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The ATLAS SCT grounding and shielding concept and implementation

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P UBLISHED BY IOP P UBLISHING FOR S ISSA M EDIALABR ECEIVED: January 13, 2012
ACCEPTED: February 25, 2012
P UBLISHED: March 13, 2012

The ATLAS SCT grounding and shielding concept
and implementationa

SUPA - School of Physics and Astronomy, University of Glasgow,
Glasgow, United Kingdomb Section de Physique, Université de Genève,
Geneva, Switzerlandc Instituto de Fisica Corpuscular (IFIC), University of Valencia and CSIC,
Valencia, Spaind Particle Physics Department, Rutherford Appleton Laboratory,
Didcot, United Kingdom
e Institute of Physics, Academy of Sciences of the Czech Republic,
Praha, Czech Republic
f Department of Physics and Astronomy, University of Uppsala,Uppsala, Sweden
g The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences,
Krakow, Polandh CERN,
Geneva, Switzerlandi
Department of Physics, Jožef Stefan Institute and University of Ljubljana,
Ljubljana, Slovenia
1 Corresponding author.

c 2012 IOP Publishing Ltd and Sissa Medialab srl

doi:10.1088/1748-0221/7/03/P03005

2012 JINST 7 P03005

R.L. Bates,a P.J. Bell,b J. Bernabeu,c,1 J. Bizzell,d J. Bohm,e R. Brenner, f
P.A. Bruckman de Renstrom,g A. Catinaccio,h V. Cindro,i A. Ciocio, j J.V. Civera,cS. Chouridou,k P. Dervan,l B. Dick,m Z. Dolezal,n L. Eklund,b L. Feld,o D. Ferrere,bS. Gadomski,b F. Gonzalez,c E. Gornicki,g A. Greenhall,l A.A. Grillo,kJ. Grosse-Knetter, p M. Gruwe,h S. Haywood,d N.P. Hessey,q Y. Ikegami,r T.J. Jones,lJ. Kaplon,h P. Kodys,n T. Kohriki,r T. Kondo,r S. Koperny,s C. Lacasta,cJ. Lozano Bahilo,t P. Malecki,g F. Martinez-McKinney,k S.J. McMahon,dA. McPherson,h B. Mikulec,h M. Mikuž,i G.F. Moorhead,m M.C. Morrissey,d K. Nagai,uA. Nichols,d V. O’Shea,a J.R. Pater,v S.J.M. Peeters,w H. Pernegger,h E. Perrin,bP.W. Phillips,d J.P. Pieron,x S. Roe,h J. Sanchez,c E. Spencer,k J. Stastny,eJ. Tarrant,d S. Terada,r M. Tyndel,d Y. Unno,r R. Wallny,y M. Weber,zA.R. Weidberg,aa P.S. Wells,h P. Wernekeq and I. Wilmutd

j Physics Division, Lawrence Berkeley National Laboratory and University of California,

Berkeley CA, United States of America
Santa Cruz Institute for Particle Physics, University of California Santa Cruz,
Santa Cruz CA, United States of Americal Oliver Lodge Laboratory, University of Liverpool,
Liverpool, United Kingdom
m School of Physics, University of Melbourne,
Parkville, Victoria 3010, Australian Faculty of Mathematics and Physics, Charles University in Prague,
Praha, Czech Republico RWTH Aachen University, I. Physikalisches Institut,
Aachen, Germany
p II Physikalisches Institut, Georg-August-Universität,
Göttingen, Germanyq Nikhef National Institute for Subatomic Physics and University of Amsterdam,
Amsterdam, Netherlands
r KEK, Oho 1-1,
Tsukuba, Ibaraki 305-0801, Japans
Faculty of Physics and Applied Computer Science, AGH-University of Science and Technology,
Krakow, Polandt Universidad de Granada and CAFPE,
Granada, Spainu Institute of Pure and Applied Sciences, University of Tsukuba,
Ibaraki, Japanv
School of Physics and Astronomy, University of Manchester,Manchester, United Kingdom
w Department of Physics and Astronomy, University of Sussex,
Brighton, United Kingdom
x Department of Physics, University of Wisconsin,
Madison WI, U.S.A.
y University of California,
Los Angeles, U.S.A.
z Karlsruhe Institute of Technology,
Karlsruhe, Germany
aa Department of Physics, Oxford University, Oxford,
United Kingdomk

A BSTRACT: This paper describes the design and implementation of the grounding and shielding
system for the ATLAS SemiConductor Tracker (SCT). The mitigation of electromagnetic interference and noise pickup through power lines is the critical design goal as they have the potential tojeopardize the electrical performance. We accomplish this by adhering to the ATLAS grounding
rules, by avoiding ground loops and isolating the different subdetectors. Noise sources are identified and design rules to protect the SCT against them are described. A rigorous implementationof the design was crucial to achieve the required performance. This paper highlights the location,2012 JINST 7 P03005

E-mail: Pepe.Bernabeu@ific.uv.es

connection and assembly of the different components that affect the grounding and shielding system: cables, filters, cooling pipes, shielding enclosure, power supplies and others. Special care istaken with the electrical properties of materials and joints. The monitoring of the grounding system
during the installation period is also discussed. Finally, after connecting more than four thousand
SCT modules to all of their services, electrical, mechanical and thermal within the wider ATLAS
experimental environment, dedicated tests show that noise pickup is minimised.
K EYWORDS : Si microstrip and pad detectors; Large detector systems for particle and astroparticle
physics; Detector grounding

2012 JINST 7 P03005–1–

Contents
The ATLAS SCT

12The ATLAS context
2.1 Grounding and shielding policy
2.2 The ATLAS caverns

4
4
4

3

Grounding and shielding concept
3.1 Small-signal path
3.2 Faraday cage and power cables
3.3 Cooling loops

5
6
8
10

4

Grounding and shielding implementation
4.1 Detector module
4.2 Low-mass tapes
4.3 Cooling pipes
4.4 Faraday cage
4.5 Heat spreader plates and segmented plates
4.6 Power cables
4.7 Patch panels
4.8 Power supplies
4.9 Other systems connected to SCT ground4.10 Ground isolation monitoring during installation

10
11
12
13
15
18
20
23
25
25
26

5

Problems and installation experience27

6Conclusions

29

1

The ATLAS SCTThe ATLAS experiment [1] is one of two general-purpose detectors at CERN’s Large Hadron
Collider (LHC). The SemiConductor Tracker (SCT) is a silicon strip detector and forms the intermediate tracking layers of the ATLAS inner detector (figure 1).
The SCT is geometrically divided into a central barrel region and two endcaps (known as ‘A’and ‘C’). The barrel region consists of four concentric cylindrical layers (barrels). Each endcap
consists of nine disks. The innermost and outermost radii of the barrel are 300 mm and 520 mm.
The endcaps innermost and outermost radii are 275 mm and 560 mm.
Barrel layers consist of one single module type, arranged in rows [2]. The endcap disks consistof modules arranged in rings [3]. One disk can have up to three rings, and the endcap modules are–1–2012 JINST 7 P03005

1

Figure 2. Picture of a barrel module (left). Three different types of endcap modules (right).

of three types: inner, middle (or short middles) and outer, corresponding to the different rings(figure 2).
The complete SCT consists of 4088 modules [4, 5]. Each module has two planes of silicon,
each with 768 active strips of p+ implant on n-type bulk [6]. The planes are offset by a small
stereo angle (40 mrad), so that each module provides space-point resolutions of 17 µ m perpendicular to and 580 µ m parallel to its strips. The implant strips are capacitively coupled to aluminium metallization, and are read out by the front-end electronics located on a printed circuitboard (known as a hybrid) on low-radiation-length material, which hosts twelve 128-channel ASIC
chips (ABCD3TA [7]). The complete SCT has 49,056 front-end ASICs and more than six million
individual read-out channels.
The SCT uses a “binary” readout architecture. For each channel, the signal is amplified,discriminated to one bit, stored in a digital pipeline memory and digitally read out. The only pulseheight information transmitted by the front-end chips is one bit per channel which denotes whether
the pulse was above a preset threshold. Further information about the size of the pulse cannot berecovered later, so the attenuation of the electromagnetic interference is crucial to the successful
operation of the detector.

–2–2012 JINST 7 P03005

Figure 1. ATLAS Inner Detector. The SCT is within the Transition Radiation Tracker (TRT) and surrounds
the Pixel detector. The SCT barrel has four layers and each of the endcaps has nine disks.

The discriminator threshold must be set at a level that guarantees uniform good efficiency
while maintaining the noise occupancy at a low level. Furthermore, the silicon part of the SCT
detector must maintain good performance even after a total ionising dose of 100 kGy, and a nonionising fluence of 2x1014 neutrons/cm2 of 1 MeV neutrons equivalent corresponding to 10 yearsof operation of the LHC at its design luminosity. The physics performance requirements, based on
track-finding and pattern-recognition considerations, are that single strip hit efficiency should begreater than 99% and noise occupancy less than 5x10−4 per channel even after irradiation.

Minimising the noise is essential to have a good discrimination margin and to achieve therequired noise occupancy. Electromagnetic interference (EMI) occurring in the input path of thedetector to the comparator appears as additional noise to that of the amplifier and detector. The
grounding and shielding task is to attenuate EMI in the detector and front-end electronics before
the discriminator.
The regions which are most susceptible to EMI are the silicon strip detector and its signal pathto the amplifier, which includes the signal return path. This small-signal path will be described
more fully in section 3.1.
If EMI attenuation design is sufficient, then the threshold increase to accommodate it should beless than 5%. Since the discriminator threshold is set at ∼1 fC of input signal charge, the detected
EMI should require a threshold increase of less than 50 aC input equivalent.
By comparing single module noise levels with module array noise measurements, the total
EMI induced noise can be estimated. The power and servicing cables extend radially from thetracker up to 120 m at all angles. The cables are in close proximity to most of the other ATLAS
systems, so EMI from all other systems is difficult to estimate and control. Noise measurements
have been taken as the SCT subsystems were assembled, verifying that system EMI should notcompromise the detector performance. However, the actual EMI value can only be learned after
designing and installing the entire ATLAS experiment, with all ATLAS subsystems operational.
The SCT detector, being part of the ATLAS experiment, is affected by mechanical and electrical constraints. Section 2 explains these constraints on the SCT grounding and shielding. The
SCT grounding and shielding concept is described in section 3. To achieve the desired EMI attenuation, many standard techniques were employed in the power supply design, cabling, filters,
detector shielding, bonding, and module design. Section 4 explains the implementation of the SCT
grounding and shielding. During the fabrication and installation of the SCT several problems arose,they are described in section 5. Finally, section 6 concludes by explaining the performance of thegrounding and shielding system.

–3–2012 JINST 7 P03005

A minimum ionising particle generates approximately 3.6 fC charge in the 285 µ m-thick silicon detector. The charge is collected typically in one or two strips, typical strip pitch is 80 µ m.
Each strip is connected to one input of the front-end chip. The signal is amplified and shaped to
20 ns peaking time and then discriminated by a single comparator. The noise level before irradiation is around 0.24 fC. In order to meet the design specifications of high efficiency and low noise,
the threshold is normally set to 1 fC. This threshold is adjustable to allow good discrimination
between signal and noise during the 10 years of operation.

2

The ATLAS context

2.1

Grounding and shielding policy• All detector systems are electrically isolated, which implies that there are no connections
between different detector systems.
• Power supplies are floating.
• There are no connections to ground other than through the unique safety ground point.
• Each subdetector is located in a Faraday cage.
In addition, electromagnetic compatibility of ATLAS electronic equipments has been insured toachieve the expected level of performance of the experiment. The ATLAS electromagnetic compatibility policy [9] defined the electrical safety aspects of the electronic systems for a proper
operation in the ATLAS electromagnetic environment.
2.2

The ATLAS caverns

In addition, the design of the SCT grounding and shielding has mechanical constraints. ATLAS
is installed in an experimental cavern at 92 m depth (UX15), where LHC collisions occur. Powersupplies, readout electronics, cooling and monitoring systems, etc. have been installed in two adjacent caverns, US15 and USA15, where there are much lower radiation levels and magnetic fields,
allowing access for maintenance. In the service caverns, the development of special field-resistantelectronics is unnecessary. Thus, the SCT electronic system spans a huge volume, with cables up
to 120 m long between the detector and the service cavern electronics as shown in figures 3 and 4.
Modules on the same barrel cylinder or the same endcap are supplied from power supplies inboth adjacent caverns. Furthermore, due to the complicated cable path inside the ATLAS detector,
modules in the same detector array going to the same power-supply cavern can have very different
cable paths. Very big loops with other subdetector electronics inside are formed (figure 5). Thisis unavoidable and the grounding and shielding design must minimise this big ground-loop effect.
ATLAS line-voltage earth potentials in USA15, UX15 and US15 can vary because they are quite–4–2012 JINST 7 P03005

The ATLAS detector consists of many complex components, installed and operating in contiguous volumes, resulting in a large amount of installed equipment with multiple interconnections
and shared services. For these reasons, if these issues are not properly taken care of already atthe design stage and systematically followed up during installation, the performance of the detector components could be adversely affected by electromagnetic interference or induced electronicnoise.
An ATLAS grounding and shielding policy was developed in the design phase of the experiment [8] to minimise possible electromagnetic interference effects. Large detector subsystems have
many connections to the surrounding world for signals, monitoring, power, cooling, etc.; if left tochance, a bewildering network of ground loops will arise. Thus, apart from safety considerations,one of the main concerns has been the prevention of ground-loop currents that would couple to thesignals of the detector systems.
The main guidelines of the ATLAS policy can be summarised in these rules:

Figure 4. On the left, a cross-section of the ATLAS cavern UX15 (30 m wide and 35 m high, 50 m long).
The SCT services arriving from the adjacent caverns are highlighted. The picture on the right is the Inner
Detector (ID) volume before detector installation. Very careful packing of the various subdetector services isrequired in the constrained space, so that isolation of each subdetector shielding system can be guaranteed.

far apart. Following the ATLAS policy of having a single connection to ground; power-supply
returns and cable shields are floating at the power-supply end in order to minimise ground loops at
low frequencies through ATLAS grounds.

3

Grounding and shielding concept

The main role of the grounding and shielding concept is the prevention of electromagnetic interference or “noise pickup”. It protects the SCT against external sources of noise and it also protects
the SCT neighbour systems against the noise that it radiates. In the SCT, the most sensitive region–5–2012 JINST 7 P03005

Figure 3. Schematic view of the underground ATLAS installation. The detector cavern is UX15, the adjacentservice caverns are USA15 and US15. PX14 and PX16 are the installation shafts for surface access, PM15
and PX15 are the two elevators. The detector is almost as large as the UX15 cavern.

USA15
PS Rack

USA15
PS Rack

PP3

PP3

PP

1

PP
1

SP

RCT

PP3
RC
T

PP1

SC T

Endc

ap

PP1

T

B

ar

re

l

1
PP

SC

HSP

1
PP

PP
1

PP3

US15
PS Rack

PP3

US15
PS Rack
PP3

PP1

PP3

PP3

is the small-signal path between sensor and amplifier. There are several potential noise sources andmeans of transmission:
1. EMI from circuits generated locally at the sensor module or from adjacent modules.
2. Electromagnetic radiation from surrounding ATLAS detectors.
3. Conducted EMI currents through the module service cables or system shielding, captured
primarily by ground loops.
4. EMI from the metal cooling tube to which each sensor module is clamped.
First, we will describe the small-signal path more fully. Next, we discuss the module design techniques to attenuate Source 1. A well-designed Faraday cage is the best means to reduce externallycaptured EMI, Source 2 and Source 3. Finally, we will comment on techniques that address Source
4, the cooling pipe relationship with the module.
3.1

Small-signal path

The EMI shielding design starts with the identification of the small-signal path. In the SCT, thesmall signal is generated in the depleted silicon sensor when an energetic particle crosses. The
detector respresents a current source. The small-signal path is the entire current loop that thephysics signal travels along to dissipate its energy.
As shown in figure 6, the small-signal path includes the silicon-strip diode and strip metal,
the front-end chip amplifier, the hybrid analogue ground plane, and the detector high voltage (HV)
backplane bias that is AC bypassed to the hybrid analogue ground. In addition, a pitch adapter andseveral bondwires are part of the path from detector to amplifier [4, 5]. EMI current induced into
the small-signal path is indistinguishable from signal, since the binary readout does not permit afiltering scheme. The hybrid ground is the zero-volt reference for the small signal.

–6–2012 JINST 7 P03005

Figure 5. The module service cables are laid in widely separated paths throughout the ATLAS volume.
Floating power supplies in the PS racks drive the cables, so the ground loops between cables are mostly
completed by substantial distributed capacitance to earth over the cable paths. In the left-hand figure, thebarrel detector sensors straddle the ground loop between the service cable arrays of Side A and Side C. In
the right-hand figure, the endcap sensors are outside the service cable ground loops.

1.5 MΩ
ABCD FRONTTRANSISTOR

AL

ST

RI
P

BIAS-

P STRIPHV BIAS+HYBRID

Figure 6. Small-signal path. The signal is generated in the silicon, routed from the strip to the amplifierin the chip. The return path includes the ground node at the chip and the hybrid, the HV capacitor and thedetector backplane.

The small-signal ground is that part of the small-signal path that multiple strips have in common. Included in this sub-category of the small-signal path are the front-end chip internal analogue
ground bus, the hybrid analogue ground, the detector-HV backplane, and their connecting wirebonds. Most SCT EMI couples into the electronics at the small-signal ground since connection to
the service-power return and digital section is essential. The front-end channels can also crosstalk
through their common small-signal ground connection. A useful diagnostic model is to look for
induced EMI currents through the small-signal ground.
Four design techniques are used to minimise EMI pickup in the small-signal path:
1. Make the small-signal ground as low resistance and low inductance as is practically possible.
EMI currents in the small-signal ground appear as pickup voltages at the front-end preamplifier transistor. Minimising the small signal-ground impedance minimises the voltage at thefront-end amplifier induced by an EMI current.
2. Reference the detector module at a single point, the power-supply connector, away fromthe small-signal ground. Supplementary connectors should be close to the power-supply
connection. This single-point tie avoids placing the small-signal ground in the loop between
two external conductors, which will usually have an AC potential across them.
3. On the hybrid, avoid the flow of currents from other services through the small-signal ground.
Logic and optical drivers on the hybrid should have their own path to the power-supply
connector, and the path should be as independent as practical from the small-signal ground.
4. Nearby conductors that are external to the module need to be shielded from the detector andsmall-signal paths. The stray capacitance to the external conductor forms a ground loop that
includes the small signal path, the power return, and the referencing path between the powersupply return and the external conductor. The module-cooling tube couples to the detector–7–2012 JINST 7 P03005

SI DETECTOR(N BULK)

Hybrid

Silicon Detector

BypassCapacitor

sensor with ∼100 pF capacitance in the simplest configuration. This direct coupling is reduced using an electrostatic shield, called the shunt shield. Unattenuated, this capacitancewould create an EMI loop through the small-signal path.
The detector module has low susceptibility to radiated EMI since the receiving antennae loop
areas are small. Two loops are indicated in figure 7. The interstrip loop area is limited by the∼80 µ m strip pitch, and the strip-to-backplane loop area is limited by the detector thickness of∼285 µ m. On the other hand, the 12 cm long strips could receive electric field interferences. Thus,
even if the susceptibility is low, the front-end electronic design should avoid local electromagnetic
radiation that could couple to the sensitive amplifiers.
3.2

Faraday cage and power cables

As seen in the previous section, an isolated module has a fair EMI resistance even without a complete shield enclosure. However, in the SCT, the detector modules overlap each other by ∼2.5% of
their area. Modules are about 6 cm wide and 6-12 cm long (figure 2), and overlap a few millimetresalong each edge. In the overlap area, the sensors are ∼2 mm apart [10]. This parasitic capacitanceto another sensor disturbs the small-signal path of the overlapping strips: overlapping sensors canhave cables widely separated in the cavern. Thus, much of the EMI design is devoted to filtering
the cable EMI, and reducing cable common-mode AC differences.
The primary purpose of the Faraday cage is to provide a low-impedance common-bonding
node for the cable shields and cable-entry filters at PP1 (Patch Panel 1). In addition, the shieldprovides a common referencing point for all conductors inside, minimising the voltage difference
between two internal parts (i.e. between two adjacent modules). Three main rules should be implemented:• Every conductor entering the Faraday cage must be shielded.
• Every cable entering the cage has its shield bonded to the cage at its entry point.
• Every conductor going inside the cage should short to the cage skin or receive capacitor
bypassing.

–8–2012 JINST 7 P03005

Figure 7. One magnetic loop antenna is comprised of a strip and its small-signal path returning throughthe detector backplane. A second loop occurs with the stray capacitance across two adjacent strips and theirsmall-signal path to the chip ground bus. Both loop areas are small, and the radiated EMI susceptibility ismodest.

Power
Supply A

Faraday Cage

Power Stray

Detector to
Detector Stray

Pickup andShielding Currents

Power Stray

Figure 8. In the absence of the Faraday shield, noise currents would flow through the modules. The shieldprotects the modules from noise currents.

Figure 8 illustrates the ground loop resulting from the overlap of modules, and the action of
the Faraday cage that shunts the loop. The tie to the ATLAS earth environment is completed by
the distributed-shield stray capacitance of the long cable runs. Noise currents generated in theexternal-cable ground loop will be diverted through the Faraday cage, attenuating the noise-current
entry into the module small-signal paths. The attenuation is directly dependent on the conductivityof the cage, which is dominated by the electrical bonding joints between the cage sections and thejoints between the cage and the cable shields.
The main tool for reducing the EMI to the detector small-signal paths is designing a highconductivity skin. Also, the cage apertures should be minimised. Only one single tie from the
Faraday cage to the safety earth should be used to avoid needless ground-loop currents in theenclosure walls.
A complete shield envelops each of the three parts of the SCT (Barrel and Endcaps A and C),each one having a single safety-earth cable. These three earth cables are tied to earth at the same
point to minimise potential differences between the three detector parts.
Given the unavoidable ground loops shown in figure 5, filtering is essential. As explainedlater, the various power cables have different lengths and thicknesses. Filter networks occur at two
patch panels, PP1 and PP3. The locations of PP1 and PP3 are detailed in sections 4.6 and 4.7. The
cable shields are AC-coupled at the power supply to avoid DC currents in the cable shields. The
barrel cage is included in the service-cable ground loop, so EMI shielding currents circulate along
the barrel. The endcap cages are outside of their respective service-cable ground loops, and noise
currents are probably modest on the endcap cages.
In both barrel and endcap, good radial connections between all PP1 shield nodes are needed,
so as to reduce noise currents on the cage skin. In the case of the barrel, noise currents between theshields of cables arriving at each end of the barrel will flow along the barrel cage. Therefore, it isimportant that the resistance of the cage skin is minimal.

–9–2012 JINST 7 P03005

Power
Supply B

3.3

Cooling loops

Each detector module is thermally coupled to a cupronickel evaporative-cooling tube that connects
adjacent modules. The cooling tubes provide little useful EMI shielding function, but the shielding
and bonding design must accommodate their high conductivity and large stray capacitance to thesensors.
A module cooling tube will enter and exit the Faraday cage at almost the same point in thecage (figure 9). The tube bonds to the cage at its entry and exit, so no isolation of the cooling tube
is needed.
The cooling loops travel very close to the detectors, with large capacitance to the silicon detector sensor, so noise currents on the tubes must be minimised. Stray capacitance between a coolingtube and a detector sensor can be as large as 100 pF. Since the cooling tube serves modules, withpower tapes entering the Faraday cage at very different points, the tube stray capacitance creates aground loop that includes the detector sensors. The potential ground loop would include a coolingtube, its module sensors with different power supply groups, and the portion of the cage between
the two power-supply entry points. Additionally, the service cable EMI from outside PP1 that PP1
does not filter will appear in the ground loop. This cooling ground-loop problem is more importantin the barrel as cooling tubes serve modules that have power tapes entering at both ends of thebarrel, so the loop area is large. A module shunt-shield between the cooling tube and the detector
sensor is needed to minimise noise currents going through the detector sensor, thereby protecting
the small-signal path. This shunt-shield is described in section 4.3.

4Grounding and shielding implementation

As mentioned above, in the SCT, all supplies are floating following the ATLAS grounding policy.
The barrel and two endcaps form three independent, isolated detector volumes with references to asingle earth. Each volume has a complete shield skin with a single-earth safety-bond point.

– 10 –

2012 JINST 7 P03005

Figure 9. Module cooling tube on one quarter of an endcap disk. It forms a loop very close to the modules
inside the Faraday cage.

VCC

VDD

220nF

220nF

POWER CONNECTOR
VCC
1µF

470pF

digitalAl strip

AGND

SiO2

~1.5M

p+

Bias-

ABCD3TA

VDD
1µF

n bulk

470pF

DGND

10nF

SILICON DETECTOR

PRINTEDCIRCUIT

Bias+
HV

5K

HVret

1K

5K

Bias+
10nF

Bias-

Figure 10. Module schematics. The small-signal path contains the detector, the analogue amplifier inthe chip, the analogue ground (AGND) and the HV bypass capacitor. Analogue ground (AGND) and digital
ground (DGND) are tied together in the printed circuit. Noise filtering is applied close to the power connector
and bypass capacitors are placed close to the chip.

This section describes how the different components of the SCT affecting the grounding andshielding have been designed and implemented following the requirements indicated in the previous section. SCT grounding and shielding conforms to the proposals in references [11, 12].
4.1

Detector module

The SCT modules were described in section 1. The small-signal path is contained in the detector
module, going from the silicon to the amplifier in the chips (figure 10).
Three power supply sources provide current to the module: the analogue power supply and its
return that power the analogue part of the chips, the digital power supply and its return that power
the digital part of the chips (pipeline, readout buffers and communication), and the silicon highvoltage and its return to bias the detector.
The possible noise arriving on analogue and digital supplies is filtered by capacitors close to
the power connector. In addition, four bypass capacitors are located close to each chip (two for the
analogue supply and two for the digital one).
The EMI-sensitive analogue path (small-signal path) should be detached from noisy sources
such as the digital circuitry. Thus, in the ABCD3TA, the analogue and digital circuits and grounds
are completely independent. Only in the discriminator section is a correct reference of analogue anddigital grounds needed. In order to isolate geographically this coupling and avoid noise currents
entering the chip, the analogue and digital ground connection is done outside the chips in the printed
circuit by means of bond wires.
The high-voltage supply has a filter network close to the power connector as well. The HV
return line is referenced to the analogue ground through a 1K Ω resistor. The high-frequency signal
passes through the HV-bypass capacitor to close the small-signal loop.

– 11 –

2012 JINST 7 P03005

backplane

Special care has been taken on the printed circuit board to provide good ground planes foranalogue and digital separately. The analogue ground layer is part of the small-signal path; returncurrents of the digital circuits are kept away in their own digital ground layer. They provide a low
impedance return to the source for noise power generated on the board; they also provide somelevel of shielding between circuit sections.
4.2

Low-mass tapes

As an extension of the power-supply cables, metal-on-kapton low-mass tapes (LMTs) are used
inside the detector volume to carry power, sensing and control signals, and high-voltage bias toeach module. Tapes in the barrel run from the modules to the first patch panel (PP1). In the endcap,
very low-mass ribbon cables with twisted-pair copper-clad aluminium power wires run from themodules to a patch panel at the edge of each disc (PP0), with LMTs from there to PP1. The PP1s
are just outside the respective Faraday cages.
Material in the ATLAS inner detector degrades the tracking performance as well as that of thecalorimeter. The significance of material grows towards the interaction point. Thus, as tapes run
inside the detector volume, they were designed to use aluminium as a conductor and are unshieldedto avoid extra material. Tapes inside the barrel shield use aluminium. However, the barrel tapes
were found to be fragile in the regions where the aluminium had been Ni plated for soldering. Thus,
it was decided to use copper conductors for the endcap tapes [13, 14].
Tapes are constructed of two glued layers of single-sided tapes (figure 11) with a width of
21 mm and a thickness of 330 µ m. Low-current lines for sensing and control have a width of
0.5 mm and high current ones for powering have a width of 4.5 mm. The high-voltage lines are
0.8 mm wide and IPC-2223 specification is obeyed for HV separation (2.5 mm for 500 V). The
range of tape lengths in the barrel is 0.7-2.1 m, in the endcap 1.3-3.1 m.
Rather large currents flow through the power tapes, typically 1.7 A in each of the 4088 tapes.
The tape design maximises conductivity because a significant voltage drop converts to undesirable
power dissipation inside the cold volume. The resistance specification is 250 mΩ/m for the 4.5 mm
power lines for the barrel and 150 mΩ/m for the endcaps. The difference arises since the endcap
tapes are longer than the barrel ones. For the 0.5 mm lines, the resistivity is 2.24 Ω/m for the barrel
and 1.7 Ω/m for the endcap.

– 12 –

2012 JINST 7 P03005

Figure 11. Cross-section of the endcap LMT with copper traces. Barrel tapes are similar, but with a 50 µ mthick layer of aluminium instead of 35 µ m of copper.

In order to reduce the sensitivity to pickup noise and given that the tapes are unshielded,
the power and return lines in the top and bottom layers are coupled back-to-back minimising thecurrent-loop area. The misalignment in the top and bottom layers had to be smaller than 0.2 mm.
The wide power and return lines have a capacitance of 1.9 nF/m and an inductance of 20 nH/m.
4.3

Cooling pipes– 13 –

2012 JINST 7 P03005

The cooling loops inside the detector volume are referenced to the SCT ground by connecting themto the shield. In order to improve the good referencing of each loop and also that between different
loops, a copper disk sheet (Cooling Reference Disk - CRD) is added at both ends of the barrel. Litz
wires connect each loop to the CRD. Each barrel cooling loop enters and exits the shield on thesame end of the barrel [2], however, its path inside the shield takes it very close to the other end. A
barrel cooling loop is referenced at both ends to both CRDs, in order to equalise the referencing of
all loops independent of which side of the barrel they enter.
The endcaps have the Radial Cable Tray (RCT) structure [3], to which the cooling loops areconnected. In addition, a copper-grounded foil was added for each endcap disk and soldered to thecooling pipes. This reduces the impedance to ground-loop currents.
As mentioned, the cooling loops inside the shield are referenced to SCT ground. The external
metal cooling tubes, running metres along the rest of the ATLAS subsystems and from the coolingplant in the adjacent cavern, are earthed to an ATLAS ground different from the SCT ground. Inorder to preserve the SCT isolation, an electrical break in the cooling tubes is implemented outside
the detector array shield in its vicinity.
The cooling pipes form conductive loops inside the detector volume. In addition, they run very
close to the detector backplanes, being a source of noise pickup into the small-signal path.
A system test was realized during the design and test phases of the SCT construction [15]. It
consisted of a barrel sector to support up to 48 modules and an endcap sector of a quarter-disk tohost up to 13 modules. It used system components (cables, fibres, cooling pipes, etc.) similar to thefinal detector. One of the aims of the system test was to investigate the effect of pickup, crosstalkand common-mode noise as functions of module location, grounding and neighbour module configuration. Many tests were developed and it influenced the design of many of the endcap and barrel
system components: power-cable routing, shielding, connections, hybrid grounding (section 4.1),
etc..
When operated in a multi-module run on the system test, the SCT modules showed similarelectrical performance compared to being operated individually on the bench. So, in order to test
the detailed performance of each grounding and shielding configuration, noise was deliberately
injected into the power tapes and cooling pipes. The best results were obtained with the module
ground connected to the module cooling block [16].
Two different options of cooling-block connection were used for barrel and endcap modules.
In the barrel modules, a shunt shield was used. It consists of an additional conductive layer inthe module mounting-block, between the cooling tube and the detector (figure 12). The layer isdirectly connected to the LMT ground entry point at the module connector. Thus, the ground loop
due to the cooling tube no longer includes the detector sensor and the small-signal path is protected.
Most of the currents in this ground loop flow through the power tape only, leaving the sensor more

BARREL

Endcapshunt shieldconnection

Figure 12. Shunt shield implementation. The connection to the module connector diverts noise pickup in
the cooling circuit to SCT ground, keeping the noise away from the small-signal path. The picture shows theconnection from the endcap cooling to the LMT power connector.

isolated from stray currents. The stray capacitance from the tube to the sensor is reduced to 1 pF or
less.
With the shunt shield in place, any variation in the potential of the cooling pipe will induce
current on the shield rather than the hybrid ground or detector backplane; hence the resulting signal
is shunted to the module ground at the power connector.
The shunt-shield flexible circuit was used over the cooling block. To optimise the capacitive
shielding, a small overhang of 1 mm was added to the shunt shield. This also had the benefit ofproviding extra protection against HV breakdown between the module and the cooling block. The
shunt shield was made from 18 µ m copper on 25 µ m polyimide and was connected to the analogue
ground on the electrical connector of the module [2]. The option of changing the grounding scheme
to connect the cooling block to the module analogue ground was made available by a tab which
could be connected to the cooling block by a screw; however, this connection was never required.
The thermal impedance of the shunt shield had to be minimised in order not to increase significantly
the temperature difference between the cooling block and the module. To this end, the shunt shields
were attached to the cooling blocks with 20 µ m of epoxy loaded with boron-nitride.

– 14 –

2012 JINST 7 P03005

ENDCAPIn the endcap modules, a similar solution was adopted in order to avoid the ground loop including the detector backplane or hybrid ground. System test results showed that in the endcap
geometry, the best performance against noise was achieved by DC-connecting the cooling block to
the power connector (figure 12).
4.4

Faraday cage

• It provides mechanical protection for the SCT assembly.
• It provides thermal insulation between the SCT and TRT, which operate at different temperatures, −7o C and +25o C respectively.
• It contains a dry-nitrogen atmosphere to avoid condensation.
• It separates the SCT and TRT gas volumes, and also the volume between SCT and TRT thatis flushed with CO2 .
• Finally, it acts as an electrically earthed shield, giving a good voltage referencing for cables
and patch panels. The apertures of the shield have been kept to the strict minimum forservices access.
The design was challenging, as all the above functions had to be accomplished with the constraints of minimal mass, gas tightness and a very strict mechanical tolerances defining the envelope. The barrel Faraday cage was realised in aluminium and the endcap ones in copper (figure 13).
Due to various constraints, the cages are not one piece. Significant care was taken to minimise impedance by use of gaskets, lap joints and conductive glue and implementing high-quality
corrosion-proof electrical joints that will not deteriorate throughout the lifetime of the SCT. In addition, the electrical shield had to be as continuous as possible; gaps and seams were minimised.
References [2, 3] explain in detail the design of the thermal enclosures.
In order to prevent corrosion in the barrel cage and avoid increased resistance across aluminium joints, all aluminium parts were treated with Alochrome 1200 to resist oxidation, thereby
providing a more reliable, long-term electrical connection. In the skin design, particular attention
was given to the Al-joint bonding technique, in order to reduce the AC impedances of the joints.
A spot-welding technique was used to weld aluminium tabs to the outer-cylinder aluminium skin(see figure 13). During assembly, the tabs were folded over the rim and bolted to an annular copperring. A silver-coated nickel epoxy, EP79, was used for some electrically conductive joints. Many
joints included commercial tin-plated Be-Cu radio-frequency gaskets.1
1 Laird Technologies, http://www.lairdtech.com, part number 0C98-0550-17.

– 15 –

2012 JINST 7 P03005

The Faraday cage is a key element in the grounding and shielding concept. It not only acts as ashield against external electromagnetic radiation, but as a low-impedance path for possible noise
currents. The aim is to keep noise currents out of the detector volume defined by the cage. The
cables and patch panels are referenced to the cage by low resistance connections.
The SCT has three independent subdetectors: Endcap A, Barrel and Endcap C. Each one
is independently earthed and has its own Faraday cage. The SCT Faraday cage functionality isprovided by the thermal enclosure. This has other functions as well as the grounding and shielding:Figure 14. Diagram of the bridging solder technique (left). Right-hand figure is a picture during the soldering. Temporary tape forms a channel for controlling the extent of the solder bridging gap between the two
Cu-polyimide sheets.

Reference [17] details the grounding and shielding implementation for the structure parts ofthe endcap. The endcap cage was formed from sheets of Cu-polyimide. Since no single sheet
was big enough, several were required and were connected using a “bridging” technique illustrated
in figure 14. The bridging was made with strips of copper foil 3 mm wide. To avoid handlingproblems associated with long strips of foil, the foil was cut into sections ∼10 cm long and joints
were made with ∼1 cm gaps between two bridges (figure 14). At the Services Thermal Feed
Through (STFT) at the end of the endcap, gaps allow services to pass through. An attempt wasmade to limit apertures to less than 1 cm x 10 cm.
To check the correct implementation of the grounding scheme, extensive measurements weremade to ensure that intended connections were indeed of low resistances and no unwanted electrical– 16 –

2012 JINST 7 P03005

Figure 13. SCT Faraday cages. The barrel one (left) was made of aluminium sheets with alochrome plating
and the endcap ones of copper foil on a foam cylinder (right).

paths had been created inadvertently. DC measurements of various items used in the assembly weremade in a systematic way in the SR1 surface building at CERN so that any corrective action could
be undertaken before the detectors were installed in ATLAS. The connections were also monitored
after installation and showed no deterioration.
Two measurement techniques were used:• A standard multi-meter was used to measure resistance with a precision of 0.1-0.2 Ω. Since a
multi-meter functions with a very small current, this method presented a low risk for devices
built with semiconductors.

In the barrel, the resistance across the different lap joints of the aluminium skin is typically∼0.16 mΩ; the resistance between the top half-cylinder and bottom half across the side panel is∼0.5 mΩ.
In the endcaps, the conducting paths had resistances less than 0.2 mΩ; while insulating paths
had resistances in excess of 1 MΩ.
The three SCT subdetectors are inserted into the TRT subdetectors, which are inside the ATLAS Liquid Argon Calorimeter and the solenoid. All these structures act as electrical shields
against radiated EMI. However, the SCT Faraday cages add additional protection. The shielding
effect against external EM fields is obtained by reflection and absorption of the EM wave.
As mentioned, reflection on the shield reduces the EM fields. The loss due to reflection on theshield at normal incidence is [18]:Ar = 20 log

|Zw |
[dB]
4|Zs |(4.1)

Zw is the characteristic impedance in air of the EM wave and Zs the one of the shield. Zw in the
air and for far field is 377 Ω. The transition from near to far field occurs at λ /2π . At a frequencyof 10 MHz, to which the chip amplifier is the most sensitive, the transition occurs at ∼5 metres. In
the near field, Zw (=E/H) is determined by the characteristics of the source and the distance.
Zs for copper and aluminium is [18]:pp|Zs,Cu | = 3.68 × 10−7 f ; |Zs,Al | = 4.71 × 10−7 f(4.2)

At a frequency of 10 MHz and far field, the SCT Faraday cages provide a reflection shielding of
98 dB and 96 dB for the copper endcap cage and the aluminium barrel cage respectively.
The loss due to absorption through a shield is [18]:
t 
[dB]
(4.3)
Aa = 8.69
δ

where t is the thickness of the shield and δ the skin depth of the material at a given frequency. The
skin depths of copper and aluminium at the most sensitive frequency are:s2δ=
; δCu,10MHz = 20.3 µ m ; δAl,10MHz = 25.4 µ m
(4.4)
ω µσ

– 17 –

2012 JINST 7 P03005• A current source set with a limit of 3 A combined with a very precise voltage measurement
enabled resistances to be determined down to 10 µ Ω (impossible with the other technique).

PP3

Power
SupplyType IV Cable

PP1
Type III Cable

HSP

Type II Cable

Barrel TRT

Endcap TRT

LMTEndcap SCT

Barrel SCT

PixelBeam pipe

where ω is the angular frequency, and µ and σ are the magnetic permeability and the electricalconductivity of the material.
The SCT barrel cage provides an absorption shielding of 171 dB for the 500 µ m aluminium
barrel shield. The endcap copper skin is thin (18 µ m), providing little absorption protection (6 dB).2
4.5

Heat spreader plates and segmented plates

The Heat Spreader Plate (HSP) and the Segmented Plates (SP) are two mechanical structures ofparticular importance for the grounding and shielding design. Both are electrically conductive ringsat the SCT cage ends providing good azimuth connection between the PP1 filters.
The HSP is a sandwich of alochromed aluminium providing shield protection to the services
running out of the barrel enclosure, in particular it protects the LMTs up to the PP1 filter (figure 15).
As mentioned previously in section 3.2, cables arrive to PP1 from very different paths; therefore, a very good electrical connection to the Faraday cage is required (figure 8). The HSP is the
structure that provides this connection. The bondings of the PP1 to the HSP and the barrel-shield
cylinder to the HSP have received particular attention in order to have very low AC impedance. In
addition, the HSP needs excellent circumferential conductivity to connect the PP1s together. These
were achieved with the use of screwed metallic pieces with radiofrequency gaskets in each joint
and star-serrated lock washers.
The safety-earth connection of the Faraday cage is made at the HSP; this is the unique groundtie of the barrel detector. Similarly, the safety-earth connection of each endcap cage is made at the
Segmented Plates.
In the endcap, the services (LMTs, cooling pipes, fibres, temperature and humidity sensors,etc.) are routed with the help of two ring structures: the Radial Cable Tray (RCT) and the Segmented Plates (SP). PP1s are located on the SP (figure 16), so the endcap SPs are the electricalequivalent of the HSP. Good connections are needed: circumferentially between the SP parts, between the PP1 and the SP and also between the SP and the endcap cage.
2 Note that for thin shields, the magnetic protection is reduced due to multiple reflections [18] by 20 log(1 −e−2t/δ )[dB], 2.4 dB for the endcap skin.

– 18 –

2012 JINST 7 P03005

Figure 15. The HSP. The schematic shows the cross-section of the inner detector with the position of the
HSP. Unshielded LMTs are protected up to PP1. The picture shows the HSP during installation when thefront cover of the HSP was not yet installed and the LMTs were being routed.

Figure 17. EMI fingerstock gasket (left) and star-serrated lock washers (centre) penetrate the metal. Litz
wire (right) consists of multiple strands insulated electrically from each other. Ordinarily the strands are
twisted or woven, but no twisting is shown in this diagram.

Each one of these connections, barrel and endcap, used EMI fingerstock gaskets and starserrated lock washers (figure 17) on every screw, ensuring penetration of surface oxide layers andthe alochrome. In addition, Litz wires were used to reduce the skin effect between critical pieces(some Segmented Plates, the cooling reference disk and others).
The inner detector subsystems (Pixel, SCT and TRT) are located inside a solenoid field of
2 Tesla. In the case of a solenoid quench, the longitudinal field would collapse and induce electromagnetic fields in closed circuits in which the flux changes. These fields would induce currents
which could damage sensitive electrical components; or produce forces in the conductors which
could damage them or exert significant forces on the SCT and surrounding structures.
The consequences of a solenoid quench were thoroughly studied [19]. Calculations predicthigh forces (∼1000 N) on the Radial Cable Tray, which is a closed low-resistance annulus. Although the structure should withstand these forces, it was decided to break the connection in one
azimuthal position. The RCT has two annuli (Front and Rear) sandwiching the radial services. An
insulating sheet was used for the Front RCT, whereas a capacitive flex circuit was used for the Rear
RCT, so that good circumferential conductivity against noise currents is kept.
All conducting material of the barrel and endcaps was grounded. This includes the carbon
fibre structures of the barrels and disks. Carbon-loaded plastic fittings were avoided to simplify thegrounding requirements.
The three inner detector subsystems are grounded at a controlled location, identified as
IDGND. IDGND is a star connection of all safety grounding cables for the inner detector subde-– 19 –

2012 JINST 7 P03005

Figure 16. The Segmented Plates. Schematics showing one half of the PP1 patch panels.

Tinned Copper Terminal and Jumper
Structure

Screw or bolt
and nut plate

Lock-nut

Washer A

Aluminum Alloy
Stainless Steel

Cadmium Plated
Stainless SteelStainless Steel or
Cadmium Plated
Stainless Steel

Cadmium Plated
Stainless Steel
Cadmium Plated
Stainless Steel

Cadmium Plated
Stainless SteelStainless Steel

Washer B
Aluminum1
Alloy
None

Washer C
Cadmium Plated
Stainless Steel
Cadmium Plated
Stainless Steel

1.Use washers having a conductive finished treated to prevent corr osion (Alodine)

Figure 19. Bolt and nut bonding or grounding to a flat surface (ref. [21]).

tectors. The IDGND point is connected to ATLAS ground (figure 18). The star configuration minimises the potential differences between subdetectors, so reducing possible noise currents throughstray capacitances.
The three SCT safety earth cables going from the Faraday cages to the IDGND point are single
core AWG 8 119/0.3 mm protected with kapton/polyimide foil. The same type of cable is used forthe rest of the Inner Detector subsystems (pixels and barrel and endcap TRTs).
Given the importance of the unique safety-earth connection to the Faraday cage, the specification AC43.13-1B document by Federal Aviation Administration for grounding in the aircraft has
been followed for reliable aluminium-to-copper bonds [20, 21] (figure 19).
4.6

Power cables

The power supply for the front-end electronics (analogue and digital), the high-voltage supply
for the silicon detectors and other control lines are delivered to each of the 4088 SCT modules– 20 –

2012 JINST 7 P03005

Figure 18. IDGND star point for the ID subsystems. The schematic on the right shows the routing of theearth cables, for the SCT and TRT barrels one cable was routed on each side, but only one is connected to
the Faraday cage (side A is connected for SCT barrel and side C is connected for the TRT barrel).

FloatingPower
Supplies
Rack

Type IV cable
15 to 75m

Type III cable
PP3

~25m

Type II cable
Crimp
SplicePP1~10m

LMT (unshielded)

SCT Module

1,5m

Cable shieldfloating

Faraday Cage

Vdd
Vdd

USA15/US15

Vdd retVdd retVcc

Vcc

Vcc retVcc retCable shield
PP1 metal box

Figure 20. SCT power supply cable description.

with a foil-shielded cable, consisting of 16 insulated conductors [22]. An uninsulated drain wire
supplements the aluminium-foil shield. The drain wire is in contact with the shield all along thecable length. It ensures the connectivity of the shield between both cable ends.
Minimising the service material inside the detector and keeping the detector as hermetic as
possible led to the use of thin cables that have higher resistance. Also, having long cables contributes to higher resistance, as well as to increasing the noise sensitivity.
The cables have two mechanical connector locations mandated by the mechanics assembly.
These are referred to as patch panels PP1 and PP3. Both provide a noise-filter function and area transition between two cable sizes. PP1 is a PCB located close to the detector array shield andthe PP3 PCB is located ∼35 m from the detector. As mentioned, space for services is restricted in
the region where the services converge on the detector volume. In order to minimise resistance inthe very restricted service volume, there are four types of module cables, with different types ofconnections between them (figure 20):
• Type IV: cable up to 75 m between the power supplies and PP3. It has large cross-section
and straight conductors to save volume.
• PP3 provides cable connection and has noise filters with a bypass capacitor and commonmode ferrite choke on each conductor. EMI sources on Type IV cable are largely attenuatedby the common mode chokes. The choke array allows the bulky Type IV cable to havestraight pairs, reducing the cross-sectional area by 30%.
• Type III: cable (∼25 m) between PP3 and Type II cable. It has a moderate cross-section andtwisted pair conductors to reduce magnetic EMI across supply pairs.
• A crimp splice at each conductor provides connection between Type III and Type II cables.
• Type II: cable (∼10 m) between Type III cable and PP1. It has a minimum cross-section,
hence the thick conductors are insulated with kapton. Special twisted groups have been used
to optimize cross-section versus magnetic rejection.

– 21 –

2012 JINST 7 P03005

Cable shield
PP3 metal box

Table 1. Power supply cable sizes and resistances. The resistances correspond to measured values on final
cables. The resistance value of the drain wire includes resistance of the shield as they are in close contact.

PowerControl
HV
Drain Wire

Type IV cableSize(mm2 ) R(mΩ/m)
4.00
4.6
0.25
72.5
0.14
137
0.75
9

Type III (24m)Type II cableSize(mm2 ) R(mΩ/m)
0.518
32
0.081
206
0.081
206
0.205
61

Type II (9m)
PP1

PP3

1 module

EMI Receiver

Figure 21. Test setup for the conducted emissions measurement.

• PP1 provides cable connection at the Faraday cage. It incorporates a simple bypass-capacitor
filter on each cable conductor. In this way, all conductors going inside the shield short to the
Faraday cage or equivalently have a capacitor bypassing.
• Type I: low-mass tapes (described in detail in section 4.2).
Table 1 summarises sizes and resistances for each conductor.
Power-supply cables were tested in the laboratory to measure their conducted electromagnetic
emissions. The setup arrangement is shown in figure 21. The test used production components thatwere being installed in ATLAS: power supply, cables and patch panels.
Results showed that SCT cables were emitting well below ATLAS limits, with the exceptionof some points around 300 KHz (figure 22). The effect of PP3 is clearly seen, achieving a goodattenuation in the important range of 1-20 MHz. Noise is slightly worse on Type II cable. The
cable shield at the power supply is left floating, as already mentioned. Grounding the shield at thepower supply (see figure 23) reduces the emission in the 150-600 KHz range, but it increases theemissions in the 1-2 MHz range. During the installation the shield was left floating at the power
supply, following ATLAS guidelines and avoiding ground loops. However, if noise deteriorates in
the future, a special card has been installed in the power supply rack, which provides a fast andeasy way to ground the shield at the power supply. Measurements after the full ATLAS installationand operation have not indicated any noise problem.
The digital transmission lines to and from each of the SCT modules are served with optical fibres [23]. The choice of optical fibre as the physical media has many advantages: the much smaller
cable size and, in the scope of the grounding and shielding design, its immunity to electromagnetic interferences, its exceptionally low loss (no repeaters are needed) and the absence of groundcurrents or other parasitic signals.

– 22 –

2012 JINST 7 P03005

Type IV (15m)Power
SupplyType III cableSize(mm2 ) R(mΩ/m)
1.00
21.2
0.14
136
0.14
136
0.75
12.8

Figure 23. Comparison of grounding or not the shield at the power supply. Plots show emissions from TypeIV and Type III cables.

4.7

Patch panels

Patch panels (PP3 and PP1) have been already introduced in previous sections. In addition to
the electrical functions, they have the mechanical function of interfacing two cable types: PP3
interfaces Type IV and Type III cables and PP1 interfaces Type II and the LMTs. The LMTs
are soldered on the PP1 PCB. PP3 is located in a dedicated rack at the outer edge of the ATLAS
detector cavern (UX15). Barrel PP1s are located on the cryostat wall close to the HSP. The endcap
PP1s are located on the SPs. All have an electrical housing as indicated on figure 20.
PP3s are individual isolated units; there is one per module. As already mentioned, they havecommon-mode inductive filters on all lines, both power and control, to attenuate any commonmode noise that may be picked up on the long run of Type IV cable. In addition, every conductor
is bypassed through a capacitor to the cable shield. The drain wire is routed on the PCB providing– 23 –

2012 JINST 7 P03005

Figure 22. Emissions from the three cable types. Module is powered, configured and data is read out during
the test. Type IV cable emissions are reduced after PP3. Noise increases slightly on Type II cable.

continuity of the cable shield from Type IV to Type III cable shield. The metal box and the cable
screens are electrically isolated to avoid contact with any other PP3 and all surrounding structuresby means of fibre-glass sheets. A wire connects the metal box to the cable shield.
The location of the PP3s inside UX15 is exposed to fringing magnetic field of the ATLAS
barrel toroids. Simulated field maps indicate that up to 600 Gauss may be present. The inductive
chokes used to attenuate common-mode pickup saturate at external fields around 100-200 G. The
metal box is designed as a magnetic shield, and additionally acts as electrostatic shield. Different
materials and configurations were tested. The best results (25 G inside the box with an external fieldof 800 G) were obtained with a box material fabricated from sheets of 0.5 mm thick grain-oriented
silicon transformer steel. Two C-section pieces are spot-welded together to form a rectangularcylinder (figure 24). The PP3 boxes enclose the cable connectors as well as the circuit board along
their whole length.
PP1s (figure 25) are printed circuits accepting low-mass tapes from the detector side and conventional cables from the power-supply side. Barrel PP1s serve 6 modules, whereas endcap ones
have two flavours: one serves two modules and the other three modules. Every conductor is AC
coupled to the Faraday cage before entering the cage by means of a capacitor. The shield of thecable is connected to the PP1 metal enclosure. The PP1 enclosure is connected to the Faraday cage
and the safety-earth cable. PP1s also provide AC coupling of analogue, digital and HV power lines
to their return lines.
In the barrel, the cable shield and the metal enclosure are connected to the digital return line
(DGND). The barrel SCT modules are grounded at PP1; the analogue and HV return are referenced
to the digital return in the module (section 4.1). In the endcap, the modules are referenced at a small
PCB called PP0 on the disks, very close to the modules. At PP0, the DGND line is connected to
the Faraday cage. The referencing of the power returns in the SCT is represented in figure 26.

– 24 –

2012 JINST 7 P03005

Figure 24. PP3 pictures. Top view with metal box open (top), the yellow wire connects the metal box to thecable shield. Front view of the PP3 (bottom), the two C-section pieces protect against magnetic fields. PP3
rack before cable installation (right).

FloatingPower
Supplies
Rack

BARRELPP3

PP1Module

AGND
DGND
HVret

Cable shieldfloating

Faraday Cage

USA15/US15
Floating
Power
Supplies
Rack

ENDCAPPP3PP1Module

AGND
DGND
HVret

PP0

Cable shieldfloating

Faraday Cage

USA15/US15

Figure 26. Referencing of the SCT power returns.

4.8Power supplies

The SCT power-supply system [24] consists of several components located in adjacent caverns
USA15 and US15. Regarding grounding and shielding, the main point is that the different power
supplies must be floating. The isolation of individual channels and individual voltages is realised by
HF transformers on the power path and by optical couplers on communication lines. As mentioned,
the Type IV cable shield is left floating at the power-supply side. In this manner, any DC groundloop is avoided.
The final grounding and shielding implementation could only be tested after full installation.
Thus, an additional card in the power-supply system has been added. A shorting card allows us
to easily ground the Type IV cable shields. This card is unconnected but is kept in case of future
problems.
4.9

Other systems connected to SCT ground

There are other mechanical and electrical systems connected to SCT ground. The mechanicalsystems (CO2 and nitrogen pipes, metallic structures, etc.) were carefully controlled to not short to– 25 –

2012 JINST 7 P03005

Figure 25. Barrel PP1s without the cover (left), with Type II cables (red) and LMT connected. Endcap PP1(right) with the electrical housing opened.

any other subdetector, keeping the SCT referenced with a single tie.
Other electrical systems, including the large area heater pads and the environmental control(temperature and humidity sensors), are connected to the SCT ground. The heater pads are used tomaintain the interface between various subdetectors at a predefined temperature. Special care has
been taken to maintain the electrical isolation of these systems. Figure 27 describes the groundingof the heater and environment control systems.
The grounding of the heater system follows the same rules as the SCT power-supply system.
It is grounded to the Faraday cage at PP1, the power supply is floating and the cable shield is only
connected at the detector end [20]. The same principle applies to the environment control system;in addition, all signals are isolated by optocouplers.
4.10

Ground isolation monitoring during installation

The installation of the three SCT subdetectors, the power-supply cables, the optical fibres, thepatch panels, the cooling pipes and other components was a very difficult task that took several
years. Furthermore, the space in the inner detector is very restricted and the different subdetectors
and their services have very little clearance between them. As a consequence of this, it was very
easy to accidentally short a part of one subdetector to a part of another one, violating subdetector
isolation and single earthing. Even with a periodic check of the isolation, in a very big system likethe SCT it was impractical to stop all activities to look for the accidental short. Furthermore, thesuccess in searching for a short was not guaranteed in such a big system; several people working at
the same time in very different locations can easily produce multiple shorts. Finding of a multiple
short is extremely difficult.

– 26 –

2012 JINST 7 P03005

Figure 27. Grounding of the heater control system (left) and the environment monitor and control system(right).

A ground isolation monitor (figure 28) was developed to detect an unintentional earth fault
path. The monitor was connected to a visual and audible alarm. The intention of it was to warn
immediately the installation person of a fault, and thus avoid the pile up of faults. People working
in the installation were trained to stop their activity when this alarm arose. For faults longer than
two minutes an email and SMS text was sent to the grounding expert.
The Bergoz IPCT (Integrated Parametric Current Transformer) is the core device in the monitor. It allows the safety-earth bonding to be maintained during the continuous monitoring. A test
current (500 mA) was injected into the SCT subdetector. The path for the reference current was
expected to be entirely in the safety earth conductor. The return path was through a dedicatedconductor. The safety earth conductor and the return path are threaded through the transformer.
In the absence of a fault, all current is flowing to the ground point and returning back. If partof the 500 mA current was diverted through a fault path from the SCT to earth, the monitor detected the event. The threshold limit to trigger the alarm was set to 1 mA difference. For the SCT
configuration, the monitor could detect fault paths with resistances lower than 1 kΩ.
Three monitor systems were built and installed to monitor both SCT endcaps and the barrel.
They were active for several years up to the ATLAS cavern closing. The monitors can not be
kept in place during ATLAS operation as the magnetic fields would saturate the transformer andrender it inoperable, regardless of the possible non-resistance to radiation of its components. In
August 2008, they were removed before first LHC operation. At the time of removal, the three
SCT subdetectors were checked to be isolated (over MΩ or over limit in the measuring device) andearthed only in a single tie.

5

Problems and installation experience

Obviously, the design and installation phases were not free of problems. The space between services of different subdetectors was very tight (figure 4, for example). A short summary is given
below listing some of the problems encountered in maintaining the electrical isolation during installation:– 27 –

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Figure 28. Ground Isolation Monitor. The transformer monitored the differential DC current. The programmable discriminator relay activated the green or red light and the audible alarm. The audible alarm
had a manual switch to cut the sound in the case of a prolonged fault. The web datalogger registered thedifferential current each minute and sent email and SMS alerts.

• Minor problems directly solved by the installation person. The ground monitor showed its
effectiveness with this type of fault. There were several each day, such as installing a pipeslightly out of its position shorting to another subdetector service.
• Problems on the SCT Barrel:– A copper-kapton foil protects the barrel services. The edge of the foil had some exposed
copper which touched TRT grounded components. The foil edge was protected withflexible kapton tape.

– The SCT cooling pipes outside the enclosure and before the electrical break were routed
in a cable tray. They were electrically protected with thermal foam. However, in somepoints the pipes were exposed and they were shorting to the cable tray, which is connected to the cryostat ground. Extra foam protection was added.
• Problems on the SCT Endcaps:– Following the reception of the endcaps from Liverpool and NIKHEF, it was found that
pipe brackets on the support cylinder were floating; therefore connections to the groundsheet were made. To ensure the isolation of the SCT from the TRT rails, the carbonloaded PEEK inserts in the support mechanisms had to be replaced by unloaded PEEK
ones.
– During the insertion of SCT Endcap C into the TRT endcap, a short via the RCT (Radial
Cable Tray) was detected. This was cured by adding a 200 µ m sheet of G10 glass fibreto the RCT surface. The same precaution was taken for Endcap A.
– PP1 boxes were protected with copper/kapton foil. SCT PP1s were located very tightly
between TRT PP1s. The kapton foil was easily scratched by the TRT PP1 boxes installation producing a fault. The SCT boxes were additionally protected with glass fibre
sheets.
– The electrical break of some cooling tubes was shorting. They had to be replaced.
• After the insertion of Endcap C into ATLAS, a connection of 2 KΩ between the SCT Endcap
C and the SCT Barrel was detected. The short was between one of the front support mechanisms and the SCT Barrel. By inserting a long (2 m) endoscope between the cryostat andthe endcap (figure 29), it was found that the short was due to a CO2 pipe. A two-metre-longextender arm with G10 glass fibre at the end was inserted into the region. The G10 was carefully placed between the endcap foot and the pipe; then the G10 and the arm were separated
leaving the G10 sheet as insulation between the two subsystems.
The experience shows that the grounding and shielding system needs to put together people
from different disciplines. On one side, electronic engineers, who are not necessarily experts in– 28 –

2012 JINST 7 P03005– The HSP had aluminium tabs connecting to the PP1 base plate to ensure good electricalconductivity between the HSP and the PP1. The back face of the tabs was not insulated
and produced multiple shorts to the Liquid Argon cryostat. They were protected withglass fibre or kapton tape.

material properties and mechanical structures, provide the concept design; while mechanical engineers have to design many structures affecting the grounding and shielding system implementation,and they are not necessarily experts in electronics. In addition, many technicians are involved withpieces of the grounding and shielding system during the installation of a detector like the SCT.
Very good communication among people involved is crucial to achieve a successful system. This
communication should start from the beginning in the design of the mechanical structures.

6

Conclusions

A great deal of effort has been made to design and implement a good grounding and shielding system for the SCT. The prevention of electromagnetic interference and noise pickup through powerlines has been an important design aspect as they are the dominant noise source jeopardising electrical performance. In accordance with ATLAS grounding rules, the design accomplishes the avoidance of ground loops and the isolation of the different subdetectors. Noise protection of the mostsensitive part, the small-signal path, has been pivotal. Protection has been developed against thevarious noise sources: noise from circuits generated locally at the module or from adjacent modules, electromagnetic radiation from surrounding detectors, currents coming through ground loops,
mainly through power cables, and noise from cooling loops inside the Faraday cage.
The implementation of the grounding and shielding design has been followed during the construction and installation of the SCT. Special attention has been given to the electrical propertiesof materials and joints. A ground isolation monitor was installed to help during the installation.
Repairs have permitted a final good implementation of the grounding and shielding system.
In order to keep good electrical performance, noise pickup in the SCT should not increase
dramatically the single-module noise. Joining more than four thousand modules together was thechallenge for the grounding and shielding system. Moreover, the SCT is part of ATLAS and its
different subdetector electronic systems. Noise pickup on the SCT from other electronic systems
had to be avoided.
The noise was measured at different stages of the SCT construction. Table 2 shows the valuesof the average measured noise for the barrel modules and the different types of endcap modules(outer, middle and inner). References [1–5, 25] report noise figures for single module tests, module
tests on single barrel layer or single endcap disk, module tests with the full barrel or endcap andfinally module tests in ATLAS. Noise increase after installation is not significant; variations are– 29 –

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Figure 29. Short before any intervention (left). G10 being inserted between the endcap foot and the barrel
pipe (centre). G10 inserted (right) and separated from the extender arm (with blue tape).

Table 2. Average noise measured in equivalent noise charge at the input (electrons) corrected to 0o C at four
subsequent stages of integration.

Single Module
Single layer/disk
Full barrel/endcap with TRT
SCT in ATLAS (Dec. 2008)

BarrelModuleNoise(e− , ±60)
1470
1480
1512
1494

Endcap Outer
ModuleNoise(e− , ±60)
1515
1591
1522
1593

Endcap Middle
ModuleNoise(e− , ±60)
1464
1527
1551
1517

Endcap Inner
ModuleNoise(e− , ±60)
1060
1069
1105
1032

Acknowledgements
We acknowledge the support of the funding authorities of the collaborating institutes including the
Spanish National Programme for Particle Physics; the Science and Technology Facilities Councilof the United Kingdom; the Polish Ministry of Higher Education and Science; the German Ministry of Science; the Swiss National Science Foundation; the State Secretariat for Education andResearch and the Canton of Geneva; the Slovenian Research Agency and the Ministry of Higher
Education, Science and Technology of the Republic of Slovenia; the Ministry of Education, Culture, Sports, Science and Technology of Japan; the Japan Society for the Promotion of Science; theOffice of High Energy Physics of the United States Department of Energy; the United States National Science Foundation; the Australian Research Council (ARC) and Department of Education,
Science and Training (DEST); the Dutch Foundation for Fundamental Research on Matter (FOM);
theMinistry of Education, Youth and Sports of the Czech Republic, Ministry of Industry and Tradeof the Czech Republic, and Committee for Collaboration of the Czech Republic with CERN; theSwedish Research Council.

References[1] ATLAS collaboration, The ATLAS experiment at the CERN Large Hadron Collider,2008 JINST 3 S08003.
[2] A. Abdesselam et al., The integration and engineering of the ATLAS SemiConductor Tracker barrel,
2008 JINST 3 P10006.
[3] A. Abdesselam et al., Engineering for the ATLAS SemiConductor Tracker (SCT) end-cap,
2008 JINST 3 P05002.
[4] A. Abdesselam et al., The barrel modules of the ATLAS semiconductor tracker,
Nucl. Instrum. Meth. A 568 (2006) 642.
[5] ATLAS collaboration, A. Abdesselam et al., The ATLAS semiconductor tracker end-cap module,
Nucl. Instrum. Meth. A 575 (2007) 353.

– 30 –

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well within the measurement uncertainties. In any case, it is far below the limit mentioned in
section 1 (50 aC = 312 e− ). In conclusion, a good SCT grounding and shielding system has been
implemented and protects the SCT from noise pickup.

[6] A. Ahmad et al., The Silicon microstrip sensors of the ATLAS semiconductor tracker,
Nucl. Instrum. Meth. A 578 (2007) 98.
[7] F. Campabadal et al., Design and performance of the ABCD3TA ASIC for readout of silicon strip
detectors in the ATLAS semiconductor tracker, Nucl. Instrum. Meth. A 552 (2005) 292.
[8] H. Williams, ATLAS policy on grounding and power distribution, ATC-GE-ER-0001.
[9] G. Blanchot, ATLAS EMC policy, ATL-ELEC-PUB-2007-003.
[10] S. Haywood, Module Overlaps in the SCT and Pixels, ATL-INDET-2002-016 (2002).

[12] M. Johnson, Grounding and shielding techniques for large scale Experiments, in proceedings of 8thWorkshop on Electronics for LHC Experiments, Colmar (2002) 44-49.
[13] V. Cindro and M. Mikuz, ATLAS SCT low mass tapes, ATLAS-IC-ES-0076.
[14] V. Cindro and M. Mikuz, Technical specifications of Cu low mass tapes, ATLAS-IC-ES-0103.
[15] P. Phillips, System performance of ATLAS SCT detector modules, in proceedings of 8th Workshop on
Electronics for LHC Experiments, Colmar (2002) 100-104.
[16] R. Bates, ATLAS SCT endcap module and system test performance,
IEEE Nucl. Sci. Symp. Conf. 2 (2003) 908.
[17] S. Haywood, ATLAS SCT end-cap grounding and shielding, engineering implementation,
ATLAS-IS-EN-0014.
[18] H. Ott, Noise reduction techniques in electronic systems, John Wiley & Sons (1976).
[19] S. Haywood, Effect of solenoid quench on SCT, ATLAS-IS-ER-0065.
[20] J. Kaplon, ATLAS inner detector grounding and shielding, engineering implementation,
ATLAS-IC-ES-0011.
[21] Department of Transportation, Acceptable methods, techniques, and practices - Aircraft inspection
and repair, US Federal Aviation Administration AC 43.13-1B (2001), http://rgl.faa.gov/.
[22] P. Wells, Specifications of SCT power cables, type II, III and IV, ATLAS-IC-CD-0001.
[23] A. Abdesselam et al., The optical links of the ATLAS SemiConductor tracker, 2007 JINST 2 P09003.
[24] J. Bohm, Power supply and power distribution system for the ATLAS silicon strip detectors, inproceedings of 7th Workshop on Electronics for LHC Experiments, Stockholm (2001) 363-367.
[25] N. Barlow, Operation of the ATLAS semiconductor tracker, in proceedings of 9th International
Conference on Large Scale Applications, 30 September–2 October (2009), Florence Italy,
PoS(RD09)004.

– 31 –

2012 JINST 7 P03005[11] V. Radeka, Shielding and grounding in large detectors, in proceedings of 4th Workshop on
Electronics for LHC Experiments, Rome, CERN/LHCC/98-36 (1998) 14-18.