TELESCOPIO NAZIONALE GALILEO
Technical Report no.79
TEST AND CHARACTERIZATION OF THE TEMPERATURE
TRANSDUCERS FOR THE CCD CAMERAS OF THE
ITALIAN NATIONAL TELESCOPE GALILEO
M. Comari, C. Corte
Osservatorio Astronomico di Trieste
June 1998
TNG Technical Report no.79 - Test and Characterization of the Temperature Transducers for the CCD Cameras
Abstract
In the cryostat of the T.N.G. C.C.D. camera, the integrated circuit temperature transducer
AD590 has been proposed for the thermostatic control and temperature monitor due to its
simple use and its prompt availability. In the data sheets (ref.1 and 2) its rated performance
temperature range is stated from -55°C to 150°C, with a note that it can be extended to 100°C with some degradation of the performance.
In this paper we investigate the behavior of this transducer at temperatures as low as -196°C;
the transducer has proved to work properly at temperatures as low as about -120°C when
powered at low voltage, even after undergoing some cyclic hard thermal shocks during
which the temperature decreased abruptly from ambient to -196°C and back to ambient.
Introduction
Four temperature transducers have to be placed in the cryostat:
one on the liquid nitrogen vessel to monitor when empty;
one on the cold finger for temperature control;
one on the cold finger for temperature monitoring;
one on the outer barrel.
For this task the AD590 integrated-circuit temperature transducer has been proposed because
it is a calibrated two-terminal temperature sensor which requires only a dc voltage supply (+
4V to +30 V), with a linear output current of 1A/K. Transmitters, linearization circuits,
precision voltage amplifiers, resistance measuring circuitry and cold junction compensation
are not needed in applying the AD590.
Calculation of the required temperature measurement accuracy
CCDs for astronomical use have to be cooled at low temperatures to reduce the dark current
and thus allow long exposures.
For a CCD the dark current is given by the following relation (ref. 3):
1)
Nd = 2.55 · 1015 N0 dpix2 T1.5 e -Eg/2kT e/s /pixel
where:
Nd is in electrons per second per pixel
N0 is the dark current in nA / cm2 at room temperature
dpix is the pixel size in cm
T is the operating temperature in K
k is the Boltzman constant (8.62 · 10-5 eV/K)
Eg is the bandgap energy in eV given by the following relation:
2)
Eg = 1.1557 - 7.021 · 10-4 T2 / (1108 + T)
The dark current for a typical, non inverted CCD is 1.8nA/cm2 at room temperature (ref.3),
and if we assume a pixel size of 15m we get 25000 e/s/pixel, which means that the CCD
saturates just in a few seconds.
If we assume a typical long exposure of half an hour and a cooling temperature of 173K,
taking into account equations 1 and 2, we obtain a dark current of 1 electron per pixel, the
associated rms noise is the square root of the dark current, therefore 1 e rms.
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TNG Technical Report no.79 - Test and Characterization of the Temperature Transducers for the CCD Cameras
If we assume an error of 5 degrees in the temperature transducer, and instead of cooling at
173K we cool at 178K, the dark current becomes 3.4 electrons per pixel, with an associated
noise of 1.8 e rms.
The lowest readout noise for a modern CCD is 2, 3 electrons rms (ref. 3).
Summing the readout noise and the noise generated by the dark current according equation 3,
we get a noise of 2.2, 3.2 electrons rms at the temperature of 173K and a noise of 2.7, 3.5
electrons rms at the temperature of 178 K.


2 = r2 d2
where:
r is the readout noise
d is the dark current noise
At the temperature of 173K and 178K the contribution of dark current noise is small
compared to readout noise. Therefore a measurement error as high as 5 degrees in the
temperature transducer is acceptable.
The survival test
We bought 10 AD590 integrated-circuits temperature transducers from Harris Semiconductor
and we used the circuit of figure 1 for testing them.
Figure 1
The test circuit.
Vs is the supply voltage and Vt is the output voltage.
4)
Vt = 1A/K 5.1K (T+273.16)K
T is the temperature measured in °C degrees.
We labelled the transducers with progressive letters from A to L and to test their survival
capabilities to hard thermal shocks, we connected them to the test circuit and dipped one at a
time in the liquid nitrogen at the temperature of -196°C and then let them return to ambient
temperature. We repeated this test two times with a Vs of 5V, two times with a Vs of 6V and
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TNG Technical Report no.79 - Test and Characterization of the Temperature Transducers for the CCD Cameras
two times with a Vs of 10 V. No device failed the test. After the test all the devices showed a
Vt at ambient temperature equal to the Vt showed at ambient temperature before the test.
In table 1 we see that at the liquid nitrogen boiling point all devices showed a Vt inconsistent
with such a temperature. Vt has been measured with a Hewlett Packard 34401 Multimeter.
D
E
V
.
A
B
C
D
E
F
G
H
I
L
Vs= 5V
NITROGEN
Boiling point -195.8 °C
Vt
T
Error
mV
°C
°C
632,5
-149,1
46,7
589,8
-157,5
38,3
686,4
-138,6
57,2
594,5
-156,6
39,2
684,6
-138,9
56,9
649,9
-145,7
50,1
668,0
-142,2
53,6
647,8
-146,1
49,7
630,9
-149,4
46,4
696,8
-136,5
59,3
Vs= 6V
NITROGEN
Boiling point -195.8 °C
Vt
T
Error
mV
°C
°C
798,3
-116,6
79,2
744,0
-127,3
68,5
878,6
-100,9
94,9
747,7
-126,6
69,3
876,0
-101,4
94,4
823,3
-111,7
84,1
853,5
-105,8
90,0
821,8
-112,0
83,8
798,2
-116,6
79,2
897,2
-97,2
98,6
Vs= 10V NITROGEN
Boiling point -195.8 °C
Vt
T
Error
mV
°C
°C
1136,1
-50,4
145,4
1078,8
-61,6
134,2
1381,3
-2,3
193,5
1057,2
-65,9
129,9
1375,4
-3,5
192,3
1193,7
-39,1
156,7
1321,0
-14,1
181,7
1195,0
-38,8
157,0
1163,3
-45,1
150,7
1482,8
17,6
213,4
Table 1
Values of Vt of the 10 devices and the corresponding T calculated by reversing equation 4 at
the liquid nitrogen boiling temperature at different voltage supply.
The characterization bench
After the encouraging results of the survival test, we set up a characterization bench to test the
transducers in a temperature range between -120°C and +100°C by means of a number of
well known fixed temperature points.
For temperatures from 0°C and lower the characterization bench is constituted by a small
dewar in whitch we froze a liquid substance with high chemical purity by means of liquid
nitrogen. When the substance began to return to the liquid state and in the dewar the
substance was 50% liquid and 50% solid, by stirring accurately we obtained a thermostatic
bath at the melting point temperature for the transducer characterization.
We got another fixed temperature point at 100°C by just boiling some distilled water in a pot.
Therefore we got five thermostatic baths at the corresponding state transition temperature.
See in table 2 the used substances and their state transition point.
All measurements were carried out at the Astronomical Observatory of Trieste, at an altitude
of 50 m above sea level.
diethil ether
acetone
mercury
water
water
melting point
melting point
melting point
melting point
boiling point
-116 °C
-94 °C
-38.9 °C
0
°C
100 °C
Table 2
The substances used for the test and their state transition point.
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TNG Technical Report no.79 - Test and Characterization of the Temperature Transducers for the CCD Cameras
Test results
The measurements were carried out using the circuit of figure 1 at the five temperatures of the
thermostatic baths by dipping the transducer in the thermostatic bath with proper electrical
insulation in the conducting fluids (i.e. mercury where also chemical insulation is a must to
avoid amalgam) achieved by dipping the transducer in an insulating varnish.
In table 3 are shown the results of the test.
D
Voltage E
supply V
.
5V A
5V B
5V C
5V D
5V E
5V F
5V G
5V H
5V
I
5V L
6V A
6V B
6V C
6V D
6V E
6V F
6V G
6V H
6V
I
6V L
10 V A
10 V B
10 V C
10 V D
10 V E
10 V F
10 V G
10 V H
10 V I
10 V L
DIETHYL ETHER
Melting point -116 °C
Vt
T
Error
mV
°C
°C
785,2 -119,2 -3,2
791,3 -118,0 -2,0
786,0 -119,0 -3,0
783,8 -119,5 -3,5
784,1 -119,4 -3,4
782,1 -119,8 -3,8
782,4 -119,7 -3,7
783,2 -119,6 -3,6
780,1 -120,2 -4,2
796,0 -117,1 -1,1
786,9 -118,9 -2,9
794,3 -117,4 -1,4
788,7 -118,5 -2,5
786,1 -119,0 -3,0
787,3 -118,8 -2,8
784,1 -119,4 -3,4
785,9 -119,0 -3,0
785,9 -119,1 -3,1
783,1 -119,6 -3,6
805,2 -115,3 0,7
847,1 -107,0 9,0
867,1 -103,1 12,9
1144,3 -48,8 67,2
821,5 -112,1 3,9
1132,2 -51,1 64,9
912,0 -94,3 21,7
1094,7 -58,5 57,5
917,5 -93,2 22,8
912,1 -94,3 21,7
1311,5 -16,0 100,0
ACETONE
MERCURY
Melting point -94°C Melting point -38.9°C
Vt
T Error Vt
T Error
mV
°C °C
mV
°C
°C
893,6 -97,9 -3,9 1177,2 -42,3 -3,4
898,6 -97,0 -2,9 1185,7 -40,7 -1,8
889,6 -98,7 -4,7 1173,8 -43,0 -4,1
894,4 -97,8 -3,8 1177,9 -42,2 -3,3
888,1 -99,0 -5,0 1173,2 -43,1 -4,2
886,5 -99,3 -5,3 1170,0 -43,7 -4,8
888,8 -98,9 -4,9 1178,3 -42,1 -3,2
891,5 -98,3 -4,3 1173,9 -43,0 -4,1
888,5 -98,9 -4,9 1170,2 -43,7 -4,8
908,5 -95,0 -1,0 1196,5 -38,5 0,4
894,4 -97,8 -3,8 1177,4 -42,3 -3,4
901,0 -96,5 -2,5 1186,7 -40,5 -1,6
891,8 -98,3 -4,3 1174,2 -42,9 -4,0
896,2 -97,4 -3,4 1178,5 -42,1 -3,2
889,9 -98,7 -4,7 1173,7 -43,0 -4,1
888,0 -99,0 -5,0 1170,8 -43,6 -4,7
890,8 -98,5 -4,5 1178,6 -42,1 -3,2
893,5 -98,0 -4,0 1174,5 -42,9 -4,0
890,3 -98,6 -4,6 1170,9 -43,6 -4,7
912,0 -94,3 -0,3 1197,3 -38,4 0,5
897,0 -97,3 -3,3 1178,2 -42,1 -3,2
907,9 -95,1 -1,1 1189,5 -39,9 -1,0
950,5 -86,8 7,2 1175,8 -42,6 -3,7
898,6 -97,0 -3,0 1180,0 -41,8 -2,9
941,4 -88,6 5,4 1175,9 -42,6 -3,7
891,6 -98,3 -4,3 1173,0 -43,1 -4,2
911,5 -94,4 -0,4 1180,6 -41,7 -2,8
899,8 -96,7 -2,7 1176,5 -42,5 -3,6
895,4 -97,6 -3,6 1172,5 -43,2 -4,3
1103,0 -56,9 37,1 1200,0 -37,9 1,0
WATER
Melting point 0°C
Vt
T Error
mV
°C °C
1373,5 -3,8 -3,8
1385,7 -1,4 -1,4
1370,5 -4,4 -4,4
1374,4 -3,7 -3,7
1370,1 -4,5 -4,5
1366,5 -5,2 -5,2
1376,5 -3,2 -3,2
1372,6 -4,0 -4,0
1364,0 -5,7 -5,7
1397,4 0,9 0,9
1373,7 -3,8 -3,8
1386,7 -1,2 -1,2
1371,0 -4,3 -4,3
1374,3 -3,7 -3,7
1370,9 -4,3 -4,3
1367,2 -5,1 -5,1
1377,0 -3,1 -3,1
1373,3 -3,9 -3,9
1364,8 -5,5 -5,5
1398,0 1,0 1,0
1374,6 -3,6 -3,6
1390,6 -0,5 -0,5
1372,5 -4,0 -4,0
1375,1 -3,5 -3,5
1373,5 -3,8 -3,8
1369,0 -4,7 -4,7
1379,2 -2,7 -2,7
1376,0 -3,3 -3,3
1366,5 -5,2 -5,2
1399,6 1,3 1,3
WATER
Boiling point 100°C
Vt
T Error
mV
°C °C
1885,6 96,6 -3,4
1893,5 98,1 -1,9
1878,0 95,1 -4,9
1883,6 96,2 -3,8
1876,7 94,8 -5,2
1878,3 95,1 -4,9
1874,1 94,3 -5,7
1891,5 97,7 -2,3
1873,9 94,3 -5,7
1881,7 95,8 -4,2
1885,7 96,6 -3,4
1901,2 99,6 -0,4
1886,7 96,8 -3,2
1888,5 97,1 -2,9
1884,1 96,3 -3,7
1879,1 95,3 -4,7
1891,5 97,7 -2,3
1891,9 97,8 -2,2
1874,4 94,4 -5,6
1920,4 103,4 3,4
1886,9 96,8 -3,2
1908,2 101,0 1,0
1888,4 97,1 -2,9
1889,8 97,4 -2,6
1887,2 96,9 -3,1
1880,1 95,5 -4,5
1893,6 98,1 -1,9
1893,2 98,1 -1,9
1875,7 94,6 -5,4
1921,5 103,6 3,6
Table 3
Measured voltages, corresponding calculated temperatures and temperature errors of the
devices in different thermostatic baths at different voltage supplies using the test circuit of
figure 1.
All devices performed well at a voltage supply of +5V and +6V and showed degraded
characteristics at low temperatures at a voltage supply of +10V.
In the temperature range from -116°C to 100°C all the 10 devices showed an error around or
lower than 5°C, the half of the 10°C maximum error of the data sheet limits (ref. 1 and 2)
in the range -55°C to 150 °C, at a voltage supply of +5V and +6V, thus satisfying the
calculated temperature measurement accuracy requirement.
From table 1 we see that the error of the temperature transducers at the liquid nitrogen boiling
temperature of -196°C at a voltage supply of 5V is lower than at 6V and the maximum read
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TNG Technical Report no.79 - Test and Characterization of the Temperature Transducers for the CCD Cameras
monitoring temperature is -136.5°C, well below -120°C. This means that the transducer can
be used for monitoring the emptying of the liquid nitrogen vessel. It is also reasonable to
assume that the devices perform better at temperatures lower than 116°C at a supply voltage
of 5V rather than 6V. For this reason we suggest to use the transducer at a voltage supply of
5V with the circuit of figure 1. At a voltage supply of 5V the rated range can be reasonably
extended as low as -120°C which is assumed to be the minimum working temperature of
CCD sensors.
The temperature error of the transducers in the working range from -120°C to 100°C can be
further reduced in a limited range around a working point by using the AD590 with the slope
trimming circuit of figure 2 or the slope and offset trimming circuit of figure 3 (ref.1 ), or by a
functionality equivalent equation implemented in the controller temperature acquisition
software.
To further reduce the error in the full range (-120°C to 100°C), an equation of calibration
obtained with the least mean squares method from measured data can be used.
Figure 2
Slope trimming circuit.
Figure 3
Slope and offset trimming circuit.
Conclusions
The ten AD590 integrated-circuits temperature transducers of the test have shown to perform
well at temperatures as low as -120°C when powered at 5V using the circuit of figure 1. No
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TNG Technical Report no.79 - Test and Characterization of the Temperature Transducers for the CCD Cameras
device failed even during the hard survival test of the thermal shock from the ambient
temperature down to -196°C.
Acknowledgments
We wish to thank Dr. Maurizio D’Alessandro of the Osservatorio Astronomico di Padova for
his helpful suggestions during this work.
References
1. Harris Semiconductor, AD590 data sheet, FN 3171 (1997).
2. Analog Devices, AD590 data sheet, CD-ROM 1997.
3. Ian S. Mc Lean, Electronic Imaging in Astronomy, Detectors and Instrumentation, WileyPraxis series in Astronomy and Astrophysics.
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