SHIELDING AND GROUNDING IN LARGE DETECTORS* Veljko Radeka Brookhaven National Laboratory, Upton, NY 11973-5000 ([email protected])

Abstract Coupler: Shield (“Faraday cage”) Shielding effectiveness as a function of shield thick- ness and conductivity vs the type and frequency of the interference field is described. induced in trans- mission lines by loop driven currents in the shield is evaluated and the importance of low shield resistance is emphasized. Some measures for preven- Cable shield connected to detector shield at the penetration: tion of ground loops and isolation of detector-readout systems are discussed. Detector Counting Area Signal, Low V, ferrite 1. INTRODUCTION High V, control lines core Prevention of electromagnetic interference (EMI), or “noise pickup”, is an important design aspect in large detectors in accelerator environments. It is of particu- ~ 5 50 m lar concern in detector subsystems where signals have insulate ac a large dynamic range or where high accuracy position electrically! power “1” interpolation is performed. Calorimeters are very sen- vacuum pumps, cooling water sitive to coherent noise induced in groups of readout cryo lines, mechanical supports channels where energy sums are formed, covering a “Solid ground” ( “earth” ) large dynamic range. There are several potential noise sources and means of transmission: ac power “2” 1) Noise from digital circuits generated locally on a major ground loops (global) single front end read-out board on the detector; (low impedance) 2) electromagnetic radiation in the space around the Figure 1. An illustration of shielding and ground loop detector generated by other detector subsystems, control concepts. power supplies, silicon-controlled rectifiers, ma- chinery, etc.; Figure 1 illustrates some of the basics of shielding 3) noise induced by penetrations into the detector en- and ground loop control. No leads of any kind should closure (e.g., cryostat) and the front end readout enter a cage without their shield being con- located on the detector; nected to the detector enclosure at the penetration, oth- 4) currents coming through ground loops, of which the erwise a lead connected to the enclosure would inject detector enclosure and the front end electronics are noise (it presents a coupler) into detector electrodes and a part, caused by any apparatus and machinery out- front end electronics. If the detector enclosure (i.e., its side the detector. front end electronics) and the readout electronics in the Problem 1) of internally generated noise is being counting area have to be connected electrically and not addressed by a careful layout and filtering on the only by optical links, then the cable shield should have board(s), shielding of preamplifiers, and by minimiz- a low resistance and a high inductance to minimize the ing digital operations on the board. noise at the receiving end due to the ground loop volt- The effects of externally generated noise in the form age. Ground loop currents in the enclosure walls and of EM radiation are best reduced by a well designed the transmission lines will be minimized by isolating Faraday shield (“cage”). The effects of noise currents the detector enclosure from the surrounding. flowing through the shields, due to ground loops, are In Section 2, some basics of shielding against exter- also reduced by the Faraday cage. nal EM fields are reviewed. In Section 3, noise pickup in Ideally, ground loops should be avoided entirely. transmission lines from ground loop voltages and currents In practice, preventing formation of ground loops means is discussed. In Section 4, potential ground loops in a large to increase a ground loop impedance over most of the detector subsystem are illustrated in the example of the frequency range as much as possible. ATLAS liquid argon calorimeter. Some practical steps for *This work is supported by the U.S. Department of Energy: isolation are described in Section 5, and in Section 6, the Contract No. DE-AC02-98CH10886. question of the safety ground is addressed. 2. SHIELDING EFFECTIVENESS A shield 0.5 mm thick provides a far field AGAINST EM RADIATION attenuation at 100 kHz of only about 21 dB by absorp- tion, and nearly 120 dB by reflection. In this case, ab- 2.1 Thick Shields (t ≥ δ) sorption becomes dominant only above ~ 5 MHz. The reflection attenuation increases with the angle of inci- Shielding properties of enclosures are analyzed in dence. detail in Ref. 1. Only some main points are emphasized here. A hermetic detector or an electronics enclosure 2.2 Very Thin Shields (t<<δ) of highly conductive material, such as copper or alumi- num, provides very high attenuation against external EM For very thin shields, multiple reflections of the mag- fields in the frequency range from a few kHz up to very netic field component within the shield reduce shield- high frequencies. The shielding effect is obtained by ing effectiveness. Total attenuation, including multiple reflection and absorption of the EM wave. Attenuation reflections, is then given by[1], by reflection of an external plane EM wave at normal Z  A = 20 logw + 20 log(2 sinh t ) (5) incidence to the shield, with wave impedance ZW is: δ 4Zs   Zw δ ≈ δ Ar = 20 log [dB] (1) For very thin shields sinh (t/ ) t/ , and by substi- 4Zs σ tuting Eq. (3) for ZS, and 1/ t for shield dc resistivity per square ρ , where ZS is characteristic impedance of the shield ¨ material, Z σt Z  ≈ w w (6) Z  = (ωµ/σ)½ = 3.7 × 10-7 f ½ [Ω] (2) Ar 20 log = 20 log . S 2√2 2√2 ρ (for copper) Thus, for very thin shields, attenuation due to re- ω = 2π µ where f is the frequency, is the permeabil- flection is determined simply by the ratio of the wave σ ity, and is the conductivity of the shielding material. impedance and the sheet resistivity of the shield. Low Z can be expressed in terms of the skin depth δ = 2/ S mass shields, such as aluminized Mylar windows on gas ωµσ ½ ( ) as, proportional chambers, can still provide very useful √2 shielding. For example, 1000 Å (0.1 µm) of aluminum, Z = s δσ . (3) which is about 1/800th of the skin depth at 1 MHz, gives ρ ≈ Ω ≈ 1/δσ is simply dc sheet resistivity per square of the ¨ 0.25 /square and Ar 533 = 55 dB. shield layer, one skin depth thick. Characteristic im- A closer inspection shows that Eq. (1) for thick ≈ Ω shields is approximately valid down to t/δ = 0.2, with pedance of the shield is very low, ZS 1m at 10 MHz; 0.1mΩ at 100 kHz. an error of 9.6 dB.

The wave impedance ZW (the ratio of the electric field and the magnetic field) depends on the nature of the source 2.3 The Role of Apertures (Gaps) in the Shield (electric or magnetic ) and the distance from the source. In the far field, i.e., distance greater than λ/2π, it Any gaps in the shield interrupt the flow of currents approaches the impedance of the free space (and air), which are essential for field attenuation provided by the 377Ω. At f = 10 MHz, λ = 30 m, so that for frequencies less shield. Attenuation by an aperture in the shield is given than 10 MHz, most detectors will be in near field condi- by[2], tions. For an “electric antenna” (high voltage and low cur- λ Aap = 20 log [dB] (7) rent), the wave impedance varies as 1/r, and for a “mag- 2L netic antenna” (high current and low voltage), it varies as where λ is the wavelength and L is the longest dimen- λ π r in the near field r < /2 . Taking the above values for ZS sion of the aperture, regardless of its shape. This indicates Ω and for ZW = 377 , the shielding effectiveness in the far significant field penetration, which increases with fre- field, due to reflection, is: quency. For example, for L = 10 cm, at 10 MHz, Aap is × 4 Ar = 9.4 10 = 99.5 dB at 10 MHz barely above 40 dB. The attenuation reduces to zero dB at = 9.4 × 105 = 119.4 dB at 100 kHz. the wave guide cutoff frequency, λ/2L = 1. The attenua- The attenuation is higher in the near field for an tion can be increased if the openings form a wave guide of electric source, and much lower for a magnetic source some length (e.g., a honeycomb structure). at low frequencies[3]. At lower frequencies (1 kHz–1 MHz), it is possible Shielding attenuation by absorption due to skin ef- to achieve very high shielding attenuation. At high fre- fect is, quencies (>10 MHz) shielding effectiveness will be ap- ωµσ ½ δ erture-limited. The importance of electrical continuity Aa = 6.2 t ( ) = 8.7 t/ [dB] (4) (for copper) of any shield cannot be overemphasized. This is also important for ground loop-driven currents (section 3). where t is thickness of the shield. a) Single ended PA C12 Ω Z Z i 0 0 Z0, Ls, rs 0 n Z0, Ls, rs Z0 Z0

iext in or vext Cb ∆ ω * v in the shield = iext ( j Ls + rs) for rs << Z0 vext Cb >> C12 * ∆v in center lead = iext · jωLs with a shield

* emf in the receiver = iext · rs v r 1 i = ext s i r n ω common mode * noise current = n = s for current in the shield Z0 /2 rs + j Ls cmr iext 2Z0 rejection ratio v r * noise current i = ext s for voltage in the loop Figure 3. Balanced transmission line with high rejection n ω 2Z0 rs + j Ls of ground loop noise. b) "Common Mode" receiver (i.e., cmr/4). A principal role of double shielding Z0/2 cmr ~ 10 −103 for terminated transmission lines is to reduce further the common mode rejection shield resistance, rS. Figure 3 illustrates a transmission Z0/2 line connection for analog signals with a very high dy- namic range (~ 5×104), which has been proven in practice. cmr Noise currents reduced by compared to a) Inductance of the shield can be artificially increased by 4 several turns on a ferrite core. The noise current in Fig. 3 c) Double Shield is given for a direct connection in place of Cb. A capaci- tance, Cb, of 100-300 pF reduces further the shield cur- rs may be rents at lower frequencies, and prevents unbalancing the much smaller transformer due to the stray capacitance, C12, at high fre- quencies. Differential amplifiers are also commonly used L , r s s instead of transformers, with somewhat lower rejection of the noise and crosstalk. figure 2. Noise pickup in cables due to currents caused The transmission line case illustrates the importance by ground loops. of a low shield resistance. The same conclusion can be reached, albeit in more complex geometry, for any Fara- 3. NOISE INDUCED IN SHIELDED day cage and, in particular, for any configuration where CONDUCTORS BY GROUND LOOP front end electronics is located in a shielded enclosure CURRENTS attached to the detector. This is the case for almost all subsystems in LHC experiments. Any gaps in the en- Derivation of noise current into the receiver at the closures are particularly important. This is where the end of a coaxial transmission line is outlined in Fig. 2. well developed technology[2] of rf gaskets may have It is based on the magnetic coupling between the center to be applied. Special attention has to be paid to gal- lead and the shield. It can be shown that the mutual vanic compatibility of the metals used, to ensure low inductance between the two is equal to the contact resistances over the lifetime of the experiment. (self)inductance of the shield[1]. The potential differ- In particular, contacts with bare aluminum have to be ence between the two ends of the shield is determined avoided. Aluminum has to be chromate or tin-plated or, by the ground loop current and the resistance and in- if that is not practical, a brush-on coating has to be ap- ductance of the shield. The induced emf in the center plied to contact surfaces. lead is equal to the voltage across the shield inductance Prevention of noise injection by ground loop cur- only. The noise current into the receiver is the result of rents is usually more difficult than shielding against EM the potential difference at the receiver end of the line. radiation. The shield resistance in relation to the characteristic im- pedance of the transmission line determines the magni- 4. POTENTIAL GROUND LOOPS IN A tude of the noise current into the receiver. In many cases LARGE DETECTOR SUBSYSTEM the ground loop voltage, vext, between the sending end and the receiving end is generated with a very low im- Large detector subsystems have a large number of pedance. The ratio of the shield resistance to the shield connections to the surrounding world for signals, moni- inductance is then a determining parameter for the re- toring, cooling, power, etc., that if left to chance, a be- ceiver noise current. wildering network of ground loops will arise. Even in For shielded, balanced transmission lines, noise re- cases where all signal transmission to and from the de- jection is improved by the common mode rejection of the tector is digital, and via optical links, power and ser- 1. Lar cryo lines 1. Coaxial Cables

2. HV supplies Shield connected to cryostat before penetrating Faraday cage 3. Data (opt. fibers) Short connection, low inductance 4. Cooling circuit (water) LAr Performed on standard feedthroughs ~20m 5. LV supplies Cryostat FEE ~70m 6. Level 1 sums ~ 5×103 channels 2. Power Supplies 7. clock Capacitors with short leads, close to cryostat low 8. slow control Accordion inductance connection (parameter input) warm cold No net DC Current in Balun to avoid saturating 9. sensors Ferrite (pass power and return). In magnetic field up to ~ 300–400 gauss, use 3D3 type ferrite B 10. solenoid cryo & supply 100 n Balun 11. mechanical supports + + 1.27 cm 1., 4., 11. insulators 10µ to floating 0 0 12. feedthrough heater (dc) power supply 2., 5. floating supplies, as in Fig.6 _ _ 100 n 6. differential transmission transformers (balun or signal) Σ i = 0 7., 8. opto-couplers (or transformers) LAr 9. insulate sensors; different techniques at various receivers 100 n 5 - 10 turns on Ferrite core 10. solenoid line to be insulated, power supply floating Cryostat 12. heater insulated, capacitors to pedestal, floating supply

Figure 4. Vital lines in the ATLAS Liquid Argon 3. Probes, HV Capacitors with short leads, close to cryostat, Calorimeter (i.e., potential ground loops). low inductance connection R > 1 kΩ can be replaced by L > 1mH when no current flows vices connections may create loops which will inject 100 n r noise currents into the critical contact areas of the de- tector and front end electronics enclosures. The solu- to electrode R tion that offers some degree of control is to isolate, elec- 100 n trically, the detector. This is illustrated for the case of LAr the ATLAS Liquid Argon Calorimeter in Fig. 4. Figure 5. Rules for Entering a Shielded Detector An objection is sometimes made that a large object, Enclosure (e.g., cryostat). such as a cryostat, has a capacitance of several nanofarads to the support structure and other sub- Figure 7 shows the connection for multiple remotely systems. However, at some intermediate frequencies, sensed power supplies. The “common” can only be at say 100 kHz, this presents several orders of magnitude the location of front end electronics (to avoid making higher impedance than the resistance of a direct con- interdependent feedback loops). nection. Noise at frequencies lower than the center fre- quency of the signal processing chain is important, since 6. THE QUESTION OF SAFETY GROUND it can be induced by various paths of currents through Prevention of ground loop currents, by increasing nonhermetic shields, into the wide band stages of front the impedance of any loop as much as possible, leads to end electronics, such as analog memories and ADCs. the following guidelines: • 5. AN OUTLINE OF SOME ISOLATION All detector subsystems will be electrically isolated; • There will be no connection to ground other than MEASURES “Safety Network”; Figure 5 illustrates some of the practical configu- • There will be no connection between different de- rations for communicating signals, power and various tector subsystems. sensor lines with the interior of a Faraday cage. The (These have been adopted as the primary guidelines intent of all of them is to divert any ground loop cur- in the ATLAS Policy on Grounding.) rents into the shield (enclosure). In examples 2 and 3, The goal of the isolation is to prevent numerous pos- the impedance of the connecting lines is increased by a sible ground loops (illustrated in Figs. 1 and 4), to al- balun transformer, or by resistors in each line where the low checking for inadvertent connections to various current in the leads is very low. “grounds” (i.e., objects which appear to be near zero Figure 6 illustrates floating dc supplies for low volt- potential, such as the experiment support structure), and age (high power) and for high voltage (very low cur- to allow for a safety connection to a single point with- rents). out creating a ground loop. 1. LV Supplies <50V Detector “Level 1 room” 50 ~ 75m GFI Signal & control PA, Receivers SCA, > 10 µH <50V ADC DAQ ~220V DC Cs “Safety <50V secondary is Floating > 100 µH supplies ground” Neutral allowed to float Figure 8. Safety ground for an isolated detector safety ground subsystem with any copper connections to the level 1 trigger electronics (and/or DAQ). 2. HV Supplies > 50V ~kV Supplies, low current

sum signals. This is the only analog transmission of Cryostat electrical signals from the calorimeter. It will be ac- ~220 V complished by differential transmission, with high com- 1-10 kΩ mon mode rejection (push-pull drivers, shielded twin lead transmission lines, differential receivers). It is rea- Safety ground sonable to require that any potential difference between the sending end and the receiving end be minimized over 3. Supplementary safety grounding: most of the frequency range. At high frequencies, the high impedance at low voltage current through the shielding braid should be mini- mized. Thus the safety ground (reference point) should be at the location of Level 1 signal receivers. Each cry- Saturable inductor ostat will have a low resistance connection to that point, as illustrated in Fig. 8 (a part of that connection could be the common braids of Level 1 cables from each crate). In case of a subsystem where all signal communi- cations (including sensors and controls) are by optical links, and floating power supplies are used, safety ground could be some other point. However, potential Figure 6. An illustration of “floating” low voltage and differences between adjacent subsystems are minimized high voltage power supplies and supplementary safety when they are connected to the same reference point. grounding. If a part of a subsystem could be inadvertently sepa- Floating rated during maintenance, and a possible safety ques- dc supplies tion arises, an additional connection could be made to + the same reference point, but via a nonlinear network + (high impedance for small signals) as in Fig. 6. Sense Load − − ACKNOWLEDGMENTS + Discussions on shielding and grounding principles + and practice with R. Chase, J. Colas, C. de La Taille, P. “Common” Sense Rehak, H. Williams, and B. Yu are gratefully acknowl- − edged. − REFERENCES Separate cables for each voltage 1. H. W. Ott, Noise Reduction Techniques in Electronic Common at the front end where the load is Systems, John Wiley & Sons, 1976. 2. Product Design & Shielding Selection Guide, 1994 In- Figure 7. Connection of remote multiple power supplies. strument Specialties, Delaware Water Gap, PA 18327. 3. The role of magnetic materials at low frequencies is Once the subsystem isolation has been ensured, a discussed in Ref.1, p.159. well-defined safety ground must be established. To what point? In the case of the LAr Calorimeter, a dominant con- sideration is to preserve from EMI the Level 1 trigger