The module is built around a big photodiode, two green LEDs, and an IR LED. The on-board mixed-signal ASIC includes an analog signal processing block, a SAR-type ADC, a digital signal processing block, an I2C communication interface, and three free programmable LED current sources.
The system drives the LEDs and measures the corresponding optical return signal with its 1.2 mm2 photodiode. The biggest challenge for measuring PPG with a wearable device is to overcome interferers like ambient light and artifacts generated by motion. Ambient light can influence the measurement results incredibly. Sunlight is not too difficult to reject but in particular light from fluorescent and energy saving lamps, which include ac components, are difficult to cancel. The ADPD174 optical module has a two-stage ambient light rejection function. After the photo sensor and input amplifier stage, a band-pass filter is integrated, followed by a synchronous demodulator, to offer best-in-class rejection for ambient light and interferers from dc up to 100 kHz. The ADC has a resolution of 14 bits and up to 255 pulse values, which can be summed to get a 20-bit measurement. Additional resolution up to 27 bits can be achieved by accumulating multiple samples.
The ADPD174 operates in two independent timeslots—for instance, to measure two separate wavelengths and can carry out the results sequentially. During each timeslot, the complete signal path is executed, starting with LED stimulation followed by photo signal capturing and data processing.
Each current source is able to drive the connected LED with currents up to 250 mA. Innovative control over the pulsing of the LED keeps the average power dissipation low and contributes significantly to the savings of power and the battery life of the system.
The advantage of this LED driving circuit is that it is dynamic and scalable on the fly. There are many factors that can affect the signal-to-noise ratio (SNR) of the received optical signal, such as skin tone or hairs between sensor and skin, which impacts the sensitivity on the receiving side. For this reason the excitation of the LEDs can be configured very easily to build an autoadaptive system. All timing and synchronization is handled by the analog front end, so there is no overhead required from the microprocessor in the system. With the ADPD174 you will be able to run a reliable heart rate monitor in normal circumstances at a power level of around a milliwatt. To find this operating point, we can tune the gain of the transimpedance amplifier (TIA) in combination with setting the maximum LED peak current. After optimizing the LED current and TIA gain, we can increase the number of LED pulses to get more signal. Be aware that increasing the LED peak current is increasing the SNR proportionally, whereas increasing the number of pulses by a factor of n, results in an SNR improvement of the root of n (√n) only.
Finding the optimum settings for your heart rate device also depends heavily on the user. The user’s skin tone has impact on the signal strength as well as device positioning, temperature, and blood flow. For calculating the power consumption, the optical front end can be seen as two separate power contributors, IADPD and ILED. IADPD is the current consumed by the input amplifier stage, the ADC, and the digital state machine. These power numbers very much depend on the sampling rate of the ADC. The LED current ILED will change with the person’s skin tone and the position of the sensor on the body. For a darker skin tone more LED current is needed, as well as for the sensor position on the body when there is very little blood flow. The average LED current is changing with the LED drive pulse width, the number of pulses, and the ADC sampling time. The average LED current is the max LED current, multiplied by the pulse width and the number of pulses. This can be seen as one timeslot and repeats every time a new sample is taken. The pulse width can be as narrow as 1 μs.
For a good heart rate measurement on the wrist, an LED peak current is required of around 125 mA, when using two pulses with 1 μs width. Considering a 100 Hz sample frequency, the average LED drive takes 25 μA. When we add 250 μA average AFE current, the optical front end is consuming 275 μA (@ 3 V = 825 μW).