FPD-IG-175, FPD-UV-3000, FPD-VIS-300 User Manual
8
VIII.
Schematic: Fast Photodiode Detectors
IX.
Glossary of Terms
Bandwidth:
The range of frequencies from 0 Hz (DC) to the frequency at which the responsivity
decreases by 3dB. Bandwidth and rise time can be approximately related by the equation:
Bandwidth ≈ 0.35/rise time for a Gaussian pulse input.
Bias Voltage:
’s junction capacitance can be modified by applying a reverse voltage. The
bias voltage reduces the junction capacitance, which improves the
temporal response. The
bias voltage also induces a dark current. When a photodiode is used with reverse bias, the shot noise of the
dark current is usually the dominant contributor to the NEP.
Dark Current (I
dark
):
When a photodiode is connected in a circuit and operated in reverse bias mode, a
small DC current (typically nanoamps) will flow, even without the presence of an optical signal.
Disconnecting the coaxial cable will prevent this current from flowing. If operated without a reverse bias
(photovoltaic mode) the dark current will be comprised of thermally generated noise without any DC
component. This thermal noise will typically be orders of magnitude lower than the shot noise of the dark
current in the same device when it is reverse biased.
Decoupling Capacitor:
A capacitor is connected in parallel with the bias voltage source. Its purpose is
twofold: 1) It maintains the bias voltage when fast signal pulses would otherwise cause the battery voltage
to droop (this would slow the response time of the
photodiode
) and 2) It also acts as a low-pass filter for
external power supplies.