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ADPD2210 Schematic ( PDF Datasheet ) - Analog Devices

Teilenummer ADPD2210
Beschreibung Low Power Current Amplifier
Hersteller Analog Devices
Logo Analog Devices Logo 




Gesamt 16 Seiten
ADPD2210 Datasheet, Funktion
Data Sheet
FEATURES
Ultralow noise, low power current amplifier
80 fA/√Hz (typical) noise floor
140 μA (typical) of supply current when active (EE = 0 μW/cm2)
100 nA (typical) of supply current in standby
Flexible output configuration
Optimized for pulsed systems
Nominal linear output: 240 μA
Space-saving 2 mm × 2 mm LFCSP package
APPLICATIONS
Photoplethysmography
Photodiode measurements
Small current pulsed amperometry
Any application requiring the ultralow noise amplification of
small currents
Ultralow Noise, Low Power
Current Amplifier
ADPD2210
FUNCTIONAL BLOCK DIAGRAM
BIAS
VCC – (2 × VBE)
POWER-DOWN
LOGIC
PWDN
10nA
OUT
IN
24 × CURRENT MIRROR
Figure 1.
GENERAL DESCRIPTION
The ADPD2210 is a low noise current amplifier designed to
allow the use of smaller photodiodes by amplifying sensor signal
currents by a factor of 24 while adding minimal noise. This
amplification provides the system sensitivity of a large photodiode
with the benefits of a smaller photodiode. A minimum linearity
of 60 dB allows accurate extraction of very small time variant
signals on top of large dc or low frequency offsets.
The ADPD2210 is optimized for pulse mode applications such
as wrist worn heart rate monitoring (HRM) or finger worn
pulse oximeter oxygen saturation (SpO2), where low power
consumption and rejection of ambient light is critical. In photodi-
ode applications, the ADPD2210 holds the sensor input to
within ±5 mV (typical) of the reference terminal, providing
near zero-bias voltage and allowing minimal dark current and
shot noise limited performance.
The ADPD2210 is designed for applications where power
conservation is critical. The ADPD2210 uses very little power,
typically 140 μA with no input to 954 μA at full scale. A power-
down pin places the ADPD2210 in standby when sensing is
inactive. This mode adds critical time for battery-powered
monitoring and can reduce battery costs in disposable applications
Using the ADPD2210 to provide sensor site amplification
reduces the effect of electromagnetic interference (EMI) in low
level wired interfaces, providing improved signal-to-noise ratio
(SNR) and rejection of interferer signals from nearby equipment.
The combination of low power, high SNR, and EMI immunity
enables low power system solutions not possible with traditional
small current sensors, such as photodiodes plus transimpedance
amplifiers (TIAs).
Rev. A
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ADPD2210 Datasheet, Funktion
Data Sheet
ADPD2210
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
PWDN 1
ADPD2210
6 VCC
OUT 2
CURRENT
AMPLIFIER
5 IN
GND 3
EPAD(7)
4 REF
NOTE
1. CONNECT THE EPAD TO GND.
Figure 2. Pin Configuration
Table 4. Pin Function Descriptions
Pin No. Mnemonic Description
1
PWDN
Power-Down Input. Tie the PWDN pin to ground for normal operation. Connecting this input to a logic high
enables standby mode. Do not leave this input floating.
2 OUT
Current Output.
3 GND
Ground.
4 REF
Voltage Reference Output. REF is nominally 1.2 V below VCC. This pin is the matched voltage reference for the
current input and is typically tied to the cathode of a photodiode. Attachment to this terminal is not required.
5 IN
Current Input (Sink). The voltage of the IN pin is forced within 5 mV of the reference input.
6 VCC
Supply.
7 EPAD Exposed Pad. Connect the EPAD to GND.
Rev. A | Page 5 of 15

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ADPD2210 pdf, datenblatt
Data Sheet
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.
ADPD2210
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.
REF
PHOTODIODE
BIAS
10nA
IN
ADPD2210
POWER-DOWN
LOGIC
24 × CURRENT MIRROR
VCC
PWDN
OUT
GND
660nm
ASIC
900nm
RF
GND
TIA
V
LED DRIVER
ADC MICROCONTROLLER DISPLAY
Figure 24. Simplified Pulse Oximeter Design
Rev. A | Page 11 of 15

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