invasive manner, but also perform real-time analysis and display, and can be stored for future use.

This technology has opened up a new fitness and healthcare consumer market for wearable devices, and has promoted the application of exercise monitors such as fitness hand straps and chest straps. In the medical field, non-invasive heart rate monitoring can be used to identify reduced blood flow to the heart and assess risks and problems such as heart attack and thrombosis. Even in hospital wards, nurses can now use simple hand-held heart rate monitors and portable analyzers to regularly “observe” indicators such as arterial blood oxygen saturation, respiration rate, and hydration level.

The market demand for non-invasive and/or wearable devices is growing rapidly, and the complexity of data collection is also increasing, increasing technical challenges in data collection, signal conditioning, and processing. This is especially true for medical applications, where data measurement must be reliable, accurate, and safe.

There are two main methods for measuring heart rate. The first is to use optical technology to detect changes in light absorption or reflectance when blood flows through blood vessels close to the skin. Optical technology can also be used to estimate the oxygen saturation (pulse oximetry) in the blood, and the main technical challenges are low power consumption, ambient light suppression and ambient noise elimination.

The second method is biopotential measurement, which uses voltage sensing electrodes to detect the electrical activity generated by myocardial tissue and transmitted to the skin. This data is used to generate an electrocardiogram (ECG), which is widely used by medical experts to determine the health of the heart. Bioimpedance measurements can also be used to determine respiration rate and relative strength. The key technical challenges associated with this method include low-power operation, motion compensation, and noise cancellation and other interference.

Engineer’s selection guide for different heart rate detection technologies

Light pulse oximeter

Fortunately, for developers, many dedicated optical data acquisition systems for heart rate monitors are now available on the market. For example, Maxim Integrated’s MAX86140 has been specifically optimized to measure optical heart rate, oxygen saturation, and muscle oxygen saturation through monitors connected to the wrist, fingers, ears, and other locations.

Optical heart rate monitoring usually requires a single light source, but a pulse oximeter requires two. In order to obtain extremely high accuracy and increase the possible measurement range, multiple light sources are usually used. The MAX86140 and MAX86141 provide single and dual optical channels, respectively.

On the transmitter side, three programmable high-current LED drivers can be configured to drive up to 6 LEDs. Since the two devices work in a master-slave mode, the LED driver can drive up to 12 LEDs. A key feature of these devices is their powerful proprietary ambient light cancellation (ALC) circuit, which is particularly suitable for ensuring accuracy under bright conditions. In addition, the system can also cope with rapid changes in light levels.

Other main functions include a low-noise signal conditioning analog front end (AFE) with a 19-bit Σ-Δ ADC, a voltage reference, and a temperature sensor. The ADC output data rate can be programmed from 8 to 8192 samples, and these devices require very little external hardware. The 128-word FIFO can provide on-chip storage for digital output data and can be connected to a microcontroller.

Engineer’s selection guide for different heart rate detection technologies

Both devices use 1.8V power supply, have independent 3.1V to 5V LED driver power supply, can provide a variety of energy-saving settings. They have flexible timing and shutdown configurations, as well as the control of each functional block, and can achieve optimized measurements at the lowest power level. At lower sampling rates lower than 128sps, dynamic power-off mode can be used. When the sensor is not in contact with the skin, the proximity mode can help reduce energy consumption.

The optical controller can be configured for various measurements. Pulses can be applied to one, two or three LED drivers in sequence, and multiple wavelengths can be measured according to the requirements of the pulse oximeter. When the LED driver is pulsed at the same time, the heart rate can be measured on the wrist-worn device. The LED drive level can be adjusted to compensate for increased noise levels such as interference with environmental signals.

Biopotential ECG measurement

The electrocardiogram can measure the heart rate, and can provide detailed information about specific signals, providing more details for professionals to perform cardiac examinations. It also allows for more reliable heart rate monitoring in fitness applications, especially when using a chest strap. Compared with optical sensors with the same level of accuracy, biopotential measurement usually requires much lower power. However, the processing of ECG signals also consumes battery power quickly. In addition, ECG readings are susceptible to movement and other sources of interference. Therefore, in fitness applications, motion compensation is particularly important, and exercise itself may also be an important source of noise.

Likewise, specific dedicated devices can be used for such applications. Maxim Integrated’s MAX30003 is a complete biopotential analog front end (AFE) solution for wearable applications, see Figure 3. This single-channel device has clinical grade ECG AFE and high-resolution ADC, which can provide 15.5 bits of effective resolution and 5μV peak-to-peak noise. In addition, it also has ESD protection, EMI filtering, internal lead biasing, DC lead disconnection detection and soft power-up sequencing. The high input impedance ensures minimum attenuation of the signal at the input during dry start.

By ensuring that the AFE has the highest possible common mode rejection ratio (CMRR), motion compensation can be achieved and the interference of motion artifacts can be eliminated. The MAX30003 has a CMRR characteristic of up to 115dB, and an optional lead bias resistor helps to increase the CMRR and can increase the input impedance. Various low-pass and high-pass filter options can be used to limit the bandwidth, which is very important for attenuating noise from static and high-frequency signals. For fitness applications, the single-power high-pass corner frequency (high-pass corner frequency) should be set to 5Hz, while for clinical applications, it can usually be reduced to 0.5Hz or 0.05Hz. For sports, the common-mode low-pass corner frequency can be set to 34Hz, which is the best level to limit clothing noise during dry start.

MAX30003 has ultra-low power consumption of 85μW under 1.1V supply voltage, which can extend battery life. The lead conduction detection function can run during standby/deep sleep mode (70nA), and the 32-word FIFO can contain up to 32 ECG data conversion results, thus saving the processing resources of the main microcontroller because it can stay asleep for a long time , Which can reduce power consumption. Similarly, the MCU can be programmed not to process potentially invalid data, and the built-in algorithm for the RR peak interval can further save power, so the MCU only consumes about 1μA of power, and if all these functions are implemented by the main MCU, it can consume 50 more Up to 100 times the power consumption.

Very usefully, Maxim provides a design platform that can be used to develop wearable health or fitness products based on this device. The MAXREFDES100 health sensor platform includes all hardware building blocks for a single PCB, and can provide an ARM mbed-based programming board as a hardware development kit (HDK). In addition to MAX30003, MAX30101 also adds pulse oximetry through optical components (including LEDs and photodetectors) and low-noise Electronic components with ambient light suppression. There is also the MAX30205 clinical-grade temperature sensor. The recommended power module is MAX14750, which can provide multiple outputs for MCU, AFE and digital interfaces.

The Links:   SEMIX703GB126V1 B141XG08-V3