Heart Rate 5 Click Board™

Product Code: MIKROE-3012

The Heart Rate 5 Click Board™ is the optical biosensor board which can be used for the heart-rate monitoring (HRM), as well as the peripheral capillary oxygen saturation monitoring (SpO2). The Heart Rate 5 Click Board™ employs a very sensitive analog front-end IC with high dynamic range, which ensures accurate and reliable readings. This analog front-end IC is coupled with the optical front end, which consists of the top-of-the-class integrated BIOFY® sensor, which features two green LEDs, one red LED, one infrared LED, and two photodiodes (PD), offering very accurate HRM and SpO2 readings. The analog front-end IC features a trans-impedance amplifier circuitry with programmable gain, which provides linear voltage changes for the 22bit ADC. It also provides individual DC offset subtraction for the LED and ambient light phases, as well as the ambient light influence cancellation at the ADC output. The signal path within the IC is kept differential, ensuring the lowest possible amount of interferences and noise.

These features allow Heart Rate 5 to achieve reliable and accurate readings. It can be used to develop applications based on the heart rate monitoring, pulse oximetry measurements, calorie expenditure, and similar health-related applications. PULSE OXIMETRY OR SPO2 Oxygen saturation in the blood can be determined by measuring the light absorption in the red/IR part of the spectrum. The oxygen saturated blood absorbs more red light and less infrared than the unsaturated blood. This fact can be used to determine the oxygen saturation of the blood. For a healthy adult person, the peripheral capillary oxygen saturation (SpO2) percentage ranges from 95% to 100%. the Heart Rate 5 Click Board™ can provide the SpO2 measurement, by simply placing the index finger over the optical sensor. HEART RATE MONITORING OR HRM While the blood passes through the capillary blood vessels, they expand and dilate. Their light reflectance index changes accordingly. This is the basis of the photoplethysmogram (PPM), a method used for the volumetric measurement of an organ, or in this case - blood vessels. The heart rate signal is calculated according to the current changes in transmission and reflection of the green light, sensed by the PD element. The Heart Rate 5 click can provide the HRM readings, by simply placing the index finger over the optical sensor. How Does The Heart Rate 5 Click Board™ Work? The Heart Rate 5 Click Board™ consists of an analog front end and the optical front end. The main task of the analog front-end IC is to drive LEDs and condition the signal received by the photodiode (PD), by eliminating the background noise and ambient light influence on the measurement. Besides that, it also provides conversion of the measurement into a digital information, which can be used by the MCU. For the conversion to be accurate, the analog front-end device must not introduce any artifacts into the readings. To achieve accurate measurements, Heart Rate 5 click employs AFE4404, an integrated analog front end (AFE) device, used for optical heart-rate monitoring and bio-sensing, from Texas Instruments. This IC supports up to three switching LEDs and a single PD element. The current from the PD element is converted to a linear voltage by the means of the integrated trans-impedance amplifier section (TIA) with a programmable gain so that it can be sampled by the AD section, which features a 22bit ADC converter. The signal chain is kept fully differential throughout the receiver channel, in order to achieve good rejection of common-mode noise, as well as the noise from the power supply. The AFE IC uses the I2C communication, with its pins routed to the corresponding mikroBUS™ I2C pins. As the optical front end, Heart Rate 5 click uses the top-of-the-class integrated BIOFY® SFH 7072 sensor, from OSRAM, which features two green LEDs, one red LED, one infrared LED, and two PDs, of which one is a broadband PD used for the HRM, while the other is the IR band cut PD, used for the SpO2 readings. These LEDs are specially designed for the HRM and SpO2 measurement applications, offering a set of calibrated wavelengths for both light-emitting diodes (LED) and photo-sensing diodes (PD). Since the SF7072 sensor offers more elements than the ALS can support, the choice is made by flipping the onboard SMD switch labeled as MODE SEL: the choice can be made between the broadband PD for the HRM readings and the IR band PD for SnO2 readings. Both poles of the switch SW1 need to stay in the same position (both to the left, or both to the right), as they are routed to each end of the respective PD element. The analog front end works with the periodically repeated operations (a pulse repetition frequency or PRF). There are four sampling phases per cycle. The four different readings are stored in separate 24bit output registers. There are also four filters on the TIA output, which are used to allow pulses from the PD to pass through the