Preamplifier Short Form Catalog in PDF Format

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Preamplifiers

This section describes the amplifier circuits recommended for Judson detectors. The PA-5, PA-6, PA-7 and PA-9 series preamplifiers are current gain amplifiers recommended for photovoltaic detectors and applications. Voltage mode preamplifiers, for use with our photoconductive detectors, include the PA-101, PA-8200 and PA-300 series preamps. Some general information on our preamplifiers follows:

Noise Sources: Figure 1 shows the various noise sources of the detector/preamp system. Values for the preamp noise sources en, in, Vos and ib are listed in the specification tables for each Judson current-mode preamplifier. The preamp noise sources, together with the detector characteristics, determine the system noise. While a complete analysis of detector system noise is beyond the scope of this guide, the effects of the various noise sources can be summarized by the following approximation:

Total e_n(f)  =  [(e_n²/Z_D²) + i_n² + (4kT/R_D) + (4kT/R_F)]^1/2 Z_F
where k is Boltzmann's constant and T is temperature in degrees Kelvin.
　

This simplified noise equation provides a good approximation of the total voltage noise density (V/Hz^1/2) at the preamplifier output. Note that the noise is dependent on the frequency f, and is normalized to a 1 Hz noise bandwidth.  The four terms in the brackets represent the four main sources of current noise:

• Preamplifier noise voltage e_n divided by the detector reactance Z_D, where

                Z_D = R_D/(1+ (2pf)² C_D² R_D²)^1/2

• Preamplifier current noise i_n

• Johnson thermal current noise from the detector shunt resistance R_D

• Johnson thermal current noise from the preamp feedback resistance R_F

The total current noise is then multiplied by the transimpedance gain Z_F, where

                Z_F = R_F/(1+ (2pf)² C_F² R_F²)^1/2

Analysis of the simplified noise equation shows the following:

• In situations where Z_D is large (>10Kohm) the preamplifier current noise i_n is more important than the voltage noise e_n. This is generally the case when using high-impedance detectors (InSb, cooled Ge, small-area Ge) at moderate frequencies. Choose a preamp with low i_n.

• In situations where Z_D is small (<1Kohm), the preamp voltage noise e_n becomes more important. This is generally true with low-impedance detectors (InAs, large-area Ge). Choose a preamp with low e_n.

• Larger R_F adds less current noise. For highest sensitivity, R_F should be greater than R_D when practical.

Preamp Noise Figure: A general method for evaluating noise performance of a preamplifier is the noise figure, N_F, which indicates what portion of the system noise is caused by the preamp.

        N_F   =  10 log₁₀ [Total Noise / Detector Noise]

A perfect preamplifier has a Noise Factor of 0 dB, indicating that the preamp noise contribution is negligible compared to the detector noise. A N_F of 0.1 to 3 dB is considered satisfactory. Preamps with N_F >3 dB add significant noise to the system.  See Fig. 6 for noise figures of Judson transimpedance preamplifiers at 1 KHz.

DC Applications - Offset Drifting:  In DC applications, the preamp input bias current I_b and input offset voltage V_os become important. In an ideal op-amp, I_b and V_os are zero. In reality they have non-zero values. Together with the detector R_D they produce a "dark current" I_D:

            I_D = I_b + (V_os/R_D)

The DC offset voltage at the preamp output is equal to I_D x R_F.  I_b and R_D each have a non-linear dependence on temperature. The offset voltage at the preamplifier output will therefore drift with temperature changes.  To minimize offsets and drifting:

For high-impedance detectors, choose a preamp with low Ib. For low-impedance detectors, choose a preamp with low V_os.  Consider stabilizing the detector temperature by using one of Judson's integral TE-cooler packages. The transimpedance (or current-mode) preamplifier circuit of Fig. 1 is recommended for most PV detector applications, for frequencies up to 1 MHz. It offers lowest noise and best linearity under a wide range of conditions.   The characteristics of the op-amp circuit maintain the diode near 0V bias.   All the photocurrent from the detector essentially flows through the feedback resistor RF. The feedback capacitance CF is added to control gain peaking (Fig. 2). The value of CF depends on the detector capacitance. It is installed at the factory to provide stable preamplifier performance with a particular detector model.  The values of RF and CF, together with the detector characteristics RD and CD, determine the overall frequency response of the system (Figs. 3, 4, 7, 8).

Figure 1	Figure 2

PA-9 Preamplifier

The PA-9 preamplifier is ideal for high-frequency performance with high-impedance photovoltaics such as cryogenically cooled InSb and Ge. The PA-9 offers low current noise and ultra-low voltage noise. However, its relatively high DC offset voltage makes it less suitable for DC applications than other Judson preamps. The PA-9 has fixed gain. When ordered with a detector, the preamp is matched to the detector for maximum gain and sensitivity. Alternatively, the customer may specify gain or minimum required bandwidth.  Bandwidth is a function of detector resistance and capacitance as well as preamp gain (Figs. 3 and 4).

Figure 3	Figure 4

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PA-5, PA-6, PA-7 Preamplifiers

Current Mode Preamplifiers convert the current output of a photovoltaic Ge, InAs, or InSb detector into a voltage output. They amplify the signal for subsequent use with oscilloscopes, lock-in amplifiers, or A-to-D converters.  Three different preamp models each offer specific advantages, depending on detector type and bandwidth requirements. A comparison of preamp noise figure as a function of detector reactance is graphed in Fig. 6.  All units (except multi-channel models) have switch-selectable gain.

The PA-7 is an excellent general purpose preamplifier for most high shunt resistance (R_D > 25Kohm) detectors, including small area J16 Series Ge and all J16TE2 Series cooled Ge. It has extremely low current noise and current offset.  For most applications, the PA-7-70 with high gain of 10⁷ V/A offers best performance and versatility. However, for applications where 10⁷ V/A gain is unusable (due to bandwidth or DC saturation), the PA-7-60 or PA-7-50 are suitable alternatives.

The PA-6 is a general purpose preamplifier recommended for intermediate shunt resistance (400ohm<R_D<50Kohm) detectors, including large area J16 Series room temperature Ge. The PA-6 has very low voltage noise and offset voltage, which significantly reduces low-frequency noise and DC drift. Standard gain settings are listed in the specification table below; custom gain settings are available.

The PA-5 is recommended for low impedance detectors (R_D<400ohm), including J12 Series room temperature InAs and J12TE2 Series InAs. It has extremely low voltage noise and low voltage offset. However, its high current noise and current offset make it unsuitable for detectors with high impedance.   Standard gain is 10⁵, 10⁴, and 10³ V/A (switch-selectable). Custom gain settings are available.

Figure 5

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Figure 6	Figure 7

Figure 8	Figure 9

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PA-7:4C, PA-7:16C and PA-7:32C Multi-Channel Preamplifiers

The PA-7:4C, PA-7:16C and PA-7:32C Series multi-channel preamplifiers are designed primarily for use with Judson's NIR Array Series and X-Y Sensors. The preamp gain is fixed as specified at the time of purchase. Standard gain settings are 107 or 106 V/A; others are available on a custom basis. While zero-volt bias is recommended for J16P Series arrays in most applications, the preamp is also available with an optional detector bias adjust. Biasing the photodiodes improves response time and high-power linearity, but also increases dark current.

Figure 10

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Voltage Mode Preamplifiers

Voltage Mode Preamplifiers may be used with photoconductive HgCdTe or with low-impedance photovoltaics such as InAs. With photoconductive detectors, a constant bias current or constant bias voltage is applied across the detector element. The element changes resistance in response to incident photons, and the resulting change in voltage is amplified by the preamp. A blocking capacitor or DC offset circuit is required to block the constant DC bias. With photovoltaic detectors, the photocurrent generated in the detector induces a voltage across the preamp input impedance. This voltage is amplified. A lower input impedance generally results in faster frequency response, but also adds more noise to the system.

PA-101 HgCdTe Preamplifier (5 Hz - 1 MHz): The Model PA-101 low-noise voltage preamplifier is recommended for all J15 Series HgCdTe detectors. An external bias resistor is used to set the constant bias current required for PC detector operation. When purchased with a detector, the preamp includes a bias resistor factory-selected for optimum detector performance. When ordering the preamp separately, please specify detector resistance and required bias current. The Model PA-101 may also be used without bias for J12 Series InAs.

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PA-8200 PbS and PbSe Preamplifier: The Model PA-8200 low-noise voltage preamplifier is recommended for all J13 and J14 Series detectors. A load resistor is selected to match the detector resistance. Preamp gain and typical bandwidth specifications are listed in the table opposite. For best results, choose the preamp model with the narrowest suitable bandwidth to keep preamp noise to a minimum.

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PA-300 HgCdTe Preamplifier (DC - 1.0 MHz): The Model PA-300 current preamplifier is designed for operation with J15D Series HgCdTe detectors. The PA-300 is designed using a bridge circuit on the front end of an operational amplifier to deliver constant bias voltage across the detector. The PA-300 is recommended for detectors used over a wide dynamic range in applications including FTIR's and laser monitoring. The PA-300 also has a first order linearity correction in the form of a positive feedback resistor.

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Preamplifier Equivalent Circuits shown below.

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Dewars, Connectors, Flanges and Electronics

Repumpable metal dewars are recommended for laboratory or R&D use. Advantages include rugged construction, low cost, long hold time, optional mounting flanges, a wide range of window materials, and shorter delivery time. The specified hold time is guaranteed for one year; re-evacuation may be required every year or two thereafter. Long life, permanently sealed glass dewars offer the advantages of small size and superior performance under mechanical vibration. Glass dewars generally require longer delivery times. Dewars with custom configurations or longer hold times can be provided.

Figure 1	Figure 2

Cold Stops: The cold stop is a field of view (FOV) limiting aperture, cryogenically cooled to block unwanted background radiation from reaching the detector. Judson defines the field of view as the angle f, which is two times the half angle defined by the cold stop and the edge of the detector (Fig. 1). Radiation at larger angles is blocked by the cold stop. Thus, the entire active area of the detector can see radiation entering from a cone angle of f. Theoretical D* improvement for BLIP detectors is more accurately determined by the angle q from the center of the detector. Note: Fig. 1 shows theoretical D* improvement for background limited (BLIP) detectors only. Actual improvement may be less than theoretical, particularly for large area detectors (1mm diameter or larger). When ordering a field of view, always specify the largest cone angle that will be needed for your optical system. Common field of view specifications include 30°, 45° and 60°. If the field of view is not specified by the customer, a standard 60° field of view aperture will be used.

Cold Filters: Judson's cold filters are mounted on the FOV and block background radiation from unwanted wavelengths. Cold filters can significantly improve D* for background limited (BLIP) detectors. Standard cold filters include the SP28 which has transmission from visible to 2.8µm. The SP35 has transmission from 1.7 to 3.5µm. Custom cold filters are also available for a wide range of wavelengths and bandwidths in the infrared. Typically, these cold filters come from stock overruns of major filter manufacturers. We can also mount cold filters supplied by the customer.

Metal Dewar Reliability: Judson's standard metal dewars are designed for long hold time and long life. Each metal dewar undergoes extensive leak testing before and after assembly. The dewars also go through extensive prebaking and postbaking after detector mounting to eliminate residual water vapor and outgassing. For FTIR applications, each unit is 100% tested for less than 1% ice band absorption after eight hours of operation. Each standard dewar comes with an SMA to BNC coax output cable and LN2 filled funnel.

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Windows: Standard windows for Judson detectors are listed below, along with the Judson model number codes. Transmission curves are shown in Figure 3. All windows are .040" thick. Windows for FTIR or other coherent light applications have a nominal 20' wedge angle (0.33°) or a 1° wedge angle to prevent fringing interference.

Figure 3

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Metal Dewar Features

• Rugged Construction
• > 10 Hour Hold Time
• One Year Warranty
• Repumpable
• Accurate Centering of Detector
• Sideview and Downview

M204 or M205 Standard Metal Dewars: The M204 sideview and M205 downview are the recommended standard metal dewars for all Judson cryogenically cooled detectors, including J16D Ge, J10D InSb, and J15D HgCdTe. An SMA to BNC coax output cable and LN2 fill funnel are included with each unit.

Figure 4

Figure 5

M200 or M201 Standard Metal Dewars: The M200 sideview and M201 downview dewars are similar to the M204 and M205 respectively, with the exception of dewar height and LN2 hold time. The M200 and M201 are 6.75" tall (Dimension "A") for >15 hours LN2 hold time.

Detector centered to within ±0.020 to O.D. of window retaining nut (at 77°K operating temperature).

M108 Metal Dewar with SMA Connectors: The M108 sideview metal dewar has two SMA connectors mounted directly to the dewar body instead of the backplate options. This configuration helps minimize the amount of space required for mounting the dewar in an optical system. The shield of the detector SMA connector is normally
grounded to the dewar body (see "Dewar Grounding and EMI"). They may be isolated from the dewar on request. The dewar ground pin allows independent grounding of the dewar body if needed.

M209 Metal Dewar for 24-Hour LN2 Hold Time: The M209 sideview metal dewar provides convenient, 24-hour hold time in a 3.5" diameter x 8" tall package (Fig. 7).

Dewar Cables: An SMA-to-BNC coaxial cable (Fig. 8) for connecting the detector SMA to a preamp is provided with each Judson metal dewar. A funnel for pouring LN2 into the dewar is also included.

Figure 6	Figure 7	Figure 8

Detector Mount/ Flat Packs for Customer LN2 Cooling: Alternate packages are available for customers wishing to mount Judson detectors in their own dewars. Special care in handling is required; the customer should be familiar with clean detector mounting and dewar evacuation techniques. The DM1 Detector Mount (Fig. 9) is used to mount detectors into standard Judson metal dewars. The assembly includes a metal base and a FOV which which can be bolted to the dewar cold finger. Flat pack carriers are also available for J10D, J15D and J16D Series detectors
(Fig. 10).

Figure 9	Figure 10

Metal Dewar Connector Options: Most Judson metal dewars are equipped with SMA connectors for detector output (Figure 11). The standard version is recommended. Where available mounting space is limited, the "B" version reduces overall dewar diameter at a nominal extra cost.

Figure 11

Dewar Grounding and EMI: The shield of the detector SMA connector is normally grounded to the dewar body. In this configuration the shield of the coax cable is common to the dewar, making the entire dewar body a shield and limiting potential EMI interference with the detector. The SMA's may be isolated from the dewar if the standard grounding scheme is suspected of causing ground loops or pick-up noise in the user's system. The cold finger ground lug is for optional use, to ground the detector separately from the dewar. Some detectors include a temperature sensor, which may also be used to ground the detector. Normally this ground is not required; the ground lug may be left unconnected.

DFM Series Mounting Flanges for Metal Dewars: DFM Series mounting flanges assist in mounting and positioning of Judson metal dewars to the customer's bench top or optical system. The DFM-1 is a dewar base mount, which is fixed to the bottom O.D. of the dewar using set screws (Fig. 12). The flange may be bolted to a horizontal surface such as a table or positioning stage. The DFM-2 ring mount and DFM-3 front mount attach to the dewar window holder.

Figure 12

VOM-1 Vacuum Valve Operator for Metal Dewars: Metal dewars require re-evacuation every one to two years to maintain the specified LN2 hold time. Dewars returned to Judson for re-evacuation receive complete service, including leak check, o-ring change, vacuum bake-out, re-evacuation and electrical retest. For customers who wish to service their own dewars, the VOM-1 is used to connect Judson metal dewars to vacuum pumping systems.

DFM-3	DFM-2	VOM-1

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RC2 Detector/Rotary Cooler Integral Assembly

The RC2 microcooler is an integral Stirling engine with the detector directly mounted to the cold finger. Its 3.5 watt power requirement makes it ideal for use with a battery. Judson recommends rotary coolers when low power dissipation and portability are important. The RC2 microcooler fits easily in the palm of the hand and cools down to 77° Kelvin ± 0.5°K. The RC2 detector cooler assembly can be used with the J10D and J15D Series detectors. To determine performance of a detector with the cooler, typical specifications from the relevant detector series should be used. This cooler assembly is available with a plug in temperature control module which provides an adjustable temperature set point and requires 12VDC input.

Applications

• Portable Infrared Radiometers
• Environmental Monitoring
• Thermal Imaging
• Range Finding
• Spectroscopy
• Infrared Instrumentation

Design Features

• MTTF
• ³ 2000 hours Dissipates 150mW heat load
• Operates at 12V DC at 0.25A
• Power requirement 3.5 watts
• Hand held

Figure 15

Controlled Temperature Operation: This cooler has been specifically designed to maintain an infrared sensor at 77K. In normal operation, a 2N2222 silicon diode chip is attached adjacent to the infrared sensor and serves as the cold finger temperature sensor. At 77K, with a forward bias current of 1mA, the voltage across the base (+) to emitter (-) junction of the diode is typically 1.060 volts. (At 295K, it is 0.7 volts.) During the initial cooling, the cooler motor operates at peak RPM. Once the set point is reached, the motor throttles back to maintain temperature established by the set point. If the temperature sensor is left in an open circuit condition, it will appear as a max. voltage to the controller circuit and cause the cooler to throttle back immediately. Conversely, if the sensor leads are shorted together, the motor will operate at full RPM and the cold finger will stabilize at a temperature at which cooler power equals the sum of radiative and conductive thermal loads. The set point is placed at 1.060 volts at the factory. This can be adjusted for some other voltage via a temperature adjust pot on the control circuit board. The adjustment range is 1.0 to 1.1 volts.

Figure 16

TABLE "A"
1 Motor Drive 0A 2 +5 Volt Hall Sensor 3 Hall Sensor 0A 4 Hall Sensor 0B
5 Hall Sensor 0C 6 +5 Volt Return 7 Motor Drive 0C 8 Motor Drive 0B

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JTC Joule-Thomson Cryostat Cooling System

Infrared detectors with Joule-Thomson Cryostat Systems have been used extensively in military applications due to their small size, fast cool down time, low maintenance and automatic operation. The Judson JTC System offers this high technology in an easy-to-use package for commercial applications.

The JTC system consists of a high-quality detector mounted with a silicon diode temperature sensor in a miniature glass dewar. The cryostat and dewar are permanently fixed in a compact aluminum housing to reduce potential mishandling of the cryostat or glass dewar.

The cryostat has no moving parts and requires no electrical power. It operates from bottled high pressure nitrogen gas. A single 14,000 (STP) liter, 6,000 psi bottle can supply 230 continuous hours of operation.

Miniature Joule-Thomson Cryostat

Applications

• Airborne thermal mapping
• Automated infrared instruments
• Thermal imaging at remote sites
• IR detector for hazardous areas

Design Features

• Continuous, maintenance-free operation
• Compact size
• No liquid nitrogen pour fill required
• Reliable
• Suitable for automatic on/off operation
• No electrical power requirements
• Intrinsically safe

Simple and rapid cooldown of detector elements is offered by Judson miniature Joule-Thomson cryostats in an appropriately designed dewar. The cryostats convert pressurized gas to cryogenic liquid at the cryostat tip which is positioned behind the dewar cold finger. The cryostats have no electrical power requirements and can be operated in any position. (Complete inversion reduces performance.)

Judson cryostats operate with high-purity nitrogen gas for 77°K detector temperature. They are capable of operating with detector/dewar heat loads up to 2 Watts in ambient temperatures of -55°C to 70°C. Cryostats for use with Argon gas (87°K) are also available.

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X-Y Position Sensors Short Form Catalog in PDF Format

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J16PS, J12PS, J10PS Position Sensors: A Ge, InAs, or InSb position sensor consists of a single element photodiode with a quadruple electrode geometry. These devices can provide linear X-Y beam position information for lasers and other infrared beams. Positioning information is determined as shown in Figure 1. The PA6:4C preamplifier is recommended for Judson position sensitive detectors.

J16QUAD, J10QUAD, J15QUAD Quadrant Detectors: A Ge, InAs, or HgCdTe quadrant detector consists of four separate detector elements arranged in a quadrant geometry with element separations of approximately 0.05mm. The PA7:4 preamplifier is available for J16Quad and J10Quad detectors.

Figure 1

Figure 2

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Temperature Controllers

TC5 Temperature Controller	TC8 Temperature Controller

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TC5 Benchtop Temperature Controller with Built-in Power Supply

The Model TC5 Temperature Controller allows safe, convenient operation of the thermoelectrically cooled detectors. The TC5 Controller is easy to use. The built-in power supply accepts AC line voltage. Detector temperature is set on the controller front panel, and a digital meter indicates current drawn by the cooler. The current-overload feature protects the cooler from damage due to overheating; cooler power is cut off if the cooler attempts to draw excess current (e.g. due to improper heat sink, improper cooler input polarity, or defective thermistor.) The controller's connector for thermistor input and cooler output mates to the HS1 Heat Sink Assembly.
Key Features

• Proportional/Integral Temperature Control
• 30 Watt Capacity
• Temperature Select Front Panel
• Easy Connections to HS1 Heat Sink/Cable Assembly
• Built-in Power Supply
• Built-in Protection against TE cooler damage
• ±15V Output for Amplifier Circuit

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TC8 Temperature Controller

The Model TC8 is a self-contained Temperature Controller for use with the thermoelectric coolers. Using a single 5 Volt power supply, the TC8 operates in conjunction with a thermistor, located in the detector assembly, to precisely measure and regulate the temperature of the cooler detector. The TC8 is also available with detector heat sink and optional hybrid amplifier in the CM21 and the CMAMP assemblies.

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Heat Sink Assemblies

CMAMP Assembly	HS1 Assembly

A thermoelectrically cooled detector requires a heat sink to dissipate the heat generated by the cooler, an amplifier to amplify the detector signal to a usable level and a temperature controller to hold the detector at a constant temperature. Judson TE cooler accessories are designed to give solutions for these problems to our customers.

HS1 Assembly (Heat Sink Assembly for TO-37, TO-66 and TO-3 Packages)
The HS1 series is designed to provide heat sinking and cabling to simplify integrating a TE cooled detector into an existing system, and is designed to mate with either the TC-5 or TC-8 temperature controllers.

HSAMP Assembly (Heat Sink and Amplifier Assy for TO-37, TO-66 and TO-3 Pkgs)
The HSAMP series is designed to provide both heat sinking and signal amplification for our TE cooled photovoltaic product line. This allows Judson to help a customer to choose the best amplifier for a particular detector and system. The HSAMP includes a hybrid PA-5, PA-6 or PA-7 preamplifier and is designed for easy connection to the customer's optical system and the TC-8 Temperature Controller.

CM21 Assembly (Heat Sink and TC8 Temperature Controller for TO-37, TO-66 and TO-3 Pkgs)

The CM21 assembly is designed to provide heatsinking and temperature control for Judson's TE cooled photoconductive and photovoltaic detector product line. It is designed to simplify integrating a TE cooled detector into an exisiting system with a customer supplied amplifier. The CM21 assembly is ideal for customers who have dsigned their own amplifier for their system.

CMAMP Assembly (Heat Sink Amplifier and TC8 Temperature Controller for TO-37, TO-66 and TO-3 Pkgs)

The CMAMP series is designed for customers that would like a fully integrated detector module. It includes heatsinking, detector signal amplification and temperature control. The CMAMP assembly requires an external power supply that provides both +5V @ 2.0 amperes and ±15 volts at 100mA. The CMAMP assembly is an ideal platform for evaluating TE cooled detectors for the first time. Custom mechanical assemblies including temperature control, detector amplification and heat sinking are available on an OEM basis.

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