THORLABS PDA36A, User Manual

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Si Switchable Gain Detector Chapter 4: Operation
Rev E, July 15, 2017 Page 5
Material
Dark
Current
Speed
Sensitivity 1
(nm)
Cost
Silicon (Si) Low High 400 – 1000 Low
Germanium (Ge) High Low 900 – 1600 Low
Gallium Phosphide (GaP) Low High 150 – 550 Med
Indium Gallium Arsenide
(InGaAs)
Low High 800 – 1800 Med
Extended Range: Indium
Gallium Arsenide (InGaAs)
High High 1200 – 2600 High
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
F
). The bandwidth of the detector can be calculated
using the following:
3
4
Where GBP is the amplifier gain bandwidth product and C
D
is the sum of the
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
OUT
) for viewing on an oscilloscope:
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
LOAD
. In an unmatched termination the length of the
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 PDA36A(-EC) is 10 volts for high impedance loads
(i.e. R
Load
> 5 k Ω ) and 5 volts for 50 Ω loads. Adjust the gain so that the
measured signal level out of the PDA36A(-EC) is below 10 volts (5 volts with a
50
Ω load) to avoid saturation.
For low terminating resistors, <5 k Ω or 1% error, an additional factor needs to be
considered. As described above the output includes a 50 Ω series resistor (R
S
).
The output load creates a voltage divider with the 50 Ω series resistor as follows:
1
Approximate values, actual wavelength values will vary from unit to unit.