Data Sheet

ADPD2210

Rev. A | Page 11 of 15

THEORY OF OPERATION

OVERVIEW

The ADPD2210 is an ultralow noise current amplifier optimized

for wearable photoplethysmography applications and featuring

very low power consumption. Essentially a current mirror with

gain, the ADPD2210 is designed to make sensor signal currents

appear 24 times larger while adding minimal noise. A laser

trimmed linearity of greater than 60 dB allows the extraction of

very small time variant signals with large dc or low frequency

components. This noise and linearity performance allows small

photodiodes to achieve performance comparable to much larger

diodes.

RECOMMENDED CONFIGURATION

In the recommended configuration, a photodiode is connected

across the REF and IN pins of the ADPD2210. The REF pin is

driven by a servo loop to stay within typically ±5 mV of the

IN pin, regardless of current generated by the optical power

incident on the photodiode junction. The current occurring at

the anode of the photodiode is sourced to the IN pin and drives

the first stage of the precision current mirror. A 10 nA static

bias is applied to the current mirror to linearize its transfer

curve at low currents and prevent the output from attempting to

go below 0 V due to unavoidable offsets.

Figure 24 shows a simplified pulse oximeter design using the

ADPD2210.

SENSITIVITY AND SNR

SNR is a measure of the ability of the sensor to separate the

signal of interest from spurious signals that occur from the

surrounding environment of the device, such as ambient light,

electromagnetic interferers, and circuit noise.

Typically, system SNR is improved by using a photodiode with

large surface area because signal increases linearly with area

while noise increases as a root sum of the square of the area.

Capacitance of the photodiode increases with area and carrier

transit time, reducing sensor bandwidth. Bandwidth can be

increased by applying a bias voltage across the diode, but this

increases dark current and, therefore, noise.

Operating at near zero-bias voltage in photoconductive mode,

the photodiode generates virtually no dark current component

except for that caused by the offset of the servo loop across the

shunt resistance of the diode and the thermal noise component

in the depletion region of the photodiode. This sets the fundamen-

tal limit of the signal resolution to the shot noise of the 10 nA

internal bias, 80 fA/√Hz relative to the input, which appears at

the output of the current amplifier and establishes the noise

floor of the ADPD2210.

PULSE MODE OPERATION

The ADPD2210 is optimized for battery-powered operation by

the inclusion of a power down pin (PWDN). When sensing is

inactive, the ADPD2210 can be quickly switched into standby

mode, reducing supply current to ~100 nA during dark periods

for pulsed or mode locked applications where the light source is

cycled to improve ambient light rejection and reduce transmit-

ter power consumption.

For multiple wavelength systems, sequentially pulsing the optical

emitters removes the need for multiple narrow bandwidth sensors.

For both multiple wavelength (SpO2) and single wavelength

(HRM) systems, pulsed operation can provide significant power

savings for battery-powered systems. Pulsed mode operation

provides a calibration signal that is necessary to compensate for

ambient light diffused throughout the tissue, which can be

extracted by measuring the sensor output while the system

emitters are off. Advanced algorithms can then extract the

signal of interest from dc offsets, noise, and interferer signals

such as motion artifacts.

Figure 24. Simplified Pulse Oximeter Design

REF

IN

PWDN

10nA

POWER-DOWN

LOGIC

24 × CURRENT MIRROR

OUT

TIA

ADC

MICROCONTROLLER

LED DRIVER

DISPLAY

BIAS

PHOTODIODE

900nm

660nm

ASIC

ADPD2210

V

GND

VCC

GND