ADPD2212 Data Sheet
Rev. 0 | Page 8 of 13
TERMINOLOGY
Optical Power
Optical power is defined as the photon energy per unit of time
measured as radiant flux (Φ) or radiant power, which is radiant
energy (Q) per unit of time.
Responsivity
Photodiode responsivity, ρ, is a constant that correlates incident
optical power (P
OPT
) with photodiode current (I
PD
) and is typically
expressed in units of amperes per watt (A/W). Responsivity is
essentially the quantum efficiency of the ability of the sensor to
convert light into electron/hole pairs and is highly dependent
upon the wavelength of the incident light as well as sensor
material and temperature.
Photodiode Area
Photodiode area is a measure of the photosensitive area of the
diode. In PIN diodes, this is the photosensitive area of intrinsic
silicon between the positive and negative doped diffusion areas.
In general, larger photodiodes demonstrate greater sensitivity as
the output signal increases linearly with photosensitive area
while noise increases at the sum of the square of the photosensitive
area. A larger photodiode area has a higher capacitance and
longer carrier diffusion paths adversely affecting bandwidth.
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.
Shot Noise
Shot noise is a statistical fluctuation in any quantized signal
such as photons of light and electrons in current. The magnitude
of the shot noise is expressed as a root mean square (rms) noise
current. Shot noise is a fundamental limitation in photodetectors
and takes the form of
Shot noise = √(2qI
PD
)
where:
q is the charge of an electron (1.602 × 10
−19
Coulomb).
I
PD
is the photodiode current.
Photoplethysmography (PPG)
Photoplethysmography uses light to measure biological
functions by sensing changes in the absorption spectra of soft
tissue due to changes in hemoglobin volume and composition.
Linearity
Linearity is a measure of the deviation from an ideal change
in output current relative to a change in optical power falling
on the sensor. Linearity is specified as the deviation from a best
straight line fit of the current output of the sensor over a speci-
fied range of optical power. Linearity is a critical specification in
PPG measurements due to the requirement of sensing small ac
signals impressed upon large dc offsets.
Static Bias
The ADPD2212 has an internal 10 nA bias that linearizes 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.
Noise Equivalent Power (NEP)
Noise equivalent power is the amount of incident light power
on a photodetector, which generates a photocurrent equal to the
total noise current of the sensor. The noise level is proportional
to the square root of the frequency bandwidth; therefore, NEP
is specified with a 1 Hz bandwidth. NEP is the fundamental
baseline of the detectivity of the sensor.
