ADPD2212 Data Sheet
Rev. 0 | Page 10 of 13
APPLICATIONS INFORMATION
The current output of the ADPD2212 provides flexibility in
interfacing to external circuitry.
POWERING THE DEVICE
The ADPD2212 is powered from a single positive 1.8 V to 5.0 V
supply. The ADPD2212 features high PSRR, but proper circuit
layout and bypassing is recommended to provide maximum
sensitivity, especially in situations where the ADPD2212 may share
reference nodes with transmitters in pulse mode applications.
Above the quiescent current of the integrated current amplifier,
there is a linear relationship to incident light as the current
amplifier amplifies the photodiode output by a factor of 24. In
typical battery-powered operation, the output of the source
LEDs is dynamically reduced to save power based on the received
signal strength of the photosensor. The extremely low noise
floor of the ADPD2212 provides very high SNR, allowing
accurate signal extraction with minimal source power and at
low incident optical power.
POWER-DOWN MODE
The ADPD2212 is optimized for battery-powered operation by
the inclusion of an extremely low power standby mode that can
be quickly switched to provide ultralow power consumption
during dark periods in pulsed or mode locked applications,
where the light source is cycled to improve ambient light
rejection and reduce transmitter power consumption. The
power-down pin is not internally pulled up or down, and must
be connected to an external logic level for proper operation of
the ADPD2212.
PULSE MODE OPERATION
The ADPD2212 is optimized for battery-powered operation by
the inclusion of a power-down pin (PWDN). When sensing is
inactive, the ADPD2212 can be quickly switched into standby
mode, reducing the supply current to 1 µA during dark periods
for pulsed or mode locked applications, where the light source
is cycled to improve ambient light rejection and reduce
transmitter 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
(heart rate monitoring) 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.
OUTPUT CONFIGURATION
The output of the ADPD2212 allows different configurations
depending on the application. The current gain of the ADPD2212
reduces the effect of surrounding interferers but, for best perfor-
mance, careful design and layout is still necessary to achieve the
best performance. The effect of capacitance on the output must
be considered carefully regardless of configuration as bandwidth
and response time of the system can be limited simply by the
time required to charge and discharge parasitics.
Because the ADPD2212 is effectively a current source, the
ADPD2212 output voltage drifts up to its compliance voltage,
approximately 1.2 V below V
CC
, when connected to an interface
that presents a high impedance. The rate of this drift is dependent
on the ADPD2212 output current, parasitic capacitance, and the
impedance of the load. This drift can require additional settling
time in circuits following the ADPD2212 if they are actively
multiplexing the output of the ADPD2212 or presenting a high
impedance due to power cycling. For multiplexed systems, a
current steering architecture may offer a performance advantage
over a break-before-make switch matrix.
3-WIRE CABLE VOLTAGE CONFIGURATION
The ADPD2212 can be used in a minimal 3-wire voltage
configuration, offering a compact solution with very few
components (see Figure 13). A shunt resistor (R
S
) sets the
transimpedance gain in front of the analog-to-digital converter
(ADC). This configuration allows flexibility in matching the
ADC converter full-scale input to the full-scale output of the
ADPD2212. The dynamic range of the interface is limited to
the compliance voltage of the
ADPD2212.
No additional amplification is needed prior to the ADC. Response
time at the lower end of the range is limited by the ability of the
output current to charge the parasitic capacitance presented to
the output of the ADPD2212.
3-WIRE CURRENT MODE CONFIGURATION
When used in the 3-wire current mode configuration with a
photodiode (see Figure 14), the ADPD2212 is insensitive to load
resistance and can be used when the signal processing is further
from the sensor. EMI noise and shielding requirements are
minimized; however, cable capacitance has a direct effect on
bandwidth, making the 3-wire current mode configuration a
better choice for unshielded interfaces. The feedback capacitance
(C
F
) value must be chosen carefully to eliminate stability and
bandwidth degradation of the ADPD2212. Large capacitance
around the feedback loop of the TIA has a direct effect on the
bandwidth of the system.
