Electrical SystemNov. 15, 2010

Monte Frandsen

Key Electronics Design Goals and Constraints

Minimal change between ground and airborne observations

Signals pass through a vacuum bulkhead

Signals (wires) transition between77K to ambient

Electronics, (pre-amps), are subject to changing environmental conditions

Packaging and connectors

Vacuum bulkhead

Size, placement, and number of wires

Thermal effects

Temperature gradients (wires)

Ambient-operating point shifts etc

2

Primary Electronics Design Tasks

Cryo-Cooler Electrical IntegrationController

Power Supply

Temperature Sensors

LWIR Detector Interface

MWIR Interface

Connectors and Packaging

3

Cryo-Cooler Integration

Cyro-cooler controller

Primarily a packaging task

Connectors and wiring

Power Supply Connector

Serial Port Connector

Cold head temperature sensor connections

Must be mounted within ??? from the cryo pump

—(waiting on info from the manufacturer)

4

Cryo-Cooler Power Supply

Boosts voltage to 48V to provide power to the Sunpower GT controller

Requirements:

Can be remotely mounting away from optical bench.

Input Voltage 28VDC

Expected Input power requirements

~30 Amp peak current draw (<10 seconds)

~15 Amps Max continuous

Output Power

48VDC

15 Amps peak (< 10 Seconds)

300 Watts maximum continous load

5

Cryo-Cooler Power Supply

Design

Based on the Vicor Maxi Power Module

Three modules paralleled to meet peak load and share the load current

Schematic based on manufacturer’s reference design

6

Temperature Sensors

RTD

Cryo Cooler cold head. Supplied by manufacturer.

Two leads directly interface to the cryo-controller.

Two lead Cryo cooler heat sink temperature

Lakeshore DT-670 Si Diodes

Expected accuracy 0.1K around 77K

Can be individually calibrated for higher accuracy as required.

Trade off between operation range vs. sensitivity

Locations (tbd)

Jusdon 2N2222 Si Diode

Part of the LWIR detector assembly

Circuit expected the be the same as the DT-670 interface

7

Temperature Sensors

LakeShore DT-670 Si Diode SensorsStandard temperature curve

4-wire measurement

Minimize lead errors due to temperature shifts.

May be able to use 2-lead system with reduced performance.

10µA Bias current

8

Temperature Sensor Interface

9

Low bandwidth-Low power

Component RequirementsPrecision amplifiers and Precision Reference

High stability

Very low drift /temperature coefficients Vos and Gain.

Resistors

Vishay precision bulk foil with TC < 1 ppm.

10.0µA

Vdd

Vss

+

–

R11

1V Ref

Rbias

Vdd

Vss

+

–

I-AmpDT-670

Rlead

Rlead Rlead

Rlead

Vout

Vdd

Vss

+

–

1Offset Trim

R1

R1

LWIR Detector Signals

Requirements12 independent Channels

Expected Sensor Bias Currents

10mA - 40mA (originally this was 1mA to 50mA)

0.5V to 3V DC bias voltage across the detectors

100 Ω typical detector impedance

1mV dynamic range.

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LWIR Pre-Amp Circuit Design

DC Servo Integrator feedback topologyProven on previous MAS sensor

Integrator nulls DC-Errors.

Also nulls any other DC sources within the limitations of the op-amp.

Transfer function is a high pass filter with a very low cut-off frequency

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Σ

1. S

Vin VoutA

β

-

+

LWIR Pre-Amp Circuit Design

AdvantagesSupports 2-wire interface

Nulls out ALL slow-time varying changes including

Vos

Lead –resistances shifts

Bias voltage shifts

Bias current shifts

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DisadvantagesNot an absolute metric (typically ratio-metric)

No common mode rejection

Practical limitations on the integrator will prevent the DC from being driven nulled to 0. (< 10mV typical)

Input signal must be chopped (AC)

In use and during calibration

Scan – “droop” or background fading pushes out the low-frequency cut-off.

LWIR Pre-Amp Circuit Design

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Simplified Circuit1. Unity Gain2. Gain~10003. Integrator4. Difference Amplifier

Rb

R5

IBIAS

Vd

Vdd

Vss

+

–

Vb

R1

R2

Vdd

Vss

+

–

Ra

Rb

R4

Rt

Vdd

Vss

+

–12

3

4

C

Vout

Key Amplifier Requirements

Amplifier 1. Unity GainUltra-Low Voltage Noise. Low 1/Fc-noise

Unity Gain Stable

Amplifier 2. High Gain (1000) InvertingUltra-Low Voltage Noise. Low 1/Fc-noise

GBW Product > 40MHz

Max-Freq ~40Khz

Amplifier 3. IntegratorUltra-Low Input Current

Precision (Low Vos)

Amplifier 4. Differential output preferred.

High capacitive loading

Short circuit protected

14

Preliminary Noise Estimates

Noise Estimate summaryAssumptions

Temperature = 300K (~27°C)

Detector modeled at 82 Ohms

Op amp noise models typically change in the .1 to 10Hz range. Parameters are selected > 10Hz.

@Amplifier 1 LT1128 en~ 5nV/sqrt(Hz) (@ 10Hz)

@Amplifier 2 (LT1037) en~ 5nV/sqrt(Hz) (@ 10Hz)

After Stage 2 gain of 1000

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e1 1.761nV

Hz=

e2 e12

er22+ ein2

2+ elt10372+:= e2 8.506

nV

Hz=

eout 1000 e2:= eout 8.506μV

Hz=

Preliminary Noise Estimates

Noise Estimate summary (continued)

With a 40kHz bandwidth, the expected mean noise output voltage~Vm ≈ eout*sqrt(Bandwidth)

Vm ≈ 1.7mV

LT-Spice Noise Simulation shows ~12mV/Hz1/2

--Still need to reconcile the differences

16

Additional LWIR Error sources

Low frequency op amp noise

Op-Amp Power SupplyPSSR (at 40Khz) 60dB @1mV ripple 1mV error

Reference Voltage noise Primarily on the Bias voltage

Common Mode tbd

CrossTalk

EMI/EMC pickup from cryo-pumpShield cables near the pump.

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LWIR Output

Full Scale output ± 5V

Dynamic range from sensorDependent on the detectors

>± 1V

Differential output op-amp with reasonable drive capabilities to drive twisted pair line.

Nominal impedance 100Ω

Based on previous reference design

50Ω on each wire.

18

MWIR Interface

Currently do not have sensor specifications

Cooled transimpedance pre-amp is part of the detector

Expected interfaceInstrumentation amplifier

Gain <100

Maximum Bandwidth 40Khz

4-wire interfacePower

Gnd

Signal

Signal-Gnd

Output Interface similar to LWIR

19

Connectors and Wiring

Pre-Amp Interface (External)Total signals --33

12 LWIR

12 MWIR (11 slots unused)

9 Temperature

6 cold

1 Hot cryo pump temperature

1 preamp

Input Power +28V

~70 connections

D38999 Series III connectors

Insert: 21-35 (79 contacts #22). Plug PN: D38999/26WG35SN

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Connectors and Wiring

Pre-Amp to Vacuum ConnectorsConnections

Wire count for 4-wire interface system (separate wires for bias and signals)

Total signals --33

12 LWIR

12 MWIR (11 slots unused)

5 Temperature

2 LWIR

1 Cryo pump cold head

3 general

116 Connections (wire count)

Vacuum Bulkhead Connectors

3-50 pin DSUB Bulkhead connectors

PN#

Preamp Connectors

3-Micro-D style (MIL-DTL-83513 Style )

Lead-times. Lead-times may force migrations to standard DSUB connectors

21

Connectors and Wiring

Cryo Pump Power Supply and Cryo Controller.D38999 Series III connectors

Inserts/Shells not yet finalized.

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