THORLABS PDA25K User manual

13050-S01 Rev H 2/10/2012 Page 1 of 6
PDA25K Operating Manual - Switchable Gain, Amplified GaP Detector
Description:
The PDA25K is an amplified, switchable-gain, GaP detector designed for detection of light signals over a 150 –
550 nm wavelength range. An eight-position rotary switch allows the user to vary the gain in 10 dB steps. A buffered
output drives 50load impedances up to 5 volt. The PDA25K housing includes a removable threaded coupler
(SM1T1) and retainer ring (SM1RR) that is compatible with any number of Thorlabs 1” threaded accessories. This
allows convenient mounting of external optics, light filters, apertures, as well as providing an easy mounting
mechanism using the Thorlabs cage assembly accessories.
The PDA25K has two #8-32 tapped mounting holes with a 0.25” mounting depth and includes a 120 VAC power
AC/DC supply. The PDA25K-EC has two M4 tapped mounting holes and includes a 230 VAC AC/DC power supply.
Specifications: General Specifications
Detector GaP PIN Gain Switch 8-Pos Rotary
Active Area 4.8 mm2(2.2 mm x 2.2 mm) On / Off Switch Slide
Surface Depth 0.12” (3.1 mm) Output BNC
Wavelength Range 150 – 550 nm Optical Head Size 2.76” x 2.06” x 0.88”
Peak Wavelength (
p
) 440 nm 70.1 x 52.3 x 22.4 mm
Peak Response 0.12 A/W @ 
p
Weight30.15 / 2.1 lbs
Output Voltage1(50Ω) 5 V (max) Accessories SM1T1, SM1RR
(Hi-Z) 10 V (max) AC Power Supply AC-DC Converter
Output Impedance 50 ΩPower Supply 31W
Max Output Current 100 mA Input Power4100-120 VAC, 50 – 60 Hz
Load Impedance 50Ω– Hi-Z (220 - 240 VAC -EC)
Gain Adj. Range 70 dB Storage Temp -55 to 125 C
Gain Steps 8x 10 dB steps Operating Temp 0 to 40 C
Figure 1 – PDA25K Spectral Responsivity
www.thorlabs.com

13050-S01 Rev H 2/10/2012 Page 2 of 6
Performance Specifications
0dB Setting 40dB Setting
Gain1(Hi-Z): 1.51 x 103V/A ±2% Gain1(Hi-Z): 1.51 x 105 V/A ±2%
(50Ω): 0.75 x 103 V/A ±2% (50Ω): 0.75 x 105 V/A ±2%
Bandwidth: 7.5 MHz Bandwidth: 100 kHz
Noise (RMS): 350 V Noise (RMS): 325 V
NEP (@ 
p
): 3 x 10-10 W/Hz NEP (@ 
p
): 1.6 x 10-11 W/Hz
Offset: 5 mV (10 mV max) Offset: 10 mV (20 mV max)
10dB Setting 50dB Setting
Gain1(Hi-Z): 4.75 x 103 V/A ±2% Gain1(Hi-Z): 4.75 x 105 V/A ±2%
(50Ω): 2.38 x 103 V/A ±2% (50Ω): 2.38 x 105 V/A ±2%
Bandwidth: 3.3 MHz Bandwidth: 32 kHz
Noise (RMS): 290 V Noise (RMS): 500 V
NEP (@ 
p
): 1 x 10-10 W/Hz NEP (@ 
p
): 1.7 x 10-11 W/Hz
Offset: 6 mV (12 mV max) Offset: 15 mV (40 mV max)
20dB Setting 60dB Setting
Gain1(Hi-Z): 1.5 x 104 V/A ±2% Gain1(Hi-Z): 1.5 x 106 V/A ±5%
(50Ω): 0.75 x 104 V/A ±2% (50Ω): 0.75 x 106 V/A ±5%
Bandwidth: 1.0 MHz Bandwidth: 11 kHz
Noise (RMS): 260 V Noise (RMS): 675 V
NEP (@ 
p
): 4 x 10-11 W/Hz NEP (@ 
p
): 1 x 10-11 W/Hz
Offset: 6 mV (15 mV max) Offset: 20 mV (75 mV max)
30dB Setting 70dB Setting
Gain1(Hi-Z): 4.75 x 104 V/A ±2% Gain1(Hi-Z): 4.75 x 106 V/A ±5%
(50Ω): 2.38 x 104 V/A ±2% (50Ω): 2.38 x 106 V/A ±5%
Bandwidth: 300 kHz Bandwidth: 3.3 kHz
Noise (RMS): 245 V Noise (RMS): 900 V
NEP (@ 
p
): 2 x 10-11 W/Hz NEP (@ 
p
): 7 x 10-12 W/Hz
Offset: 8 mV (15 mV max) Offset: 40 mV (200 mV max)
Note 1: The PDA25K has a 50series terminator resistor (i.e. in series with amplifier output). This forms a voltage divider with
any load impedance (e.g. 50load divides signal in half).
2: All measurements performed with a 50load unless stated otherwise.
3: Detector Package / Power Supply.
4: Although the power supply is rated for 31W the PDA25K actual usage is <5W over the full operating range.
Setup
Unpack the optical head, install a Thorlabs TR-series ½” diameter post into one of the #8-32 (M4 on -EC
version) tapped holes, located on the bottom and side of the head, and mount into a PH-series post holder.
Connect the power supply 3-pin plug into the power receptacle on the PDA25K.
Plug the power supply into a 50 – 60 Hz, 100 – 120 VAC outlet (220 – 240 VAC for -EC version).
Attach a 50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 50resistor (Thorlabs p/n T4119) for maximum
performance. Connect the remaining end to a measurement device such as an oscilloscope or high speed DAQ
card. Caution: Many high speed oscilloscopes have input impedances of 50. In this case, do not install a 50
terminator. The combined loads will equal 25which could allow ~135 mA of output current. This will damage
the output driver of the PDA25K.
Power the PDA25K on using the power switch located on the top side of the unit.
Install any desired filters, optics, adapters, or fiber adapters to the input aperture. Caution: The PDA25K was
designed to allow maximum accessibility to the photodetector by having the front surface of the diode flush with
the 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 PDA36A) inside the 1” threaded coupler before installing
the fiber adapter.
Apply a light source to the detector. Adjust the gain to the desired setting.

13050-S01 Rev H 2/10/2012 Page 3 of 6
Operation
Thorlabs PDA series are ideal for measuring both pulsed and CW light sources. The PDA25K includes a reverse-
biased PIN photo diode, mated to a switchable gain transimpedance amplifier, and packaged in a rugged housing.
The thin profile housing allows the PDA25K to fit into small spaces. All controls and connections are orthogonal to
the light path providing easier access and minimizing the light path footprint.
Output
The maximum output of the PDA25K is 10 volts for high impedance loads (i.e. RLOAD > 5k) and 5 volts for 50
loads. Adjust the gain so that the measured signal level out of the PDA25K is below 10 volts (5 volts with a 50load)
to avoid saturation. If necessary, use external neutral density filters to reduce the input light level. The BNC output
signal is buffered with an amplifier capable of driving 50loads. A 50series resistor is included on the output to
impedance match a 50coax cables. For best performance, Thorlabs recommends operating the PDA25K with a
50terminating load located at the end of the coax cable. While this is not necessary, it eliminates ringing and
distortion due to impedance mismatches.
Gain Adjustment
The PDA25K includes a low noise, low offset, high gain transimpedance amplifier that allows gain adjustment over a
70dB 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 10dB steps. It is important to note that the bandwidth will decrease as the gain
increases. See the specifications above to choose the best gain vs. bandwidth for a given input signal.
Light to Voltage Conversion
The Spectral Responsivity, (), can be obtained from Figure 1 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 GaP
detector with the transimpedance gain as shown below:
E.g. 1 –Output [V/W] = transimpedance gain [V/A] x () [A/W]
For low terminating resistors, <5kor 1% error, an additional factor needs to be included in the above formula. As
described above the output includes a 50series resistor (RS). The output load creates a voltage divider with the
50series resistor as follows:
E.g. 2 – Scale Factor = RLOAD / (RLOAD + RS)
Where RLOAD is the terminating resistor and RS= 50. For a standard 50terminator, the gain will be scaled by ½ as
shown below:
Scale Factor = 50/ (50+ 50) = 0.5
E.g. 3 – Output [V/W] = transimpedance gain [V/A] x () [A/W] x Scale Factor
Maintaining the PDA25K
There are no serviceable parts in the PDA25K 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 isopropyl alcohol and optical grade
wipes. If you suspect a problem with your PDA25K please call Thorlabs and technical support will be happy to assist
you.

13050-S01 Rev H 2/10/2012 Page 4 of 6
PDA25K Drawing
1.89
48
1.70 43.2
2.06 52.3
0.79 20.1
2.10 53.3
2.76 70.1
0.535-40 INTERNAL THREAD MATES WITH SM05 LINE
#8-32 X 0.25" THREAD
1.08 27.4
0.80 20.3
ROTARY GAIN SWITCH 8 POSITION
POWER SWITCH
0.50 12.7
0.88 22.4
A
1.035-40 EXTERNAL THREAD MATES WITH SM1 LINE
OUTPUT BNC WITH
0 TO 10v RANGE
LED POWER
INDICATOR
POWER SUPPLY
CONNECTOR
0.12
3.1
DETAIL A
SCALE 2 : 1
DETECTOR SURFACE

13050-S01 Rev H 2/10/2012 Page 5 of 6
Regulatory
As required by the WEEE (Waste Electrical and Electronic Equipment Directive) of the European Community and the
corresponding national laws, Thorlabs offers all end users in the EC the possibility to return “end of life” units without
incurring disposal charges.
This offer is valid for Thorlabs electrical and electronic equipment:
Sold after August 13, 2005
Marked correspondingly with the crossed out “wheelie bin” logo (see right)
Sold to a company or institute within the EC
Currently owned by a company or institute within the EC
Still complete, not disassembled and not contaminated
As the WEEE directive applies to self-contained operational electrical and electronic products, this end of life take
back service does not refer to other Thorlabs products, such as:
Pure OEM products, that means assemblies to be built into a unit by the user (e. g. OEM laser driver cards)
Components
Mechanics and optics
Left over parts of units disassembled by the user (PCB’s, housings etc.).
If you wish to return a Thorlabs unit for waste recovery, please contact Thorlabs or your nearest dealer for further
information.
Waste Treatment is Your Own Responsibility
If you do not return an “end of life” unit to Thorlabs, you must hand it to a company specialized in waste recovery. Do
not dispose of the unit in a litter bin or at a public waste disposal site.
Ecological Background
It is well known that WEEE pollutes the environment by releasing toxic products during decomposition. The aim of
the European RoHS directive is to reduce the content of toxic substances in electronic products in the future.
The intent of the WEEE directive is to enforce the recycling of WEEE. A controlled recycling of end of life products
will thereby avoid negative impacts on the environment.
Wheelie Bin Logo

13050-S01 Rev H 2/10/2012 Page 6 of 6
Thorlabs Worldwide Contacts
USA, Canada, and South America
Thorlabs, Inc.
56 Sparta Avenue
Newton, NJ 07860
USA
Tel: [phone]
Fax: [phone]
www.thorlabs.com
www.thorlabs.us (West Coast)
Email: [e-mail]
Support: [e-mail]
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85221 Dachau
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Fax: [phone]
www.thorlabs.de
Email: [email protected]om
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Cambridgeshire CB7 4EX
Great Britain
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Fax: [phone]
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Email: [e-mail]
Support: [e-mail]
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78600 Maisons-Laffitte
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Email: [email protected]
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412 63 Göteborg
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Higashi-Ikebukuro Q Building, 1F
2-23-2, Higashi-Ikebukuro,
Toshima-ku, Tokyo 170-0013
Japan
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Lane 2891, South Qilianshan Road
Putuo District
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China
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Email: [email protected]