1-INTRODUCTION
The future LHC particle accelerator will have more than 10 000 sensors in order to be able to control the
superconductors magnets helium bath temperature. The low signal amplitude obtained when measuring
cryogenics temperatures, and the fact that most of the sensors will be located inside the machine tunnel,
requires the design of a radiation hardened signal conditioner with very low cost, high precision, and small
physic volume.
Cryogenics temperature measurements are often performed using sensors with a semiconductor behaviour
and negative temperature coefficient, that mean that they have a low resistance value at ambient temperature
and some K at super fluid helium temperatures fig.1. The advantage of this type of sensors, is that can
cover all the future LHC machine temperature range and a lot of them will be probably used.
The STMS1 signal conditioner has been implemented to work with semiconductor behaviour temperature
sensors, and the basic aim of the design has been to obtain, at the lowest price per unit as possible, the
required temperature precision that the future LHC particles accelerator constrains impose. The signal
conditioner can be used trough a digital fielbus transmission system, a scanner or with industrial PLCs (
Program Logic Controllers ) thanks to the different output configurations.
This conditioner represent a good step to a final design in the sense that the experimental results show a
very good readout temperature precision, and the influence of the power supply and ambient temperature
fluctuations over the circuit are insignificant. Eventually is important to indicate that a big effort has to be
done to ensure a suitable radiation hardened design.
2- BLOCK DIAGRAM
The fig.1 shows the signal conditioner’s block diagram
FIG.2
To better exploit the temperature range, the conditioner works in three different current ranges depending
on the sensor values, Rthm. The maximum current is used at ambient temperature where the sensor
resistance value Rthm is low, while at cryogenics temperatures the minimum current range is used. The
current reductions with the temperature and the change of the current’s sign avoid respectively Joule and
thermocouple effects. The current source needs a precision square wave input voltage that feeds a
differential voltage to current converter, and an electronic control system in order to select the correct
range. The control system also has a range indicator.
The current feeds the sensor Rthm and the differential voltage generated through it, is then recovered by an
instrumentation operational amplifier. The amplified square signal is rectified, filtered and corrected. This
corrected output signal is the reference to the control system and can be connected to an isolated 4 - 20 mA
transducer if necessary.
To rectify the signal instead of a normal diodes bridge, we have used operational amplifies connected as
shown in fig.3. The switch control in phase wit the input square signal change to a positive gain of one
amplifier when the input signal is positive, and to the inverse gain of one amplifier when the signal is
negative, so the total output signal is positive. The small phase errors and high frequency noise is then
filtered to obtain a stable output signal, proportional to the sensor value depending on the range. With this
configuration we eliminate as much as possible ambient temperature effect over the circuit.
FIG.3
3-RANGE CONTROL
An ALTERA PLD ( Program Logic Device ) is the responsible of the range indicator and range control of
the circuit. The reference to the PLD controller is the corrected 0 – 10 V output analog signal that is send
through a couple of comparators to the PLD control inputs. The fig.4 shows the basic state machine
implemented inside the PLD. One comparator is connected to the input “ xin ” and the other to input
“ yin ”( the comparators have CMOS compatible outputs and can be connected directly to the PLD ).
The control system works basically as an over - voltage detector, it means that “ xin ” will be high only
when the output voltage surpass a minimum threshold limit, And “ yin ” will be in high only when the
output voltage surpass a maximum threshold limit in a specific current range.
It’s clear that is not possible to have both logic inputs in high, and that “ xin ” and “ yin ” only will be in
high to indicate respectively a transition to a inferior or superior range. The fig.5 shows the 0 – 10 V output
and the hysteresis transitions between the three different current ranges.
FIG.4
FIG.5
4-PRECISION AND ADJUSMENTS
The circuits contain five potentiometers to be able to obtain a good readout precision and low dispersion
between the different box. The first one is used to adjust the clock frequency to 20 Hz. The second
potentiometer correct the differential offset generated at the rectifier input. This offset voltage involve small
amount of ripple over the DC rectified voltage level, and at the lowest values of the sensor, in each range,
this ripple is an important part of the signal and could induce readout errors fig.6.
FIG.6
FIG.7
The third potentiometer corrects the current send through the sensor to obtain a symmetrical voltage wave
at the operational amplifier output fig.7. The non-symmetrical current effects can be appreciated at the
highest values of the sensor in each range. The last two potentiometers correct the general offset and gain of
the conditioner.
With all this adjustments and using high precision components at the critical places, the conditioner carry
out the LHC readout temperature design constrains ( maximum error T / T  0.25 % from ambient
temperature to 1.8 K ) keeping a very low dispersion between them.
5- AMBIENT TEMPERATURE DEPENDENCE
Since the signal conditioner is basically an analog system, it is important to know the ambient temperature
o
effects over the circuit. To simulate this effect, we place some of the circuits, adjusted at 25 c , inside an
o
o
o
oven, and then we shift the temperature from 0 c to 50 c in step of 5 c , using a high precision decade
box to simulate the sensor.
The graph.1 shows the sensor theoretical relative error Vs ambient temperature shifts using the data
provided by the manuals, for a signal conditioner with the 0 – 10 V output implemented. The curves over
the predictions represent the temperature constrain T / T  0.25 % translated to a relative resistance error
for different sensors.
The graph.2, graph.3 and graph.4 the tested result by one of the box. The graph.5, graph.6 and graph.7
show the tested with the 4 – 20 mA output implemented.
DR/R (DT/T=.25%) [ Rmax = 25 K ] (output 0-10V without isolation)
0.50%
DR/R [%]
0.40%
0.30%
0.20%
0.10%
0.00%
1
10
T [K]
100
1000
o
Graph.1: Theoretical sensor dR / R [ % ] shift Vs ambient temperature dependence. Blue = T = 5 c ,
o
o
green = T = 10 c , red = T = 15 c .
BOX 3 [ 100 uA ]
( 0 - 10 V )
BOX 3 [ 10 uA ]
( 0 - 10 V )
0.20%
0.20%
dR / R [ T = 50 ]
dR / R [ % ]
dR / R [ T = 40 ]
0.10%
dR / R [ T = 35 ]
0.00%
dR / R [ T = 25 ]
dR / R [ T = 15 ]
-0.10%
dR / R [ T = 40 ]
0.10%
dR / R [ T = 35 ]
0.00%
dR / R [ T = 25 ]
dR / R [ T = 15 ]
-0.10%
dR / R [ T = 10 ]
dR / R [ T = 10 ]
dR / R [ T = 0 ]
dR / R [ T = 0 ]
-0.20%
-0.20%
0
100
200
300
400
500
0
R [ ohm ]
1000
2000
3000
4000
5000
R [ ohm ]
BOX 3 [ 1 uA ]
( 0 - 10 V )
0.20%
dR / R [ T = 50 ]
dR / R [ % ]
dR / R [ % ]
dR / R [ T = 50 ]
dR / R [ T = 40 ]
0.10%
dR / R [ T = 35 ]
0.00%
dR / R [ T = 25 ]
dR / R [ T = 15 ]
-0.10%
dR / R [ T = 10 ]
dR / R [ T = 0 ]
-0.20%
0
10000 20000 30000 40000 50000
R [ ohm ]
o
Graph.2, graph.3 and graph.4: The sensor dR / R [ % ] shift Vs ambient temperature dependence [ c ] for a
conditioner with 0 – 10 V output.
BOX 4 [ 100 uA ]
( 4 - 20 mA )
BOX 4 [ 10 uA ]
( 4 - 20 mA )
0.30%
0.30%
dR / R [ T = 50 ]
0.10%
dR / R [ T = 35 ]
0.00%
dR / R [ T = 25 ]
-0.10%
dR / R [ T = 15 ]
dR / R [ T = 10 ]
-0.20%
dR / R [ T = 50 ]
0.20%
dR / R [ % ]
dR / R [ T = 40 ]
dR / R [ T = 40 ]
0.10%
dR / R [ T = 35 ]
0.00%
dR / R [ T = 25 ]
-0.10%
dR / R [ T = 15 ]
dR / R [ T = 10 ]
-0.20%
dR / R [ T = 0 ]
-0.30%
dR / R [ T = 0 ]
-0.30%
0
100
200
300
400
0
R [ ohm ]
1000
2000
3000
4000
5000
R [ ohm ]
BOX 4 [ 1 uA ]
( 4 - 20 mA )
0.30%
dR / R [ T = 50 ]
0.20%
dR / R [ % ]
dR / R [ % ]
0.20%
dR / R [ T = 40 ]
0.10%
dR / R [ T = 35 ]
0.00%
dR / R [ T = 25 ]
-0.10%
dR / R [ T = 15 ]
dR / R [ T = 10 ]
-0.20%
dR / R [ T = 0 ]
-0.30%
0
10000 20000 30000 40000 50000
R [ ohm ]
o
Graph.5, graph.6 and graph.7: The sensor dR / R [ % ] shift Vs ambient temperature dependence [ c ] for a
conditioner with 4 – 20 mA output.
6-BOX DISPERSION
Another important parameter it is the measured relative error sensor’s resistance between different
conditioners compared with the ideal average resistance. The resulting graphics give us the dispersion
between the different conditioners, and a statistical idea about the stability and adjustment precision of the
system. The sensor was simulated using a decade box, and each conditioner was adjusted individually at
o
25 c . The acquisition of the values was made maintaining the temperature as stable as possible.
The graph.8 to 10 shown the dispersion between three conditioners with the 0-10 V output implemented. It
is clear that the dispersion between the conditioners is very low < 0.02 % for the three current ranges.
10 uA
( output 0-10 V )
100 uA
( output 0-10 V )
0.02%
dR / R ( BOX 1)
0.00%
dR / R ( BOX 2)
-0.01%
dR / R ( BOX 3)
dR / R [ % ]
0.01%
0.01%
dR / R ( BOX 1)
0.00%
dR / R ( BOX 2)
-0.01%
dR / R ( BOX 3)
-0.02%
-0.02%
0
100
200
300
0
400
1000
2000
3000
4000
Rthm [ Ohm ]
Rthm [ Ohm ]
1 uA
( output 0-10 V )
dR / R [ % ]
0.02%
0.01%
dR / R ( BOX 1)
0.00%
dR / R ( BOX 2)
-0.01%
dR / R ( BOX 3)
-0.02%
0
10000
20000
30000
40000
Rthm [ Ohm ]
Graph.8, graph.9 and graph.10: The dR / R [ % ] = ( Raverage – Rcal ) / Raverage measured for different
0-10 V signal conditioners.
9-POWER SUPPLY DEPENDENCE
It is important to know the power supply effects over the circuit. To simulate this effect, we adjust some
o
of the circuits at 25 c , and then we shift the power supply voltage in  1V over the nominal 24 V, using a
high precision decade box to simulate the sensor. The ambient temperature over the circuits was maintained
as stable as possible.
The graph.11 to 13 shown the power supply shift effect over one 0-10 V output conditioner, and the
graph.14 to 16 the same power supply dependence but with a 4 – 20 mA output conditioner.
10 uA
( output 0 - 10 V )
0.06%
0.04%
0.02%
0.00%
-0.02%
-0.04%
-0.06%
dR/R ( 23 V )
dR/R ( 24 V )
dR/R ( 25 V )
0
100
200
Rthm [ Ohm ]
300
400
dR / R [ % ]
100 uA
( output 0 - 10 V)
dR / R [ % ]
dR / R [ % ]
0.02%
0.06%
0.04%
0.02%
0.00%
-0.02%
-0.04%
-0.06%
dR/R ( 23 V )
dR/R ( 24 V )
dR/R ( 25 V )
0
1000
2000
Rthm [ Ohm ]
3000
4000
1 uA
( output 0 - 10 V )
0.06%
dR / R [ % ]
0.04%
0.02%
dR/R ( 23 V )
0.00%
dR/R ( 24 V )
-0.02%
dR/R ( 25 V )
-0.04%
-0.06%
0
10000 20000 30000 40000 50000
Rthm [ Ohm ]
Graph.11, graph.12 and graph.13: The sensor dR / R [ % ] shift Vs power supply dependence [ 24  1V] for
a conditioner with 0 – 10 V output.
dR/R ( 23 V )
dR/R ( 24 V )
dR/R ( 25 V )
0
100
200
300
dR / R [ % ]
10 uA
( output 4 - 20 mA )
0.15%
0.10%
0.05%
0.00%
-0.05%
-0.10%
-0.15%
0.15%
0.10%
0.05%
0.00%
-0.05%
-0.10%
-0.15%
400
dR/R ( 23 V )
dR/R ( 24 V )
dR/R ( 25 V )
0
Rthm [ Ohm ]
1000
2000
3000
4000
Rthm [ Ohm ]
1 uA
( output 4 - 20 mA )
dR / R [ % ]
dR / R [ % ]
100 uA
( output 4 - 20 mA )
0.15%
0.10%
0.05%
0.00%
-0.05%
-0.10%
-0.15%
dR/R ( 23 V )
dR/R ( 24 V )
dR/R ( 25 V )
0
10000 20000 30000 40000 50000
Rthm [ Ohm ]
Graph.14, graph.15 and graph.16: The sensor dR / R [ % ] shift Vs power supply dependence [ 24  1V] for
a conditioner with 4 – 20 mA output.
8-LINEARITY
The signal conditioner should present a linear output response in each current range depending on the
sensor value as shown in fig.5. The tests indicate a good linear behaviour for the first two current ranges,
but a small non-linearity for the 1 A current range. This non-linearity can be corrected using a third grade
approximation instead of the linear one. The graph.17 to 19 shown the relative resistance sensor error when
a linear or a cubic approximation has been used in each range. This non-linearity can be explained
considering that the input impedance of the instrumentation amplifier is modified a little bit when the sensor
resistor values are big.
100 uA
( output 0-10 V )
10 uA
( output 0-10 V )
0.04%
dR / R linear
0.00%
dR / R cub
-0.01%
-0.02%
dR / R [ % ]
0.01%
0.02%
dR / R linear
0.00%
dR / R cub
-0.02%
-0.04%
0
100
200
300
400
0
1000
Rthm [ Ohm ]
2000
3000
4000
5000
Rthm [ Ohm ]
1 uA
( output 0-10 V )
dR / R [ % ]
dR / R [ % ]
0.02%
0.15%
0.10%
0.05%
0.00%
-0.05%
-0.10%
-0.15%
dR / R linear
dR / R cub
0
10000 20000 30000 40000 50000
Rthm [ Ohm ]
Graph.17 graph.18and graph.19:The sensor dR / R [ % ] = ( R – Rcal ) / R. With R calculated using a
linear and a cubic equation for each range.
9-TEMPERATURE MEASUREMENTS
Some temperature sensors were installed inside the CERN facilities CTM3 cryostat to measure the helium
bath temperature (references) and the vacuum degradation effect over sensors mounting with different
thickness. Each sensor was connected to the acquisition system through the conditioner. The cryostat was
cool down from ambient temperature to 1.9 K ( graph.20 ). The graph.21 shown the transition between
4.2 K to 1.9 K.
Thanks to the super fluid helium characteristics the cryostat was maintained well thermalised during
several days at 1.9 K. The readout temperature dispersion, provided by the sensors installed in the cryostat,
give an idea about the long term stability and the relative temperature precision that can be obtained using
the conditioner in real conditions. The graph.22 shows the readout temperature for different sensors. The
dispersion in the measurement is less than 5 mK at 1.9 K.
180
160
140
120
T20Ca
T[K]
T21Ca
T22Ca
100
T23Ca
T24Ca
80
T25Ca
T26Ca
T27Ca
60
40
20
0
30/ 11/ 98
1/ 12/ 98
2/ 12/ 98
3/ 12/ 98
4/ 12/ 98
5/ 12/ 98
6/ 12/ 98
7/ 12/ 98
8/ 12/ 98
9/ 12/ 98
time
graph.20: CTM3 cryostat cool down
5
4.5
4
3.5
T20Ca
T[K]
T21Ca
T22Ca
T23Ca
3
T24Ca
T26Ca
T27Ca
2.5
2
1.5
1
4/ 12/ 98
4/ 12/ 98
5/ 12/ 98
5/ 12/ 98
6/ 12/ 98
time
graph.21: 4.2 K to 1.9 K temperature transition
6/ 12/ 98
7/ 12/ 98
7/ 12/ 98
2
1.99
1.98
T20Ca
T21Ca
1.97
T[K]
T22Ca
T23Ca
T24Ca
1.96
T26Ca
T27Ca
1.95
1.94
1.93
6/ 12/ 98
6/ 12/ 98
6/ 12/ 98
6/ 12/ 98
6/ 12/ 98
7/ 12/ 98
7/ 12/ 98
7/ 12/ 98
time
graph.22: CTM3 1.9 K measured temperature through different sensors.
To be able to control the real temperature error through the conditioners, manual measurements were taken
at ambient temperature and 1.8 K, using a high precision current source and a multimeter. The manual
values were then compared with the measurements acquired, using the conditioners, through the automatic
system. The table 1 shows the results, considering the manual acquisitions as reference. The maximum error
was obtained for the 3 wires cabling sensor ( the conditioner has been design to work with 4 wires ), and the
15 mm mounted sensor at 1.8 K. The 15 mm sensor error at 1.8 K can be explain looking at it
corresponding resistor column. The manual measured resistance is  26 K, and the conditioner has been
adjusted to work with 25 K maximum resistance range, Therefore at 1.8 K the conditioner is out of range.
When the conditioner is working properly yield very good results, as shown for the same sensor at ambient
temperature in the same table.
Instruments
I:Keithley 220
U: HP34401A
Date:
Measurement in situ
Time Mounting
16:00 2.5mm
5.5mm
7.5mm
10.5mm
12.5mm
15.5mm
Disk
HeII
HeII
Date:
Time
Place
2.5mm
15:50 5.5mm
7.5mm
10.5mm
12.5mm
15.5mm
Disk
HeII
HeII
Time
Place
2.5mm
16:00 5.5mm
7.5mm
10.5mm
12.5mm
15.5mm
Disk
HeII
HeII
26/11/98
Tag number
T19T
T20T
T21T
T22T
T23T
T24T
T25T
T26T
T27T
Pvac:2E-6mBar
Thermometer Name
CX_LS_X4224
CX_LS_X4225
CX_LS_X4227
CX_LS_X4228
CX_LS_X4229
CX_LS_X5013
CX_LS_X05600
CX_LS_X05587
CX_LS_X05594
14/12/98
Current [uA] +/100
100
100
100
100
100
100
100
100
V+[mV]
27.78
5.131
4.9262
4.7995
4.455
5.5264
5.6476
5.2879
5.523
V-[mV]
27.782
5.1352
4.9283
4.8023
4.45625
5.5288
5.6501
5.291
5.526
R [Ohm]
277.81
51.331
49.2725
48.009
44.55625
55.276
56.4885
52.8945
55.245
T [K]
36.211
287.636
290.104
289.752
290.14
290.077
70.6761
290.07
289.769
T.ctm [K]
36.137
287.680
290.180
289.960
290.180
290.190
70.714
290.080
289.870
DT/T@300K [%]
0.2044%
-0.0153%
-0.0262%
-0.0718%
-0.0138%
-0.0390%
-0.0536%
-0.0034%
-0.0349%
1
1
1
1
1
1
1
1
1
V+[mV]
11.755
13.8267
11.765
10.907
6.7183
26.2036
0.48136
6.7777
9.819
V-[mV]
11.7586
13.8275
11.7678
10.9036
6.722
26.1997
0.48436
6.781
9.8159
R [Ohm]
11756.8
13827.1
11766.4
10905.3
6720.15
26201.65
482.86
6779.35
9817.45
T [K]
1.7930
1.8057
1.8038
1.8055
1.8075
1.8062
24.8383
1.8008
1.8014
SCAN 15:59:04
T.ctm [K]
1.7953
1.8075
1.8058
1.8076
1.8085
1.8105
24.8360
1.8016
1.8029
DT/T@1.8K [%]
-0.1272
-0.1025
-0.1103
-0.1158
-0.0537
-0.2358
0.0093
-0.0472
-0.0810
1
1
1
1
1
1
1
1
1
V+[mV]
11.7370
13.8170
11.7586
10.8940
6.7130
26.1746
0.4817
6.7740
9.8140
V-[mV]
11.7490
13.8170
11.7600
10.8900
6.7180
26.1695
0.4846
6.7770
9.8100
R [Ohm]
Temperature
11743.00
1.79394
13817.00
1.80620
11759.30
1.80427
10892.00
1.80648
6715.50
1.80811
26172.05
1.80703
483.15
24.82050
6775.50
1.80133
9812.00
1.80198
SCAN 16:10:43
T.ctm [K]
1.7954
1.8079
1.8062
1.8085
1.8087
1.8111
24.8090
1.8018
1.8030
DT/T@1.8K [%]
-0.0814%
0.0941%
0.1070%
0.1118%
0.0326%
0.2252%
-0.0463%
0.0261%
0.0566%
Pvac:2E-6mBar
Tag number
T19T
T20T
T21T
T22T
T23T
T24T
T25T
T26T
T27T
Thermometer Name
CX_LS_X4224
CX_LS_X4225
CX_LS_X4227
CX_LS_X4228
CX_LS_X4229
CX_LS_X5013
CX_LS_X05600
CX_LS_X05587
CX_LS_X05594
Current [uA] +/-
Tag number
T19T
T20T
T21T
T22T
T23T
T24T
T25T
T26T
T27T
Thermometer Name
CX_LS_X4224
CX_LS_X4225
CX_LS_X4227
CX_LS_X4228
CX_LS_X4229
CX_LS_X5013
CX_LS_X05600
CX_LS_X05587
CX_LS_X05594
Current [uA] +/-
* 3-wires measurment
Table 1