Operation; Gain Switch; Fiber Adapters And Other Accessories; Maintaining The Pda400 - THORLABS PDA400 Operating Manual

•
Attach a 50Ω coax cable (i.e. RG-58U) to the output of the PDA. When running cable lengths longer than 12" we
recommend terminating the opposite end of the coax with a 50Ω resistor (Thorlabs p/n T4119) for maximum
performance.

Operation

•
The PDA400 gain is adjusted using a small slotted screwdriver to turn the internal, gain-setting rotary switch. An
access hole labeled GAIN is provided on the rear panel for this purpose. The gain is set to 0dB, when the slot is
aligned counterclockwise as far as it will go. Each clockwise click of the switch increases the gain by 10 dB. Do
not use excessive force when adjusting the gain switch.
•
The PDA400 is switched on by the POWER toggle switch on the rear of the head.
•
The light to voltage conversion can be estimated by factoring the wavelength-dependent responsivity of the
InGaAs detector with the transimpedance gain
(e.g. output in volts / watt = transimpedance gain (V/A) x responsivity (A/W) )
•
The maximum output of the PDA400 is 10 volts for high impedance loads (5V for 50Ω loads). Adjust the gain so
that the measured signal level out of the PDA400 is below 10 volts (5 volts with a 50Ω load) to avoid saturation.
If necessary, use external neutral density filters to reduce the input light level.
•
For maximum linearity performance when measuring focused beams, fiber outputs, or small diameter beams, do
not exceed a maximum intensity of 10mW/cm
•
Because of the finite gain-bandwidth performance common to all amplifier circuits, the bandwidth of the PDA400
goes down with increased gain settings.

Fiber Adapters and Other Accessories

Thorlabs sells a number of accessories that are compatible with the 1" thread on the PDA housing including FC,
SMA, and ST fiber adapters, stackable lens tubes for mounting optics, and cage assemblies that allow the PDA to
be incorporated into elaborate 3-D optical assemblies.
Caution: The PDA400 was designed to allow maximum accessibility to the photodetector by having the front surface
of the diode extend outside of the PDA housing. When using fiber adapters, make sure that the fiber ferrule does not
crash into the detector. Failure to do so may cause damage to the diode and / or the fiber. An easy way to
accomplish this is to install a SM1RR retaining ring (included with the PDA55) inside the 1" threaded coupler before
installing the fiber adapter.
Also available in the PDA series are and higher bandwidth silicon and InGaAs models.

Maintaining the PDA400

There are no serviceable parts in the PDA400 optical head or power supply. The housing may be cleaned by wiping
with a soft damp cloth. The window of the detector should only be cleaned using optical grade wipes. If you suspect
a problem with your PDA400 please call Thorlabs and an engineer will be happy to assist you.
2
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Gain Switch

Gain
position (dB)
1 0
2 10
3 20
4 30
5 40
Table 1. Gain Settings
2151-D02 REV F 8/15/2005
Page 2 of 5
Transimpedance Gain
(V/A)
4
1.5 x 10
4
4.7 x 10
5
1.5 x 10
5
4.7 x 10
6
1.5 x 10