THORLABS PDAPC1, User Manual, Page: 11 / 18

Si Switchable Gain Detector, OEM Chapter 4: Operation
Rev A, April 5, 2021 Page 9
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
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
The maximum output of the PDAPC1 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 PDAPC1 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 PDAPC1 includes a low noise, low offset, high gain transimpedance amplifier
that allows gain adjustment over a 70 dB range. The gain is adjusted by selecting
the correct gain switch settings (see page 3). 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 11 to choose the best
gain versus bandwidth for a given input signal.
1
Approximate values; actual wavelength values will vary.