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Heartrate monitoring in wearable design

May 23, 2019
4 Mins read
Introduction A very noticeable feature in modern wearables and smartwatches these days is the presence of an optical photoplethysmograph (PPG) heart-rate sensor. PPG is a non-invasive method of measuring the variation in blood volume in tissues using a light source and a detector. In this article, we will talk about some component-level heart-rate measurement (HRM) sensors which will be useful for designers and developers looking to develop wearable devices that can monitor and record heart rate and blood oxygen levels. What is PPG? Figure 1 The above is a typical PPG waveform capture, distinctive systole peaks, and dicrotic notches are clearly visible in each heartbeat PPG and its principle of operation are well established as a medical device and has been more commonly referred to as a pulse oximeter. Pulse oximetry is a non-invasive method for monitoring a person's blood peripheral oxygen saturation (SpO2), which is vital for patients undergoing anesthesia, premature or new-born babies, sedated or comatose patients in a hospital where a low blood oxygen condition could potentially be fatal. Fortunately, healthy conscious individuals have a blood oxygen level typically above 95% and pulse oximetry is not useful in a daily situation. PPG simply refers to the signal waveform that is measured in pulse oximetry but has been simplified to only record the characteristic pulsatile waveform of an individual’s heart rate. Typical PPG sensors on smartwatches or wrist-worn wearables today feature a reflectance-type PPG, where the sensor typically consists of an LED and a detector or photodiode. Light is emitted into the tissue; the intensity of the reflected light is then measured by the photodiode. The intensity of light is absorbed or reflected varies with the pulsing of the blood with each heartbeat. Fluctuations from the light corresponding to the pulsatile blood flow caused by the beating of the heart. The plot for this variation against time is referred to be a photoplethysmographic or PPG signal. Since the change in blood volume is synchronous to the heartbeat, this technique can be used to calculate the heart rate. Challenges with discrete designs Figure 2 The above shows a typical block diagram representation of a PPG system In a typical PPG design, an LED driver drives an LED which emits light into the skin, and an analog sensor comprising of a photodiode picks up the reflected signal. The signal is then filtered or cleaned through an Analog Front End (AFE) and is then sampled by an ADC which is then fed to the microcontroller (MCU) for interpretation of the data. Although the PPG principle is well understood, implementing a traditional PPG design in a wearable is still challenging due to several factors – movement artifacts, ambient light noise, cross-talk between the LED and photosensors and because most of the LED’s light is reflected from body tissue, rather than the blood vessels mean that the variation caused by the pulsing of the heart is represented by a tiny AC signal on top of a large DC base signal. Detection of this AC signal calls for highly sensitive analog circuitry with a wide dynamic range that implements some level of band-pass or narrow-band filters, optical signal modulation and a programmable gain amplifier (PGA). Analog design and schematic of a pulse oximeter are described here. Figure 3 Left: Integrated PPG solutions from Omron SFH 7050 with a red LED emitter and the NJL5310R with a green LED emitter. Centre: THESIS-designed Quad-Infrared (IR) LED emitter with a single photodiode detector in the centre as a discrete solution. Right: One of THESIS PPG designs have discrete LED drivers, LEDs, Photodiode sensor in the centre and specific discrete transimpedance op-amp filters and amplifiers with programmable gain ADC. Over the years, we’ve had extensive experience in designing high-fidelity discrete PPG sensor systems, however the traditional approach described requires several discrete components each taking up precious PCB real-estate in a space-constraint wearable. Fortunately, with major growth in this segment today; there are a variety of discrete designs and integrated HRM sensors to choose from. An integrated PPG/Pulse oximeter module will contain to a varying degree of functional blocks and can be as simple as a LED-photodiode combination (e.g. SFH 7050), to a full-fledged purpose-built all-in-one heart-rate monitor with LEDs, photo-detectors, optical processing circuitry, an analogue front end (AFE) and heart-rate algorithms built into the component (e.g. AMS AS7000). The ground-up design of a PPG sensor could be achieved with the use of discrete LEDs, photodiodes and signal chain components, but this would require much more design time and risk than the use of a compact, low-power, proven sensor solution-on-a-chip. The wide variety of HRM sensors now widen the options available to designers depending on the application that is being designed for. Depending on the complexity desired and the cost of the solution, an HRM module can be dropped into a product design without major integration or design effort or software development. We’ve compiled a table of HRM sensors as of May 2019 that are available to designers today. Discussion From the above tables, we can see a large variety of choices broadly categorized into four types of integrated HRM components with varying levels of analog and digital block integration: 1. Entire HRM solution in one package with onboard MCU and algorithms (e.g. AMS AS7000) 2. Complete HRM + SpO2 solution with I2C output (e.g. Maxim MAX30101, MAX30105) 3. A photodiode, AFE and LED drivers built-in, but no LEDs (e.g. Rohm BH1792, Silabs Si1144-AAGX) 4. Analog LED and photodiode pair, no LED driver nor ADC circuitry (e.g. OSRAM’s and NJL5310, NJL5501 solutions) Depending on the level of design control required and cost concern for the designer, one could simply use the first or second type of integrated HRM solution from AMS or Maxim. Or if one’s existing system already has an AFE/ADC on-board, you can now choose between Rohm’s, Silabs or Osram’s solutions.   Summary Heartrate monitors (HRM) have proven to be very useful in determining various physiological factors and have contributed to the explosive growth of smartwatches and wellness wearables. Challenges with miniaturizing many discrete components to consume minimum power and fit into an extremely small package in wearables have proven challenging. In this article, we have reviewed several integrated HRM solutions that are already optimized with analog and/or digital circuit blocks that help to simplify the design and shorten time to market. Using these components, one can concentrate on the features that differentiate the product, and drastically reduce time-to-market. In the fast-paced health and fitness wearables market, that can be the difference between success and failure. Thesis have designed various HRM/PPG systems for various applications, come talk to us to find out more on how we can help you with your design.
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