1NETWORK MEETING, APRIL 25, 1998

A new approach for the electronic readout of a large array of bolometers

Gianluigi PESSINA

The Cryogenic Underground Observatory for Rare Events

2NETWORK MEETING, APRIL 25, 1998

THE EXPERIMENT AND THE SYSTEMTHE DETECTOR CONSISTS OF AN ARRAY OF LARGE MASS TeO2(330g EACH) BOLOMETERS;

NTD GERMANIUM THERMISTORS ARE USED, TO CONVERT THE TEMPERATURE INCREASE INTO ELECTRICAL SIGNAL;

THE MEASUREMENTS ARE EXPECTED TO BE VERY LONG (ββDECAY AND DARK MATTER STUDY). CONSEQUENTLY THE SYSTEM MUST BE ABLE TO RUN MAINTAINING VERY GOOD STABILITY;

THE STEPS FORESEEN IN THE CONSTRUCTION ARE: 20 CHANNELS (THE PRESENT), 100 CHANNELS AND 1000 CHANNELS;

NTD Ge Ther.

THE READOUT SPECIFICATIONSAS USUAL FOR THESE APPLICATIONS, THE NOISE MUST BE LOW;

VERY GOOD STABILITY OF THE DRIFT AND ALSO REJECTION TO TRANSIENTS IN SUPPLY VOLTAGES ARE REQUIRED;

EACH ELECTRICAL AND DYNAMICAL DETECTOR PARAMETER MUST BE ADJUSTED REMOTELY SINCE THE NUMBER OF CHANNELS IS LARGE;

THE EXPERIMENT CAN NOT STOP, MALFUNCTIONING PARTS MUST BE EASILY CHANGED.

3NETWORK MEETING, APRIL 25, 1998

THE READOUT CHAIN

NTD Ge Ther.

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PREAM. AMP. ANTIALIASING FILT.

ADC

INSIDE THE FRIDGE

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OFFSETADJUST

BOLOMETERS HAVE SELDOM THE SAME CHARACTERISTICS. FOR EACH ONE THE PROPER OPERATING POINT AND DYNAMIC RANGE MUST BE SET PROPERLY.

IT IS NECESSARY TO MAKE UNIFORM THE THE ARRAY RESPONSE.THE ELEMENTS OR PARAMETERS INDICATED IN RED MUST BE ADJUSTABLE INDEPENDENTLY FOR EACH DETECTOR.

4NETWORK MEETING, APRIL 25, 1998

READOUT CHAIN DETAILS: BIAS AND LOAD RESISTORS

TO THE DETECTOR

ROOM TEMPERATURE

BOTH THE DETECTOR BIAS VOLTAGE AND THE LOAD RESISTORS VALUE CAN BE SELECTED INDEPENDENTLY FOR EACH BOLOMETER.

THE SETTING IS MADE WITH BISTABLE RELAYS (INDICATED IN RED), WHOSE COIL IS ACTIVE ONLY WHEN FIRED.

COMMON

LOAD RESISTORS ARE LOCATED AT ROOM TEMPERATURE.

THE SELECTABLE RANGE IS FROM 2GΩ TO 100GΩ (BY SRT RES. TECHN. GMBH).

THEIR MINIMUM EQUIVALENT SHOT NOISE IS ABOUT 0.5pA

OPTICALLINK

5NETWORK MEETING, APRIL 25, 1998

READOUT CHAIN DETAILS: THE PREAMPLIFIER (A)

cP

cP

THE SIGNAL BANDWIDTH OF A LARGE MASS BOLOMETER IS LIMITED TO A FEW Hz.

THIS PERMITS TO TOLERATE A RELATIVELY LARGE PARASITIC SHUNTING CAPACITANCE OF THE CONNECTING WIRES.

THIS MEANS THAT THE PREAMPLIFIER CAN BE LOCATED AT ROOM TEMPERATURE; OBTAINING 2 IMPORTANT RESULTS:

1 SAFE OF POWER DISSIPATION INSIDE THE CRYOSTAT;

2 EASY ACCESSIBILITY TO THE FRONT-END.

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CROSS-TALK SIGNAL

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THE PREAMPLIFIER IS A DIFFERENTIAL VOLTAGE SENSITIVE ONE. THE CONFIGURATION ALLOWS TO OBTAIN:

SUPPRESSION OF THE MICROPHONISM OF THE ELECTRICAL LINK;

SUPPRESSION OF THE SIGNAL CROSS-TALK COMING FROM THE COUPLING BETWEEN THE CONNECTING WIRES OF ADJACENT DETECTORS.

6NETWORK MEETING, APRIL 25, 1998

READOUT CHAIN DETAILS: THE PREAMPLIFIER (B)

10

20

30

40

50

60

70

80

90

1 10 100 1000 10000 100000

HzC

MR

R (d

B)

COMON MODE REJECTION

PSRR >145 dB

CMRR= 78 dB

THE PREAMPLIFIER HAS BEEN DESIGNED TO SHOW GOOD NOISE AND DYNAMICAL PERFORMANCES.

1

10

100

0.1 1 10 100 1000

Hz

nV/

Hz

N.Avg:30

0.1 Hz N: 29 nV/√Hz1 Hz N: 5.5 nV/√HzWN: 3.4 nV/√HzAT THE INPUT THERE ARE A PAIR OF

JFET BIASED AT THEIR OPTIMUM OPERATING POINT FOR OBTAINING BOTH LOW NOISE AND MINIMAL LEAKAGE CURRENT.

-0.050

0.050

0.150

0.250

0.350

0.450

0 5 10 15 20

PEACK TO PEAK OUTPUT VOLTAGE (V)

INTE

GR

AL E

RR

OR

(%) LINEARITY

DYNAMICAL PERFORMANCE ARE VERY ACCURATE THANKS TO THE LARGE OPEN LOOP GAIN OF THE PREAMPLIFIER

7NETWORK MEETING, APRIL 25, 1998

READOUT CHAIN DETAILS: THE PREAMPLIFIER (C)

-80

-60

-40

-20

0

20

40

60

80

-1 4 9 14 19 24Time (hs)

Vin

(uV)

-10

0

10

20

30

40

50

60

70

Tem

pera

ture

(C)

Vin (uV)SimulatedTemperature

(-10,40)C:0.38uV/C

VO

PTATCURRENT

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FOR VERY LONG MEASUREMENTS THE READOUT IS ASKED TO SHOW NOT ONLY LOW NOISE, BUT VERY LOW DRIFT. WE SUCCEED IN OBTAIN THIS CHARACTERISTIC:

A CIRCUIT SOLUTION HAS BEEN ADOPTED TO COMPENSATE FOR THE ACTUAL DRIFT OF THE PREAMPLIFIER.

THE ORIGINAL PREAMPLIFIER DRIFT CAN BE ADJUSTED FROM 10-30µV/CO, TO LESS THAN 300nV/CO.

BUT THIS IS NOT ALL….

MEASUREMENT MADE INSIDE A CLIMATIC ROOM

8NETWORK MEETING, APRIL 25, 1998

READOUT CHAIN DETAILS: THE SUPPLY VOLTAGE (A)

THE SYSTEM MUST SHOW GOOD STABILITY ALSO AGAINST SUDDEN CHANGES OF THE MAIN POWER, OFTEN PRESENT INSIDE THE GRAN SASSO TUNNEL.

IN AN ACTUAL EXPERIMENTAL SITUATION WRONG OR MISSING CONNECTIONS CAN HAPPEN AND MAY DAMAGE THE READOUT SYSTEM.

FOR THIS REASON A SPECIAL SUPPLY VOLTAGE FOR THE WHOLE READOUT HAS BEEN DESIGNED.

ITS SAFETY FEATURES ARE:

FULL PROTECTION AGAINST POLARITY INVERSION;

FULL PROTECTION AGAINST EXCESSIVE INPUT VOLTAGE AMPLITUDE;

SHORT CIRCUIT CURRENT-FOLDBACK PROTECTION.

ONE SUPPLY VOLTAGE WILL FEED THE FRONT-END OF 7 CHANNELS, FOR GETTING ANYWAY THE MAXIMUM LOAD REGULATION.

9NETWORK MEETING, APRIL 25, 1998

BIPOLAR OUTPUT VOLTAGE : ±10V;

MINIMUM DROPOUT INPUT/OUTPUT: 0.5V;

UNREGULATED INPUT: 12V ± 1V, -12V ±1V;

RATED CURRENT: ±1.5A;

POWER SUPPLY REJECTION RATIO: 140dB;

THERMAL STABILITY: ±6ppm/CO;

OUTPUT NOISE NOISE: 100nV/√Hz @ 1Hz.

READOUT CHAIN DETAILS: THE SUPPLY VOLTAGE (B)THE CHARACTERISTICS OF THE SUPPLY ARE:

1.00

10.00

100.00

1000.00

1 10 100 1000

Hz

nV/

Hz

05

10152025303540

0 1 2 3 4

Time (hs)Tem

pera

ture

(C)

-10.0015-10.001-10.0005-10-9.9995-9.999-9.9985-9.998

+10V SUP. VO

L.

Tempe Vout

Drift MAX < 6 ppm/C@ ±1.5 A Load Current

MEASUREMENT MADE INSIDE A CLIMATIC ROOM

-0.6

-0.4

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0.2

0.4

0.6

0.8

1

0 2 4 6 8 10 12TIME (h)

Pre.

Out

. (m

V)

99.51010.51111.51212.51313.5

Vcc

Supp

ly (

V)

Vout Vcc

PSRR=137dB

LOAD CURRENT: 1.2A

10NETWORK MEETING, APRIL 25, 1998

SECOND STAGE AMPLIFIER AND ANTIALIASING FILTER

AMP. ANTIALIASING FIL.

THE GAIN OF THE SECOND STAGE AMPLIFIER CAN BE CHANGED BY MEANS OF MOS SWITCHES.

THE OVERALL GAIN CAN BE SET WITH 3 BITS OF SENSITIVITY FROM 200 TO 10.000

THE ANTIALIASING FILTER IS A 4 POLES ACTIVE BESSEL (FLAT TOP RESPONSE) FILTER.

FREQUENCY BANDWIDTH CAN BE ADJUSTED WITH 3 BITS OF SENSITIVITY FROM 12Hz TO 150Hz.

11NETWORK MEETING, APRIL 25, 1998

LAST BUT NOT THE LEAST: OFFSET ADJUSTMENT

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TO THE DETECTOR

A/DD/AIT IS ESSENTIAL TO BE ABLE TO REGULATE THE OUTPUT OFFSET VOLTAGE REMOTELY:

BOLOMETER CHARACTERIZATION CAN BE MADE EASILY, FOR THE WHOLE ARRAY;

THE CALIBRATION OF THE OFFSET ALLOWS TO EXPLOIT THE WHOLE DYNAMIC RANGE OF THE SYSTEM;

AN AUTOMATIC PERIODIC OFFSET ADJUSTMENT ALLOWS THE FULL KNOWLEDGE OF THE STORY OF THE RUN AND AVOID THE PRESENCE OF AN OPERATOR.

RESOLUTION: 15 BITS,

CORRESPONDING TO ±50mV OF INPUT OFFSET VOLTAGE CORRECTION AT THE MAXIMUM GAIN OF 10.000.

12NETWORK MEETING, APRIL 25, 1998

SUMMARY

IN THIS SHORT TALK SOME INFOs. ON OUR READOUT APPROACH FOR THE REALIZATION OF A LONG MEASUREMENT, WITH AN ARRAY OF MANY CHANNELS OF BOLOMETERS, HAVE BEEN GIVEN;

THE PREAMPLIFIER (A DIFFERENTIAL VOLTAGE ONE) WAS DESIGNED FOR HAVING LOW SERIES AND PARALLEL NOISE AND VERY LOW DRIFT;

A SUPPLY VOLTAGE WAS ALSO DESIGNED FOR INCREASE THE STABILITY FURTHER;

THE READOUT WAS MADE REMOTELY PROGRAMMABLE FOR WHAT CONCERN THE SELECTION OF:

THE DETECTOR BIAS VOLTAGE;LOAD RESISTOR;THE OVERALL VOLTAGE GAIN;THE READOUT BANDWIDTH;OFFSET ADJUSTMENT

(UNDER DEVELOPMENT: LINEAR OPTICAL LINK OF THE DETECTOR SIGNAL)