Page 36 - EngineerIt August 2021

MEASUREMENT

at the receive side to cancel out ambient light interferers. This increased dynamic range is essential to measure SpO 2
Integrating multiple pulses increases the PD signal amplitude under low perfusion scenarios. This integrated optical AFE has
and lowers the average current consumption. Increasing the eight onboard low noise current sources and eight separate PD
total PD area also increases CTR as more of the reflective light inputs. The digital timing controller has 12 programmable timing
is captured. slots that enable the user to define an array of PD and LED
For heart rate PPG measurement, a combination of a sequences with specific LED current, analog and digital filtering,
single large PD and multiple power efficient green LEDs has integration options and timing constraints.
been adopted by many HR device manufacturers, to be used A key benefit of the ADPD4100 is the increase in SNR/
on places where there is limited blood flow. Green LEDs are μW, which is an important parameter for battery-powered
chosen due to their high rejection of motion artifacts. However, continuous monitoring. This key metric has been addressed by
this comes at the cost of power. Green LEDs have a higher increasing the AFE dynamic range while also lowering the AFE
forward voltage than red and IR and high absorbance in human current consumption. The ADPD4100 now boasts a total power
tissue, meaning a higher LED power is required to return consumption of only 30 μW for a 75 dB, 25 Hz continuous PPG
meaningful cardiac information. measurement including the LED supply. Increasing the number
As SpO 2 requires multiple wavelengths and most systems still of pulses per sample (n) will result in a (√n) increase in SNR while
incorporate high efficiency green LEDs for the HR PPG, the most increasing the LED drive current will have a proportional increase
common configuration for HR and SpO 2 PPG systems is a single in SNR. 1 μW total system consumption will return 93 dB SNR for
green, red, and IR LED array surrounded by multiple PDs, as seen a continuous PPG measurement using a 4 V LED supply.
on the ADI VSM watch in Figure 5. PD to LED spacing has been Automatic ambient light rejection reduces the burden on the
optimised to reduce backscatter and the baffle design reduces host microprocessor while achieving 60 dB of light rejection. This
LED to PD crosstalk. is achieved using LED pulses as fast as 1 μs in conjunction with a
Multiple prototypes of the ADI VSM watch were trialled to band-pass filter to reject interference. In certain operating modes,
verify the most efficient PD to LED spacing for our HR PPG and the ADPD4100 automatically calculates the photodiode dark
SpO 2 measurement. current or LED off state. This result is subtracted from the LED on
state before conversion in the ADC to remove ambient light as
Motion artifacts well as gain errors and drift within the photodiode.
Motion artifacts provide one of the greatest design challenges to The ADPD4100 is supported with the EVAL-ADPD4100-4101
a PPG measurement system. When motion is present, the width wearable evaluation kit along with the ADI vital signs monitoring
of the arteries and veins change due to pressure. The amount study watch. This hardware seamlessly connects to the ADI wave
of light absorbed by the photodiode changes and this is present tool application to enable bioimpedance, ECG, PPG heart rate
on the PPG signal because photons are absorbed or reflected and multi-wavelength PPG measurements for SpO 2 development.
differently than when a body is at rest. Embedded in the study watch is an automatic gain control
For an infinitely wide photodiode area covering an infinitely (AGC) algorithm for the ADPD4100 that tunes the TIA gain and
long deep tissue sample, all photons will eventually be reflected LED current to deliver optimum AC signal dynamic range for all
to the photodiode. In this case, no artifact due to motion will be LED wavelengths selected.
detected. This, however, cannot be achieved; the solution is
to increase the photodiode area while taking capacitance into Alternative ADI solutions
account — lowering AFE and providing filtering for motion artifacts. Finger- and earlobe-based SpO 2 readings are the easiest to
The normal frequency for a PPG signal is between 0.5 Hz to 5 design for, as the signal-to-noise ratio is higher than wrist- or
Hz while motion artefacts are typically between 0.01 Hz to 10 Hz. chest-based positioning due to the reduction in bone and tissue,
Simple band-pass filtering techniques cannot be used to remove which also reduces the DC component contribution.
motion artifacts from the PPG signal. To achieve high accuracy For such applications, the ADPD144RI module and the
motion cancellation, an adaptive filter needs to be supplied ADPD1080 are suitable devices.
with highly accurate motion data. For this purpose, Analog The ADPD144RI is a complete module with an integrated red
Devices has developed the ADXL362 3-axis accelerometer. This 660 nm LED and 880 nm IR LED and four PDs in a
accelerometer provides 1 mg resolution with up to 8 g of range
while consuming only 3.6 μW at 100 Hz and is available in a 3
mm × 3 mm package.
ADI solution: ADPD4100
The positioning of the pulse oximeter generates several
challenges. Wrist-worn SpO 2 devices provide additional
design challenges as the AC signal of interest is only 1% to
2% of the total received light on the PD. To achieve medical
grade certification and distinguish between slight variations in
oxyhaemoglobin levels, a higher dynamic range on the AC signal
is required. This can be achieved by reducing ambient light
interference and decreasing LED driver and AFE noise. ADI has
addressed this problem with the ADPD4100.
The ADPD4100 and ADPD41001 achieve up to 100 dB SNR. Figure 6. ADPD410X block diagram.

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