Dark Current; Bandwidth And Response; Terminating Resistance - THORLABS PDA10D User Manual

InGaAs Amplified Detector

4.3. Dark Current

Dark current is leakage current which flows when a bias voltage is applied to a
photodiode. The PDA with Transimpedance Amplifier does control the dark current
flowing out. Looking at the figure above, it can be noted that Point B is held at
ground and the amplifier will try to hold point A to "Virtual Ground". This minimizes
the effects of dark current present in the system.
The dark current present is also affected by the photodiode material and the size
of the active area. Silicon devices generally produce low dark current compared to
germanium devices which have high dark currents. The table below lists several
photodiode materials and their relative dark currents, speeds, sensitivity, and
costs.
Material
Silicon (Si)
Germanium (Ge)
Gallium Phosphide (GaP)
Indium Gallium Arsenide
(InGaAs)
Extended Range: Indium
Gallium Arsenide (InGaAs)

4.4. Bandwidth and Response

A load resistor will react with the photodetector junction capacitance to limit the
bandwidth. For best frequency response, a 50 Ω terminator should be used in
conjunction with a 50 Ω coaxial cable. The gain of the detector is dependent on
the feedback element (R
the following:
Where GBP is the amplifier gain bandwidth product and C
photodiode junction capacitance and the amplifier capacitance.

4.5. Terminating Resistance

A load resistance is used to convert the generated photocurrent into a voltage
(V ) for viewing on an oscilloscope:
OUT
1
Approximate values, actual wavelength values will vary from unit to unit
Rev G, December 19, 2017
Dark
Current
Low
High
Low
Low
High
). The bandwidth of the detector can be calculated using
F
3
Sensitivity
Speed
High 400 – 1000
Low 900 – 1600
High 150 – 550
High 800 – 1800
High 1200 – 2600
4
Chapter 4: Operation
1
Cost
(nm)
Low
Low
Med
Med
High
is the sum of the
D
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