Si Switchable Gain Detector
Chapter 4: Operation
Rev D, April 21, 2020
Page 5
Material
Dark Current
Speed
Sensitivity
1
Cost
Silicon (Si)
Low
High
400 - 1000 nm
Low
Germanium (Ge)
High
Low
900 - 1600 nm
Low
Gallium Phosphide (GaP)
Low
High
150 - 550 nm
Med
Indium Gallium Arsenide
(InGaAs)
Low
High
800 - 1800 nm
Med
Extended Range: Indium
Gallium Arsenide (InGaAs)
High
High
1200 - 2600 nm
High
4.4. Bandwidth and Response
For the PDA detectors, the gain of the detector is dependent on the feedback
element (R
f
). The bandwidth of the detector can be calculated using the
following:
𝑓(−3𝑑𝐵) = √
𝐺𝐵𝑃
4𝜋𝑅
𝑓
× 𝐶
𝐷
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
The maximum output of the PDA100A2 is 10 V for high impedance loads (i.e.
R
Load
> 5 kΩ) and 5 V for 50 Ω loads. Adjust the gain so that the measured signal
level out of the PDA100A2 is below 10 V (5 V
with a 50 Ω load) to avoid
saturation.
For low terminating resistors, <5 kΩ or 1% error, an additional factor needs to be
considered. The output of the PDA includes a 50
Ω series resistor (R
s
). The
output load creates a voltage divider with the 50
Ω series resistor as follows:
𝑆𝑐𝑎𝑙𝑒 𝐹𝑎𝑐𝑡𝑜𝑟 =
𝑅
𝐿𝑜𝑎𝑑
𝑅
𝐿𝑜𝑎𝑑
+ 𝑅
𝑠
𝑉
𝑂𝑈𝑇
=
(𝜆) ∗ 𝑇𝑟𝑎𝑛𝑠𝑖𝑚𝑝𝑒𝑑𝑎𝑛𝑐𝑒 𝐺𝑎𝑖𝑛 ∗ 𝑆𝑐𝑎𝑙𝑒 𝐹𝑎𝑐𝑡𝑜𝑟 ∗ 𝐼𝑛𝑝𝑢𝑡 𝑃𝑜𝑤𝑒𝑟 (𝑊)
Note that we already include the scale factor in our specification for the gain at
50 Ω. Refer to the table in Chapter 6 for additional performance specifications.
4.6. Gain Adjustment
The PDA100A2 includes a low noise, low offset, high gain transimpedance
amplifier that allows gain adjustment over a 70 dB range. The gain is adjusted by
rotating the gain control knob, located on the top side of the unit. There are 8
gain positions incremented in 10 dB steps. It is important to note that the
bandwidth will decrease as the gain increases. See the specifications table on
page 7 to choose the best gain verse bandwidth for a given input signal.
1
Approximate values; actual wavelength values will vary.
