Test & measurement ADI SolutIon: ADPD4100
The positioning of the pulse oximeter generates several challenges. Wrist-worn
SpO2 devices provide additional design challenges as the AC signal of interest is only one to two per cent of the total received light on the PD. To achieve medical grade certification and distinguish between slight variations in oxyhemoglobin 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 100dB SNR. This increased dynamic range is essential to measure
SpO2 under low perfusion scenarios. This integrated optical AFE has eight onboard low noise current sources and eight separate PD inputs. The digital timing controller has 12 programmable timing slots that enable the user to define an array of PD and LED sequences with specific LED current, analog and digital filtering, integration options, and timing constraints. A key benefit of the ADPD4100 is the
increase in SNR/μW, which is an important parameter for battery-powered continuous monitoring. This key metric has been addressed by increasing the AFE dynamic range while also lowering the AFE current consumption. The ADPD4100 now boasts a total power consumption of only 30μW for a 75dB, 25Hz continuous PPG measurement including the LED supply. Increasing the number of pulses per sample (n) will result in a (√n) increase in SNR while increasing the LED drive current will have a proportional increase in SNR. 1μW total system consumption will
Figure 6. ADPD410X block diagram.
return 93dB SNR for a continuous PPG measurement using a 4V LED supply. Automatic ambient light rejection reduces
the burden on the host microprocessor while achieving 60dB of light rejection. This is achieved using LED pulses as fast as 1μs in conjunction with a band-pass filter to reject interference. In certain operating modes, the ADPD4100 automatically calculates the photodiode dark current or LED off state. This result is subtracted from the LED on state before conversion in the ADC to remove ambient light as well as gain errors and drift within the photodiode. The ADPD4100 is supported with the
EVAL-ADPD4100-4101 wearable evaluation kit along with the ADI Vital Signs Monitoring
Study Watch. This hardware seamlessly connects to the ADI Wavetool application to enable bioimpedance, ECG, PPG heart rate, and multiwavelength PPG measurements for
SpO2 development. Embedded in the study watch is an automatic
gain control (AGC) algorithm for the ADPD4100 that tunes the TIA gain and LED current to deliver optimum AC signal dynamic range for all LED wavelengths selected.
AlternAtIve ADI SolutIonS
Finger- and earlobe-based SpO2 readings are the easiest to design for as the signal-to- noise ratio is higher than wrist- or chest- based positioning due to the reduction in bone and tissue, which also reduces the DC component contribution. For such applications, the ADPD144RI module
and the ADPD1080 are suitable devices. The ADPD144RI is a complete module
with an integrated red 660nm LED and 880nm IR LED and four PDs in a 2.8mm × 5mm package. The spacing between the LEDs and PD have been optimised to give the
best signal-to-noise ratio for SpO2 high accuracy PPG measurements. This module allows users to quickly skip the design challenges associated with LED and PD placement and spacing to achieve optimal power to noise ratios. The ADPD144RI mechanically has been optimised to reduce optical crosstalk as much as possible. This provides a solid solution, even when the sensor is placed under a single glass window. The ADPD1080 is an integrated optical AFE
with three LED drive channels and two PD current input channels in a 17-ball, 2.5mm × 1.4mm WLLCSP. This AFE is ideal for custom design low channel count PPG products where board space is critical.
Figure 7. ADPD4100 simultaneous red (right) and IR (left) PPG measurement. Instrumentation Monthly November 2021 Analog Devices
www.analog.com 21
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