ADPD2210

Data Sheet

Rev. A | Page 12 of 15

APPLICATIONS INFORMATION

POWERING THE ADPD2210

The ADPD2210 is powered from a single positive 1.8 V to 5 V

supply, although performance below 2.5 V is limited by the

reduced dynamic range of the device. Above the quiescent

current (140 μA), the supply current has a linear relationship to

features a 25 nA/V (typical) PSRR, but proper circuit layout and

bypassing is recommended to provide maximum sensitivity,

especially in designs where the ADPD2210 may share reference

nodes with transmitters in pulse mode applications.

EXPOSED PAD CONNECTION

The exposed pad (EPAD) on the ADPD2210 acts as an electri-

cal, thermal, and mechanical platform for the amplifier and

must be connected to a quiet GND. External cooling is not

required due to the extremely low power consumption of the

ADPD2210. Analog Devices, Inc., recommends removal of

active traces beneath the device to eliminate potential coupling

of external signals into the sensitive internal nodes of the

ADPD2210.

POWER-DOWN

The power-down pin does not have an internal pull-up/pull-down

circuit and must be connected to an external logic level for

proper operation.

In the recommended configuration and when it is not in power-

down mode, the ADPD2210 presents an approximate 90 Ω load

to the photodiode anode. This load limits the photovoltaic

effect from a silicon photodiode at full scale to approximately

900 μV. In the power-down state, the ADPD2210 presents a

high impedance at the IN pin and the photovoltaic effect from a

photodiode is limited to the open-circuit voltage of the

photodiode.

In applications where the ADPD2210 is fed from a current source,

initiation of power-down mode causes the voltage at the IN pin

to slew up to the compliance voltage of the current source. The

rate at which the voltage slews depends on the current sourced

and capacitance at the IN pin. If the compliance voltage of the

voltage of the IN pin, the ADPD2210 requires additional settling

emitter voltage.

REFERENCE OUPUT

The REF pin is sensitive to loading and is not intended to drive

more than 1 μA. When the ADPD2210 REF output is connected

to the cathode of the photodiode, loading of the REF pin is limited

to the offset voltage (±5 mV), divided by the shunt resistance

(typically >1 GΩ) of the photodiode.

In applications where the REF output is used to provide an

external reference or a guarding voltage, the REF output must

be buffered. Failure to buffer the REF pin may adversely affect

linearity above 4 μA.

LAYOUT CONSIDERATIONS

Working with very low currents requires special attention in

layout to prevent error currents due to leakage, especially in

instrumentation applications where the ADPD2210 may be

located at a distance from the current source. In applications

that rely on dynamic signals, parasitic capacitance must be

controlled as seemingly insignificant capacitance becomes

problematic with nanoampere scale signals.

OUTPUT CONFIGURATION

The output of the ADPD2210 allows different configurations

depending on the application. The current gain of the ADPD2210

reduces the effect of surrounding interferers but, for best perfor-

mance, careful design and layout is still necessary to achieve

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 ADPD2210 is effectively a current source, the

ADPD2210 output voltage drifts up to its compliance voltage,

approximately 1.2 V below VCC, when connected to an inter-

face that presents a high impedance. The rate of this drift is

dependent on the ADPD2210 output current, parasitic capaci-

tance, and the impedance of the load. This drift can require

additional settling time in circuits following the ADPD2210 if

they are actively multiplexing the output of the ADPD2210 or

presenting a high impedance due to power cycling. For multi-

plexed systems, a current steering architecture may offer a

performance advantage over a break-before-make switch matrix.

ACCURACY IN CLINICAL APPLICATIONS

Even with perfectly calibrated electronics, it is important to

note there is no absolute in photoplethysmography measure-

ments because they are affected by other variables, including

high levels of carboxyhemoglobin or methemoglobin, density of

other chromophores such as melanin, and conditions that may

affect perfusion such as peripheral artery disease, shock, or

hypothermia. It is important that photoplethysmography, though

well suited for real-time monitoring, be supported in a clinical

environment with more accurate laboratory procedures such as

blood gas analysis.