FPD-IG-175, FPD-UV-3000, FPD-VIS-300 User Manual
9
Noise Equivalent Power (NEP):
NEP
refers to the amount of optical input power that would generate an
electric signal that is equal in magnitude to the electrical noise (in the dark). It is a function of the device
responsivity and the dark current. Shot noise (
I
shot
) is a source of noise generated by current; in the case
of reversed biased diodes operating at high speeds and low signal levels, it is the dominant contributor to
the total noise. NEP can be calculated from the shot noise and responsivity. For example, for the FPD-
UV-3000 with a dark current of <10nA and responsivity of 0.58 A/W at 890 nm, the shot noise will be:
𝐼
𝑠ℎ𝑜𝑡
= √2𝑞 𝐼
𝑑𝑎𝑟𝑘
= √2 × 1.6 × 10
−19
𝐴𝑠 × 10 × 10
−9
𝐴 = 0.057 𝑝𝐴√𝑠 = 0.057 𝑝𝐴/√𝐻𝑧
𝑁𝐸𝑃
𝜆
=
𝐼
𝑠ℎ𝑜𝑡
𝑅
𝜆
=
0.057 𝑝𝐴/√𝐻𝑧
0.58 𝐴/𝑊
= 0.098 𝑝𝑊/√𝐻𝑧
at 890 nm
q
= electron charge =
1.6 × 10
−19
𝐴𝑠
Photodiode:
A semiconductor device that converts photons into an electrical current.
Responsivity:
In amps per watt (A/W), responsivity is the current output of the photodiode per input
power, and is determined by the diode structure. Responsivity varies with wavelength and diode
material.
Responsivity (Irradiance):
In V per W/cm
2
is the voltage output as a function of the light input
irradiance. This is equal to the responsivity in units of A/W multiplied by the photodiode sensitive area
and the effective impedance load seen by the photodiode.
Rise Time & Fall Time:
Rise Time is the time required by a signal to change from a specified low value
to a specified high value (typically 10% to 90%). Fall Time is the time taken for the amplitude of a pulse
to decrease from the specified high value to the specified low value. In photodiodes, the junction
capacitance is a major factor contributing to the rise time and fall times. Reverse biasing of the
photodiode is a method used to decrease the junction capacitance and improve the rise and fall times.
FPD-IG-175, FPD-UV-3000, FPD-VIS-300 User Manual
18 June 2020
Rev 1.2-2
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