ADPD2210 Data Sheet
Rev. A | Page 10 of 15
TERMINOLOGY
Amperometry
Amperometry is a technique used in chemistry and biochemis-
try to detect ions in a solution by measuring very small currents
between polarized electrodes. Methods of amperometry include
direct, pulsed, and amperometric titration, where a substance
(titrate) known to react with the analyte (the substance being
measured) is added in measured quantities and the effect on the
ionic concentration of the analyte is measured.
Dark Current
Dark current is the current flowing in a photodiode with no light
incident upon the diode junction. In reversed bias operation,
the dominant source of dark current is current generated by the
bias voltage across the bulk resistance of the semiconductor
material (shunt resistance). In zero bias operation, thermal
generation of charge carriers in the depletion region becomes
the dominant source of dark current.
Linearity
Linearity is a measure of the deviation from an ideal change in
output current relative to a change in input current. Linearity is
specified as the deviation from a best straight line fit of the
amplifier current output over a specified range of input current.
Linearity is a critical specification in photoplethysmography
due to the requirement of sensing small ac signals impressed
upon large dc offsets.
Noise Equivalent Power (NEP)
Noise equivalent power is the amount of incident light power
on a photo detector, which generates a photocurrent equal to
the total noise current of the sensor, expressed as A/√Hz. The
NEP is the fundamental baseline of the detectivity of the optical
sensor.
Offset
Offset in the ADPD2210 is defined as the differential voltage
between the reference output and the input of the ADPD2210.
The ADPD2210 holds the input terminal voltage to within
±5 mV (typical) of the reference terminal.
Photoconductive Mode
Photoconductive operation of a photodiode occurs when
photons entering the silicon generate electron/hole pairs that
are swept by the electric field to the opposite terminal. These
carriers are presented at the terminals of the photodiode as a
current proportional to the luminous flux incident on the
junction of the photodiode.
Photoplethysmography
Photoplethysmography uses light to measure biological functions
by sensing changes in the absorption spectra of soft tissue
caused by differences in hemoglobin volume and composition.
Common applications of photoplethysmography include
transmission SpO2 pulse oximetry and reflectance HRM.
Shot Noise
Shot noise is a statistical fluctuation in any quantized signal
such as photons of light and electrons in current. The magni-
tude of the shot noise is expressed as a root mean square (rms)
noise current. Shot noise is a fundamental limitation in photo
detectors and takes the form of
Shot noise = √(2qI
PD
BW)
where:
q is the charge of an electron (1.602 × 10
−19
Coulomb).
I
PD
is the photodiode current.
BW is the bandwidth.
Static Bias
The ADPD2210 has an internal 10 nA bias that is used to linearize
the input current mirror at low input levels and prevents transient
reverse bias of the amplifier input stage. This bias is fixed and
appears on the output as a 240 nA typical offset.
Thermal (Johnson) Noise
All resistors generate a noise component based on temperature,
including the shunt resistance in a photodiode due to genera-
tion of carriers within the bulk semiconductor. The magnitude
of this generated noise current is calculated as follows:
Photodiode Thermal Noise Current =
RSH
fkT4
where:
k = 1.38 × 10
−23
joules per °K. k is the Boltzmann constant.
T is the absolute temperature in degrees Kelvin (273 K = 0°C).
Δf is the noise measurement bandwidth.
RSH is the shunt resistance of the photodiode
Thermal noise generated in the bulk semiconductor outside the
depletion region of the photodiode appears as a broadband ac
signal. Thermal noise generated within the depletion region
appears as a dc current but is typically an insignificant compo-
nent of dark current relative to the bias/shunt resistance
component.
