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Data Sheet

ADPD144RI

Rev. A | Page 9 of 34

THEORY OF OPERATION

INTRODUCTION

The ADPD144RI is a highly integrated, optical module

designed for reflective PPG measurements. The module

combines red and infrared LED emitters, a four segment optical

detector, and a mixed signal, photometric front end with digital

control and signal processing in a compact device for optical

measurements.

The ADPD144RI uses synchronous detection of optical pulses

to improve the rejection of ambient light while also using much

less operating power than nonsynchronous architectures. The

integrated LED emitters produce light pulses synchronous with

the active sampling period of the AFE. The AFE comprises a

programmable TIA, a band-pass filter and an integrator. The

processed analog signals are digitized by a 14-bit ADC and

summed by the 20-bit burst accumulator. Four channels of

simultaneous sampling are matrixed into two independent time

slots (one for each LED wavelength). The number of pulses per

sample is adjustable and accumulation and averaging can be

applied to multiple samples to increase the dynamic range to

27 bits.

A high speed I2C interface allows data to be read from output

registers directly or through a first in, first out (FIFO) buffer.

Configuration control of the ADPD144RI is provided through

the I2C interface.

OPTICAL COMPONENTS

Photodiode

The ADPD144RI integrates a 1.2 mm2 deep-junction photodiode

optimized for red and infrared sensitivity. The optical sensing

area is segmented into four physical photodiodes. These diodes

are multiplexed into two or four separate optical channels within

the analog signal processing block. Data processing is performed

on each channel independently, and output data is reported in

sets of four 16-bit or 32-bit registers (one per optical channel)

or summed into a single register, depending on programmed

output data format.

LEDs

The ADPD144RI module contains two LED emitters at the

optimum 660 nm red (LED1) and 880 nm IR (LED2) wavelengths

for measurement of blood oxygenation (SpO2). A common

anode connection to VLED supplies both integrated LEDs

while the cathode of each LED is connected to a programmable

current sink. The use of current sinks to pulse the LEDs allows

a single supply voltage to power LEDs with different forward

voltage requirements automatically compensating for the

different forward voltages required by different LED families.

When using the integrated LEDs, do not connect the LEDX1

and LEDX2 pins.

NOTES

1. ALL DIMENSIONS ARE TYPICAL AND SHOWN IN mm.

2. LED DIMENSIONS:

880nm LED: 0.350mm × 0.350mm

660nm LED: 0.340mm × 0.340mm

3. PHOTODIODE AREA: 1.2mm2 (0.3mm2 PER PD).

880nm

LED

660nm

LED

2.80

1.36

5.00

0.83

0.66

1.40

0.55

1.48

2.95

4.53

1.70

3.50

3.11

2.71

2.32

1.10

Figure 11. Optical Component Locations

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