Detecting Your Heart Rate

Smart watches and wearable fitness trackers have become must-have replacements for the humble watch. Whatever our exercise regime, whether daily or weekly, our wearable device is capable of tracking our heart rate continuously, from resting to an intensive workout. Monitoring the heart rate also allows other data points to be derived, such as the calories expended and the duration of the different sleep phases.

There are two methods of detecting the human heart rate: electrocardiogram (ECG) biopotential, and optical detection.

The one most used in clinical applications is the ECG, where detection of the electrical signals generated within the human heart yields the most reliable and informative picture of its operation. Sensing the differential biopotential signals and amplifying them yields a number of different signal patterns for clinicians to investigate. These are termed the QRS complex – see Figure 1. The peak-to-peak ‘R’ signal interval is used to calculate the heart rate of the wearer. Electrodes are attached to the chest; these are predominantly the wet type for clinical use, where a conductive gel is applied. For sports and fitness use the electrodes tend to be dry – no gels or liquid are used – and are integrated within a fabric chest strap along with a module containing sensor electronics, a method of short-range wireless communication such as Bluetooth, and a coin-cell battery.

From the electronic design perspective, maintaining a reliable electrode connection is paramount so that signals of sufficient amplitude and quality are detected. Unfortunately, while dry electrodes provide a more convenient method of attachment, they can present a high impedance circuit during the early stages of exercise, resulting in signal attenuation. This lasts until the body has exercised sufficiently to sweat. As a consequence, the analogue front-end circuitry of the heart rate detection device needs to offer a similar high impedance input so that the maximum input signal is achieved. Failing to do this can result in heart rate detection errors, commonly referred to as a ‘dry start’. An example IC is the MAX30003, a complete biopotential ECG analogue front-end IC from Maxim Integrated – see the functional block diagram in Figure 2.

Click image to enlarge

Figure 2: MAX30003 functional block diagram (source: Maxim Integrated)

The MAX30003 is an ultra-power device and can be used for both clinical and sports performance applications. It features a single biopotential detection channel that uses two electrodes. The device has a comprehensive set of analogue front-end features such as ESD/EMI protection, a lead off/on check, and several lead bias, polarity and calibration options. Since there is an electrical connection to the body, ESD protection from overdriving the inputs, for example during defibrillation, is essential. Likewise, being able to detect if the leads are connected to the body allows gating of the heart rate display when not connected – see Figure 3.

Click image to enlarge

Figure 3: Analogue front-end input functional blocks (source: Maxim Integrated)

Another factor in obtaining reliable heart rate detection, particularly for those designs intended for fitness and sports activities, is motion rejection. Motion artifacts manifest themselves on the ‘R’ signal as a result of body movement during exercise, the movement of clothes against the body, and the movement of the chest strap’s electrodes against the body. Reducing and eliminating such interference is down to a series of low- and high-pass filters, and takes place in the analogue signal path prior to the high-resolution analog-to-digital conversion process, as seen in Figure 2.

The other method of heart rate measurement, and the one used for most popular wrist-mounted fitness trackers such as those from Fitbit, is an optical approach. Unlike their ECG-based counterparts, wrist-mounted devices are more comfortable to wear all day and night, and this allows the designer to incorporate other useful functions such as timers, alarms and a GNSS receiver as a way of product differentiation. There are more opportunities to tailor the product’s aesthetic design to serve different applications and use cases, too. One or more LEDs flash light, typically green at a wavelength of approximately 560 nm, through the skin and the reflected signal is detected by a photodiode, the light having been amplitude modulated by the wearer’s heart rate. This technique, termed photoplethysmography (PPG), is used to detect changes in blood volume flow – exhibited as heart rate pulses – in the lower blood dermis skin layer. The greater the blood volume change, the more reflected light is received – see Figure 4.

Click image to enlarge

Figure 4: PPG using an LED and a photodiode (source: Maxim Integrated)

Unfortunately, unlike heart rate monitors worn in clinical situations, where the patient’s movement is predictable, detecting the heart rate of a wearer engaged in a sports activity is more challenging. Motion artifacts – invalid signals that can alter the heart rate readings – are introduced in a number of different ways that will be explained shortly. Also, the ambient light condition has the potential to be erroneously detected by the photodiode, further impacting the heart rate detection reliability.

When selecting an IC for a wrist-mounted application it is recommended to carefully select a device that includes functionality to counter motion artifacts and reject ambient light. An example is theMaxim Integrated MAX8614x optical pulse oximeter and heart rate sensor series. Comprising two devices, the MAX86140 – a single-optical-channel device, and the MAX86141 – a dual-channel IC, they are complemented by the MAX86140EVSYS evaluation platform – see Figure 5. These ultra-low-power ICs bring consumption down to 10 µA during low-power operation and feature three programmable high-current LED drivers. On the receive side a low-noise analogue front end includes a 19-bit ADC and ambient light rejection algorithms.

Click image to enlarge

Figure 5: Maxim Integrated MAX86140 and MAX86141 functional block diagram

There are two primary sources of motion artifacts that can interfere with the desired PPG signal; movement of the wearable device on the wrist and the motion of the blood resulting from body movement. These need to be isolated from the PPG signal, and one simple approach is to use an accelerometer within the wearable device to sense the wearer’s motion. Calculation of the motion frequency allows suppression of the motion artifacts in the PPG signal. If this motion is regular, for example when cycling on a flat road, the frequencies to suppress can be relatively constant. However, with irregular motion it is harder to detect the corresponding frequencies. For these, the use of multiple optical signal paths together with a suitable algorithm is a viable approach. A single LED and two photodiodes provide the most power-efficient approach while introducing optical signal diversity. The Maxim Integrated’s MAX86141 device takes this approach to provide motion compensation using an accelerometer and a dual redundant optical signal path. Each optical path has its own algorithm, the outputs of which are fused to provide a more reliable and accurate heart rate measurement.

When it comes to ambient light rejection the physical design of the LED and photodiode arrangement have a significant impact on the rejection characteristics. Protecting the photodiode from receiving most sources of ambient light can be achieved by placing it furthest from the edge of the wearable device, typically between the two LEDs. In a typical design, such as the Fitbit Blaze, the LEDs and the photodiode are located on a raised layer off the base. Operating instructions for the device inform the wearer to tighten the wrist strap during exercise so that the raised layer is closely coupled to the skin, providing the optimal direct light path arrangement for reliable heart rate measurement. Needless to say, crosstalk between the photodiode and the LEDs across the raised layer needs to be avoided through the use of a physical barrier. Unwanted ambient light sources can include office lighting, computer displays and televisions. The frequencies of such light sources are potentially within the same bandwidth as the heart rate measurement, so need careful algorithm and physical rejection techniques. Another source of unwanted ambient light could potentially come from the fitness strap’s display. The industrial design of the heart rate monitor needs to ensure there are no direct or indirect light paths from the display to the photodiodes.

Prototyping a health and fitness monitor is extremely straightforward thanks to the two platforms provided by Maxim Integrated. The MAX86140 EVSYS – see Figure 6 – provides a complete evaluation system for the MAX86140 (single optical channel) and MAX86141 (dual optical channels) devices. An accelerometer is included on the board together with on-chip Maxim Integrated proprietary ambient light cancellation algorithm. Comprehensive GUI software is also provided – see Figure 7 – that allows complete configuration of the evaluation platform.

Click image to enlarge

Figure 6: Maxim Integrated MAX86140 EVSYS evaluation kit (source: Maxim Integrated)

Click image to enlarge

Figure 7: Maxim Integrated MAX86140 EVSYS GUI evaluation software

The second evaluation method is a complete fully functioning wrist health and fitness monitor, the MAX-HEALTH-BAND– see Figure 8. The monitor includes a MAX86140 and a MAX20303power-management IC designed specifically for wearable devices. The monitor provides both raw data and algorithm output from the sensors via Bluetooth to a smartphone app to assist in algorithm development. The raw PPG and accelerometer data is also available, and the MAX-HEALTH-BAND includes software to monitor steps, classify the type of activity and track heart-rate variability.

Click image to enlarge

Figure 8: Maxim Integrated MAX-HEALTH-BAND (source: Maxim Integrated)

Incorporating a heart rate monitor into a wearable device helps brand manufacturers differentiate their product offering. Extending that to detect steps walked and to calculate calories burnt adds further functionality. The integrated devices highlighted in this article, together with the evaluation kits, provide a reliable and proven method of quickly prototyping a design.

Mouser Electronics

www.mouser.com