THORLABS PDA10JT, User Manual

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Amplified MCT Detector Chapter 5: Operation
Rev G, March 11, 2019 Page 7
5.3. Bandwidth Filter Adjustment
The PDA10JT also includes an adjustable low pass filter with settings from 1.25
kHz to 160 kHz in 8 steps. This filter allows the user to optimize the PDA10JT to
operate at the lowest amount of high frequency optical and electrical noise. The
filter is adjusted by rotating the filter control knob, located on the side of the unit.
To adjust the filter, follow the steps below.
1. Determine the maximum bandwidth required.
2. Set the filter bandwidth switch setting just above the desired
bandwidth.
Note that system signal bandwidth is 80 KHz. The 160 KHz setting is provided to
eliminate high frequency noise introduced above this limit.
5.4. Thermoelectric Cooler
The thermoelectric cooler built into the detector is factory set to cool the detector
to -30 °C with thermistor feedback to stabilize the temperature. The housing is
used as a heat sink and a fan is used to increase the cooling capacity. It is
important to note that the cooling fan will keep the heat sink at room temperature.
Without it, the heat sink will heat up causing a higher temperature drop between
the detector element and the heat sink, resulting in larger TEC currents. Without
the fan, the TEC current will operate at its limit (~820 mA) and the detector element
will no longer be temperature stabilized. Offsets will increase and fluctuate, and
output noise will increase. For best results, do not block, limit airflow, or stop the
cooling fan. This operation is automatic and requires no input or adjustment by the
user.
5.5. Light-to-Voltage Conversion
The Spectral Responsivity,

(

), can be obtained from Figure 6 on page 13 to
estimate the amount of output voltage to expect. The light-to-voltage conversion
can be estimated by factoring the wavelength-dependent responsivity of the
HgCdTe detector with the gain as shown below:
V
out
(V) = Gain 𝑉 𝑉
⁄ ∗ ℜ(λ) 𝑉 𝑊 ⁄ ∗ Input Power (W)
For low terminating resistors, <5 k
Ω or 1% error, an additional factor needs to be
included in the above formula. 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:
Scale Factor
=
𝑅
𝐿𝑂𝐴𝐷
(𝑅
𝐿𝑂𝐴𝐷
+ 𝑅
𝑆
)
Where, R
LOAD
is the terminating resistor and R
S
= 50
Ω. For a standard 50 Ω
terminator, the gain will be scaled by ½ as shown below:
Scale Factor
=
50 Ω
(50 Ω + 50 Ω)
= 0.5
𝑉
𝑜𝑢𝑡
(𝑉) = 𝐺𝑎𝑖𝑛 𝑉 𝑉
⁄ ∗ ℜ(𝜆) 𝑉 𝑊 ⁄ ∗ Input Power (W) ∗ 𝑆𝑐𝑎𝑙𝑒 𝐹𝑎𝑐𝑡𝑜𝑟