Grounding and Interfacing 1187can be paralleled across the same output line with negli-
gible drop in level. About 60: has been suggested as
the optimum Zo for driving up to 2000 ft of typical
shielded twisted pair cable in these balanced interfaces.^5
32.3.3 Line Drivers and Cable CapacitanceA line driver and cable interact in two important ways.
First, output impedance Zo and the cable capacitance
form a low-pass filter that will cause high-frequency
roll-off. A typical capacitance for either unbalanced or
balanced shielded audio cable might be about 50 pF/ft.
If output impedance were 1 k: (not uncommon in
unbalanced consumer gear), response at 20 kHz would
be –0.5 dB for 50 ft, –1.5 dB for 100 ft, and –4 dB for
200 ft of cable. If the output impedance were 100:
(common in balanced pro gear), the effects would be
negligible for the same cable lengths. Low-output
impedance is especially important when cable runs are
long. Also be aware that some exotic audio cables have
extraordinarily high capacitance.
Second, cable capacitance requires additional
high-frequency current from the driver. The current
required to change the voltage on a capacitance is
directly proportional to the rate of change or slew rate
of the voltage. For a sine wave,(32-6)
where,
SR is slew rate in volts per second,
f is frequency in hertz,
Vp is peak voltage.(32-7)
where,
I is current in A,
SR is slew rate in V/μs,
C is capacitance in μF.For example, we have a cable with a slew rate of
1V/μs at 20 kHz for a sine-wave of 8 Vp or 5.6 Vrms,
which is also +17 dBu or +15 dBV. For a cable of 100 ft
at 50 pF/ft, C would be 5000 pF or 0.005μF. Therefore,
peak currents of 5 mA are required to drive just the cable
capacitance to +17 dBu at 20 kHz. Obviously, increasing
level, frequency, cable capacitance, or cable length will
increase the current required. Under the previous condi-
tions, a cable of 1000 ft would require peak currents of
50 mA. Such peak currents may cause protective current
limiting or clipping in the op-amps used in some line
drivers. Since it occurs only at high levels and high
frequencies, the audible effects may be subtle.
Of course, the load at the receiver also requires
current. At a +17 dBu level, a normal 10 k: balanced
input requires a peak current of only 0.8 mA. However,
a 600: termination at the input requires 13 mA.
Matching 600: sources and loads not only places a
current burden on the driver but, because 6 dB (half) of
signal voltage is lost, the driver must generate +23 dBu
to deliver +17 dBu to the input. Unnecessary termina-
tion wastes driver current and unnecessary matching of
source and load impedances wastes head room!32.3.4 Capacitive Coupling and ShieldingCapacitances exist between any two conductive objects,
even over a relatively large distance. As we mentioned
earlier, the value of this capacitance depends on the
surface areas of the objects and the distance. When
there are ac voltage differences between the objects,
these capacitances cause small but significant currents
to flow from one object to another by means of the
changing electric field (widely referred to as electro-
static fields although technically a misnomer since
static means unchanging).
Strong electric fields radiate from any conductor
operating at a high ac voltage and, in general, weaken
rapidly with distance. Factors that increase coupling
include increasing frequency, decreasing spacing of the
wires, increasing length of their common run,
increasing impedance of the victim circuit, and
increasing distance from a ground plane. For some of
these factors, there is a point of diminishing returns. For
example, for parallel 22-gauge wires, there is no signifi-
cant reduction in coupling for spacing over about 1 in.^6
Capacitive coupling originates from the voltage at the
source. Therefore, coupling from a power circuit, for
example, will exist whenever voltage is applied to the
circuit regardless of whether load current is flowing.
Capacitive coupling can be prevented by placing
electrically conductive material called a shield between
the two circuits so that the electric field, and the
resulting current flow, linking them is diverted. A shield
is connected to a point in the circuit where the offending
current will be harmlessly returned to its source, usually
called ground—more about ground later. For example,
capacitive coupling between a sensitive printed wiring
board and nearby ac power wiring could be prevented by
locating a grounded metal plate (shield) between them,
by completely enclosing the board in a thin metal box, or
by enclosing the ac power wiring in a thin metal box.SR= 2 SfVpISRC= u