ADPD188GG datasheet(13/61 Pages) AD

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Data Sheet

ADPD188GG

Rev. B | Page 13 of 61

THEORY OF OPERATION

INTRODUCTION

The ADPD188GG is a complete, integrated, optical module

designed for photoplethysmography (PPG) measurements.

The module contains two optical detectors. Photodiode 1 (PDET1)

has 0.4 mm2 of active area and is connected to Channel 3 of the

ASIC. Photodiode 2 (PDET2) has 0.8 mm2 of active area and is

connected to Channel 4 of the ASIC. The two photodiodes can be

combined into a single detector with 1.2 mm2 of active area.

Both photo-diodes are coated with an infrared (IR) cut filter

that maximizes ambient light rejection without the need for other

light cancellation techniques.

The module combines the dual photodetector with two green

LEDs, and a mixed-signal, photometric, front-end ASIC into a

single compact device for optical measurements. The on-board

ASIC includes an analog signal processing block, an ADC, a

digital signal processing block, an I2C and SPI communication

interface, and three, independently programmable, pulsed LED

current sources.

The core circuitry stimulates the LEDs and measures the

corresponding optical return signals in discrete data locations.

Data can be read from output registers directly or through a

first in, first out (FIFO) buffer.

This highly integrated system works well in environments where

ambient light is poorly controlled and the signal modulation

ratio is low. As a result, the device produces high SNR for

relatively low LED power.

OPTICAL COMPONENTS

Photodiode

The ADPD188GG integrates a 1.2 mm2 deep junction photodiode.

The optical sensing area is a dual detector that is connected to

Channel PD3 and Channel PD4 in the ASIC. The photodiodes

are accessible from Time Slot A or Time Slot B. The responsivity

of the ADPD188GG photodiode is shown in Figure 6.

LEDs

The ADPD188GG module integrates two green LEDs.

Table 10. LED Dominant Wavelength

LED Color

Driver

Typical Wavelength (nm)

Green (2×)

LED1

525

In addition to the integrated LEDs, the ADPD188GG has the

ability to drive external LEDs.

1.67

1.42

5.0

PD1

PD2

Figure 13. Optical Component Locations

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