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Shielding and Guarding
How to Exclude Interference-Type Noise
What to Do and Why to Do It - A Rational Approach
by Alan Rich
This is the second of two articles dealing with interference noise.
In the last issue of Analog Dialogue (Vol. 16, No. 3, pp. 16-19),
we discussed the nature of interference, described the relationship
between sources, coupling channels, and receivers, and considered
means of combatting interference in systems by reducing or elimi-
nating one of those three elements.
One of the means of reducing noise coupling is shielding. Our pur-
pose in this article is to describe the correct uses of shielding to re-
duce noise. The major topics we will discuss include noise due to
capacitive coupling, noise due to magnetic coupling, and driven
shields and guards. A set of guidelines will be included, with do's
and don'ts.
From the outset, it should be noted that shielding problems are al-
ways rational and do not involve the occult; but they are not al-
ways straightforward. Each problem must be analyzed carefully.
it is important first to identify the noise source, the receiver, and
the coupling medium. Improper shielding and grounding, based
on faulty identification of any of these elements, may only make
matters worse or create a new problem.
You can think of shielding as serving two purposes. First, shielding
can be used to confine noise to a small region; this will prevent
noise from extending its reach and getting into a nearby critical cir-
cuit. However, the problem with such shields is that noise captured
by the shield can still cause problems if the return path the noise
takes is not carefully planned and implemented by understanding
of the ground system and making the connections correctly.
Second, if noise is present in a system, shields can be placed around
critical circuits to prevent the noise from getting into sensitive por-
tions of the circuits. These shields can consist of metal boxes
around circuit regions or cables with shields around the center
conductors. Again, where and how the shields are connected is
important.
CAPACITIVELY COUPLED NOISE
If the noise results from an electric field, a shield works because
a charge, Q2, resulting from an external potential, V], cannot exist
on the interior of a closed conducting surface (Figure 1).
+ + . v..=o
Figure 1. Charge tl, cannot create charge inside a closed
metal shell.
Reprinted from Analog Dialogue 17--1 1983
Coupling by mutual, or stray, capacitance can be modeled by the
circuit of Figure 2. Here, v,, is a noise source (switching transistor, T
TTL gate, etc.), C, is the stray capacitance, Z is the impedance of
a receiver (for example, a bypass resistor connected between the
input of a high-gain amplifier and ground), and Vno is the output
noise developed across Z.
NOISE I COUPUNG IRECEIVER
SOURCE , MEDIUM I
Figure 2. Equivalent circuit of capacitive coupling between
a source and a nearby impedance.
A noise current, in = Vn/(Z + ch), will result, producing a noise
voltage, Vno = Vn/(l + ZCs/Z). For example, if Cs = 2.5 pF,
Z = 10kn (resistive), and V,, = 100mV at 1.3 MHz, the output
noise will be 20 mV (0.2% of 10V, i.e., 8 LSBs of 12 bits).
It is important to recognize the effect that very small amounts of
stray capacitance will have on sensitive circuits. This becomes in-
creasingly critical as systems are being designed to combine cir-
cuits operating at lower power (implying higher impedance levels),
higher speed (implying lower nodal stray capacitance, faster edges,
and higher frequencies), and higher regolution (much less output
noise permitted).
When a shield is added, the change to the situation of Figure 2 is
exemplified by the circuit model of Figure 3. With the assumption
that the shield has zero impedance, the noise current in loop A-B-
D-A will be 1UZcsi, but the noise current in loop D-B-C-D will
be zero, since there is no driving source in that loop. And, since
no current flows, there will be no voltage developed across Z. The
sensitive circuit has thus been shielded from the noise source, V,,.
CD c.,Ci)
\\\\\\ m
(i) /\ SHIELD
Figure 3. Equivalent circuit of the situation of Figure 2, with
a shield interposed between the source and the
impedance.
Guidelines for Applying Electrostatic Shields
oAn electrostatic shield, to be effective; should be connected to
the reference potential of any circuitry contained within the shield.
If the signal is earthed or grounded (i.e., connected to a metal chas-
sis or frame, and/or to earth), the shield must be earthed or ground-
ed. But grounding the shield is useless if the signal is not grounded.
shield return. This, in turn, develops a shield voltage common to
both the analog and digital shields. An equivalent circuit is shown
in Figure 10, in which V(t) is a 5-volt step from a rn. logic gate,
R02 is the 13-ohm output impedance of the logic gate, Cw,, is the
470-pF capacitance from the shield to the center conductor of the
shielded cable, and R, and L, are the 0.1-ohm resistance and 1-
microhenry inductance of the 2-foot wire connecting the shield to
the system ground.
ad C...
M813 ohms
1titl =5ultl
L, = 1;."
INPUT VOLTAGE =5 volt “Op
Figure 10. Equivalent circuit for generating shield voltage.
The shield voltage, Vs(t), can be solved for by conventional circuit-
analysis techniques, or simulated by actually building and care-
fully making measurements on a circuit with the given parameters.
For the purpose of demonstration, the calculated response
waveform, illustrated in Figure 11, with a 5-volt initial spike,
resonant frequency of 7.3 MHz, and damping time constant of
0.15p.s, is sufficient to illustrate the nature of the voltage thatap-
pears on the shield and is capacitively coupled to the analog input.
If the voltage is looked at with a wideband oscilloscope, it will look
like a noise "spike." We can see that this transient will couple a
fast damped waveform of significant peak amplitude to the analog
system input.
v.11) l
Figure 11. Computed response of circuit of Figure 10.
Even in a purely digital system, noise glitches can be caused to ap-
pear in apparently remote portions of a system having the kind of
situation shown. This can often explain some otherwise inexplica-
ble system bugs.
In quite a few cases, the proper choice of shield connection among
the many possibilities may not be immediately obvious, and the
guidelines may not provide us with a clear choice. There is no alter-
native but to analyze the various possibilities and choose the ap-
proach for which the lowest noise may be calculated.
For example, consider the case illustrated in Figure 12, in which
the measurement system and the source have differing ground po-
tentials. Should we connect the shield to A: the low side at the
measurement-system input, B: ground at the system input, C:
ground at the signal source, or D: the low side at the source
A is a poor choice, since noise current is allowed to flow in a signal
MEASUREMENT
SYSTEM
Figure 12. Possible grounds where system and source have
differing ground potentials. '
conductor. The path of the noise current due to kv, as it returns
through C4, is shown in Figure 13a.
B is also a poor choice, since the two noise sources in series, Vm
and kv, produce a component across the two signal wires, de-
veloped by the source impedance in parallel with C2, in series with
ch, as shown in Figure 13b.
C is poor, too, since Vrs1 produces a voltage across the two signal
wires, by the same mechanism as (B), as Figure 13c shows.
D is the best choice, under the given assumptions, as can be seen
in Figure 13d. It also tends to confirm the grounding guideline to
connect the shield at the signal's reference potential.
I SHIELD
" = (Va, + Val e,-tz;
b. Return path B.
v, -t-rl"
Vu=Vm c7535
c. Return path C.
d. Return path D.
Figure 13. Equivalent circuits.
NOISE RESULTING FROM A MAGNETIC FIELD
Noise in the form of a magnetic field induces voltage in a conduc-
tor or circuit; it is much more difficult to shield against than elec-
0The shield conductor of a shielded cable should be connected
to the reference potential at the signal-reference node (Figure 4).
OH the shield is split into sections, as might occur if connectors
are used, the shield for each segment must be tied to those for the
v... it LOAD
REFERENCSPOT‘ENTIAL
Figure 4. Grounding a cable shield.
adjoining segments, and ultimately connected (only) to the signal-
reference node( (Figure) 5).
Figure 5. Shields must be interconnected if interrupted.
OThe number of separate shields required in a system is equal to
the number of independent signals that are being measured. Each
signal should have its own shield, with no connections to other
shields in the system, unless they share a common reference poten-
tial (signal "ground"). If there is more than one signal ground (Fig-
ure 6), each shield should be connected to its own reference
potential.
MEASUREMENT
SYSTEM
Figure 6. Each signal should have its own shield connected
to its own reference potential.
PDon't connect both ends of the shield to "ground". The Poten-
tial difference between the two "grounds" will cause a shield cur-
rent to flow (Figure 7). The shield current will induce a noise vol-
tage into the center conductor via magnetic coupling. An example
of this can be found in Part 1 of this series, Analog Dialogue 16-3,
page 18, Figure 10.
"" "GROUND 2"
Figure 7. Don't connect the shield to ground at more than
one point.
oDon't allow shield current to exist (except as noted later in this
article). The shield current will induce a voltage in the center
conductor.
oDon't allow the shield to be at a voltage with respect to the refer-
ence potential (except in the case of a guard shield, to be de-
scribed). The shield voltage will couple capacitively to the center
conductor (or conductors in a multiple-conductor shield). With
J'igure 8.
a noise voltage, V,, on the shield, the situation is as shown in
SHIELD VOLTAGE. V.
a. Shield at potential Vs.
b. Equivalent circuit.
Figure 8. Don't permit the shield to be at a potential with re-
spect to the signal.
The fraction of V, appearing at the output will be
Vt I+-,-.-,-,,,-.,-,)------
(ZarfReqCSC)
where v, is the open-circuit signal voltage, R0 is the signal's source
impedance, Csc is the cable's shield-to-conductor capacitance, and
Req is the equivalent parallel resistance of K, and RL. For example,
ifV, = IV at 1.5MHz, Csc = 200pF (10 feet of cable), Ro = 1000
ohms, and RL = 10k0, the output noise voltage will be 0.86 volts.
This is an often-ignored guideline; serious noise problems can be
created by inadvertently applying undesired potentials to the
shield.
0101020 by careful study how the noise current that has been cap-
tured by the shield returns to "ground." An improperly returned
shield can cause shield voltages, can couple into other circuits, or
couple into other shields. The shield return should be as short as
possible to minimize inductance.
Here is an example that illustrates the problems that can arise in
relation to these last two guidelines: Consider the improperly con-
figured shield system shown in Figure 9, in which a precision volt-
age source, VI, and a digital logic gate share a common shield con-
nection. This situation can occur in a large system where analog
and digital signalsare cabled together.
ANALOG SYSTEM
DIGITAL SYSTEM
2 FT " GA WlRE
Figure 9. A situation that generates transient shield
voltages.
A step voltage change in the output of the logic circuit couples
capacitively to its shield, creating a current in the common 2-foot
tric fields because it can penetrate conducting materials. A typical
shield placed around a conductor and grounded at one end has lit-
tle if any effect on the magnetically induced voltage in that
conductor.
As a magnetic field, B, penetrates a shield, its amplitude decreases
exponentially (Figure 14). The skin depth, 8, of the shield material,
is defined as the depth of penetration required for the field to be
attenuated to 37% (exp (-1)) of its value in free air. Table 11 lists
typical values of 8 for several materials at various frequencies. You
. can see that any of the materials will be more effective as a shield
at high frequency, because 8 decreases with frequency, and that
steel provides at least an order of magnitude more effective shield-
ing at any frequency than copper or aluminum.
FLUX DENSITY
t THICKNESS
Figure 14. Magnetic field in a shield as a function of penetra-
tion depth.
Figure 15 compares absorption loss as a function of frequency for
two thicknesses of copper and steel. Ve-inch steel becomes quite ef-
fective for frequencies above 200 Hz, and even a 20-mil (0.5 mm)
thickness of copper is effective at frequencies above 1 MHz. How-
ever, all show a glaring weakness at lower frequencies, including
50-60-Hz line frequencies-the principal source of magnetically
coupled noise at low frequency.
0.0205".
Amman Loss - «a
" ttt' 10' "' 10' IO. 107
FREQUENCY - H:
Figure 15. Absorption loss vs. frequency fortwo thicknesses
ofeopper and steel.
For improved low-frequency magnetic shielding, a shield consist-
ing of a high-permeability magnetic material (e.g., Mumetal)
Table 1. Skin depth, 8, vs. frequency
6 for Copper ti for Aluminum 8 for Steel
Frequency (in.) (mm) (in.) (mm) (in.) (mm)
60Hz 0.335 8.5 0.429 10.9 0.034 0.86
100Hz 0.260 6.6 0.333 8.5 0.026 0.66
1kHz 0.082 2.1 0.105 2.7 0.008 0.2
lOkHz 0.026 0.66 0.033 0.84 0.003 0.08
lOOkHz 0.008 0.2 0.011 0.3 0.0008 0.02
IMHz 0.003 0.08 0.003 0.08 0.0003 0.008
'Table l and Figures " and 16 are from Ott, H.W., Noise Reduction Tech-
niques in Electronic Systems (New York: John Wiley lk Sons, 0 1976).
-si' "5/d2'f," ---"
I "6 menueucv f"
15 v,,',',','','.',', ALUMINUM "WET- s,i)u I‘m COPPER
, Lit: I l "-'''''" I
2 / "usae'esess'''"
i," {N :'j':'ii''"
E ",,W'' 1'iystii:;,):rr'';"
p''C,,,,W "1C,Jw''' 'i'i'ta) 7m: Ily'
" / l I
/ , 'l'
I COPPER
s ' La
---" -3----eGrian,
' -.,..,ver.r.::'c2"cu'--'" J
0 " 20 " to so so "
THlCKNESS - mils
Figure 16. Shielding, attenuation of Mumetal and other
materialsatseveralfrequencies.
should be considered. Figure 16 compares a 30-mil thickness of
Mumetal with various materials at several frequencies. It shows
that, below 1 kHz, Mumetal is more effective than any of the other
materials,'While at 100kHz it is the least effective. However,
Mumetal is not especially easy to apply, and if it is saturated by
an excessively strong field, it will no longer provide an advantage.
As you can see, it is very difficult to shield against magnetic fields,
i.e., to modify the coupling medium by shielding. Therefore, the
"most effective approaches at low frequency are to minimize the
strength of the interfering magnetic field, minimize the receiver
loop area, and minimize coupling by optimizingwiring geomet-
ries. Here are some guidelines:
OLocate the receiving circuits as far as possible from the source
of the magnetic field.
oAvoid running wires parallel to the magnetic field; instead, cross
the magnetic field at right angles.
OShield the magnetic field with an appropriate material for the
frequency and field strength.
OUse a twisted pair of wires for conductors carrying the high-level
current that is the source of the magnetic field. If the currents in
the two wires are equal and opposite, the net field in any direction
v». PW” Rurao
a. Correct connection with balanced cu rrents.
b. Incorrect connection forming ground loop.
Figure 17. Connections to a twisted pair.
over each cycle of twist will be zero (Figure 17a). For this arrange-
ment to work, none of the current can be shared with another con-
ductor, for example, a ground plane. Figure 17b shows what can
happen if a ground loop is formed; if part of the current flows
through the ground plane (depending on' the ratio of conductor re-
sistanpe to ground resistance), it will form a loop with the twisted
pair, generating a field determined by i3( = i1 - iz).
The ground connection between A and B need not be as simple as
a short circuit to cause trouble. Any stray unbalanced capacitance
or resistance from Rload circuits to the ground plane will also,
unbalance the currents and produce a net current through the
wires and the ground plane, producing a ground loop and a related
magnetic field. For this reason, it is also good practice to run the
twisted pair close to the ground plane to tend to balance the
capacitances from each side to ground, as well as to minimize loop
OUse a shielded cable with the high-level source circuit's return
current carried in the shield (Figure 18). If the shield current, I, is
equal and opposite to that in the center conductor, the center-con-
ductor field and the shield field will cancel, producing a zero net
field. In this case, which seems to violate the "no shield current"
rule for receiver circuits, the concentric cable is not used to shield
the center lead; instead, the geometry produces cancellation.
Figure 18. Use of shield for return currentto noisy source.
This scheme can be usefully employed in an ATE system where ac-
curate measurements must be performed on devices with high
power-supply currents that may be noisy. For example, Figure 19
shows the application of this technique to the connections for the
high-current logic supply for an a/d converter under test-at the
end of a test cable.
NOISV Vex: cue
TO lock: swrrcnmc
Vac [+5V)
' Mf dc -=
DIG GND
vc-ff"")- +15v
+ TSV -=- V - tSV
- 15V w..-
Ji-F/k-iz-z).- ANALOG GND
ANALOG INPUT
Figure 19. Application of circuit of Figure 18 in a test system.
OSince magnetically induced noise depends on the area of the re-
ceiver loop, the induced voltage due to magnetic coupling can be
reduced by reducing the 1oop's area. What is the receiver loop In
the example shown in Figure 20, the signal source and its load are
connected by a pair of conductors of length L and separation D.
The circuit (assuming it has a rectangular configuration) forms a
loop with area D . L.
V I--;---- L -----l
V... (siiiii,j)j'jj'i,i,i ',,iii, m
r ffd 1_r'
Figure 20. Area of a loop that receives magnetically coupled
noise.,
The voltage induced in series with the loop is proportional to the
area and the cosine of its angle to the field. Thus, to minimize
noise, the loop should be oriented at right angles to the field, and
its area should be minimized.
The area can be reduced by decreasing the length of and/or de-
creasing the distance between the conductors. This is easily accom-
plished with a twisted pair, or at least a tightly cabled pair, of con-
ductors. It is good practice to pair conductors so that the circuit
wire and its return path will always be together. To do this, the
designer must be certain of the actual path that the return current
takes in getting back to the signal source. Quite often, the current
returns by a path not intended in the original design layout.
If wires are moved (for example, by a technician troubleshooting
some other problem), the loop area and orientation to the field
may change, so that yesterday's acceptable noise level may be
transformed to tomorrow's disastrous noise level. Which may lead
to a service call . . . and another repetition of the cycle. The bot-
tom line: Know the loop area and orientation, do what must be
done to minimize noise-and permanently secure the wiring!
DRIVEN SHIELDS AND GUARDING
We have discussed the role of a current-driven shield carrying an
equal and opposite current to reduce generated noise by reducing
the magnetic field around a conductor.
Guarding is similar, in that it involves driving a shield, at low impe-
dance, with a potential essentially equal to the common-mode vol-
tage on the signal wire contained within the shield. Guarding has
many useful purposes.. It reduces common-mode capacitance, im-
proves common-mode rejection, and eliminates leakage currents
in high-impedance measurement circuits.
Figure 21 shows an example of an op amp with negligible bias cur-
rent connected as a high-impedance non-inverting amplifier with
gain. The purpose of the cable is to shield the high input-impe-
dance signal conductor from capacitively coupled noise and to
minimize leakage currents. The signal comes from a 10-megohm
source, and the cable is assumed to have 1000 megohms of leakage
resistance (which may change as a function of temperature,
humidity, etc.) from conductor to shield. If connected as shown,
the equivalent input circuit is an attenuator which loses 1% of the
INCORRECT
CONNECTION
v. I um [l 'thu, v,
v-U-ar''-.',, F. N -,ggp-,d = "str.
Figure 21. Op amp connected as high-impedance non-
inverting amplifierwith gain, with shielded input lead.
