Bandwidth And Response; Terminating Resistance - THORLABS PDA10CS User Manual

InGaAs Switchable Gain Detector
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
using 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
Depending on the type of the photodiode, load resistance can affect the
response speed. For maximum bandwidth, we recommend using a 50 Ω coaxial
cable with a 50 Ω terminating resistor at the opposite end of the cable. This will
minimize ringing by matching the cable with its characteristic impedance. If
bandwidth is not important, you may increase the amount of voltage for a given
light level by increasing R
coaxial cable can have a profound impact on the response, so it is recommended
to keep the cable as short as possible.
The maximum output of the PDA10CS(-EC) is 10 volts for high impedance loads
(i.e. R > 5 kΩ) and 5 volts for 50 Ω loads. Adjust the gain so that the
Load
measured signal level out of the PDA10CS(-EC) is below 10 volts (5 volts with a
50 Ω load) to avoid saturation.
For low terminating resistors, <5 kΩ or 1% error, an additional factor needs to be
considered. As described above the output includes a 50Ω series resistor (R
The output load creates a voltage divider with the 50Ω series resistor as follows:
1
Approximate values, actual wavelength values will vary from unit to unit
Rev G, July 15, 2017
Dark
Current
Low
High
Low
Low
High
). The bandwidth of the detector can be calculated
F
3
. In an unmatched termination the length of the
LOAD
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
Page 5
).
S