THORLABS PDB440A, Operation Manual, Page: 10 / 29

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PDB440x and PDB450x detectors
3.3 Electrical Outputs
The Thorlabs PDB440x and PDB450x detectors has three SMA output connectors:
·
M
·
MONITOR -
· RF OUTPUT
RF OUTPUT delivers an output voltage proportional to the difference between the photo cur-
rents of the two photodiodes This voltage can by calculated to:
with: Â ( l ) - responsivity of the photo diode at given wavelength
P
opt,1
and P
opt,2
- optical input power
G - transimpedance gain of the RF output
The responsivity  ( l ) for a given wavelength can be read from the individual curves in section
Technical Data to estimate the RF OUTPUT voltage. Please note that the given responsivity
curves represent typical values - individual responsivity may deviate.
The maximum output voltage swing of the RF OUTPUT can be found in the Technical Data
section.
The optical input saturation power (see the PDB440x and PDB450x detectors individual tech-
nical data in the appendix) of the balanced detector is the minimum value, as it is given for the
wavelength with the detector highest responsivity. At other wavelengths, saturation will be
reached at higher input power levels. The output signal should not exceed the maximum output
voltage to avoid saturation. Therefore the optical input power (or the power difference between
the optical inputs) should not exceed CW Saturation Power listed in Specifications.
MONITOR Outputs
The signal monitor outputs ( and MONITOR- ) allow to observe the input power level
and can be used as individual power indicators. These outputs can also be used to measure
low frequency modulated signals on each detector separately. The maximum output voltage
swing of the
MONITOR output is +10 V for high impedance loads (+1.5 V into 50
W
). Saturation
of MONITOR outputs will occur at optical input power level greater than 1 mW, depending on
the detector's wavelength response.
MONITOR outputs can be used to roughly adjust equal input power levels on each detector for
balanced operation. While the DC component of the RF OUTPUT in balanced mode is zero,
the MONITOR outputs provide capability to independently observe the individual optical input
power. MONITOR outputs of the unit are also convenient to use for free-space beam align-
ment.
The amplifier offset voltage is factory set to zero at 23°C ambient temperature. A small drift dur-
ing a short warm-up period (~5min) may occur. For exact DC light level measurements a con-
stant temperature environment is recommended.
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