Grounding and Interfacing 120732.6.3 Solving Interface Problems
32.6.3.1 Ground Isolators
A device called a ground isolator solves the inherent
common-impedance coupling problem in unbalanced
interfaces. Broadly defined, a ground isolator is a differ-
ential responding device with high common-mode rejec-
tion. It is not a filter that can selectively remove hum,
buzz, or other noises when simply placed anywhere in
the signal path. To do its job, it must be installed where
the noise coupling would otherwise occur.
A transformer is a passive device that fits the defini-
tion of a ground isolator. Transformers transfer a
voltage from one circuit to another without any elec-
trical connections between the two circuits. It converts
an ac signal voltage on its primary winding into a fluc-
tuating magnetic field that is then converted back to an
ac signal voltage on its secondary winding (discussed in
detail in Chapter 11).
As shown in Fig. 32-43, when a transformer is
inserted into an unbalanced signal path, the connection
between device grounds via the cable shield is broken.
This stops the noise current flow in the shield conductor
that causes the noise coupling, as discussed in Section
32.5.4. As discussed in Chapter 11, the highest noise
rejection is achieved with input-type transformers
containing Faraday shields. A transformer-based
isolator for consumer audio signals using such trans-
formers, the ISO-MAX® model CI-2RR, is shown in
Fig. 32-44. To avoid bandwidth loss, such isolators
must be located at the receive end of interconnections,
using minimum-length cables between isolator outputs
and equipment inputs. Conversely, isolators using
output-type transformers, such as the ISO-MAX®
model CO-2RR and most other commercial isolators,
may be freely located but will achieve significantly less
noise rejection.
Ground isolators can also solve most of the problems
associated with balanced interfaces. The ISO-MAX®
Pro model PI-2XX shown in Fig. 32-45 often improves
CMRR by 40 dB to 60 dB and provides excellent
CMRR even if the signal source is unbalanced. Because
it also features DIP switches to reconfigure cable shield
ground connections, it can also solve pin 1 problems.
Because it uses input-type transformers, it attenuates RF
interference such as AM radio by over 20 dB. Again, to
avoid bandwidth loss, it must be located at the receive
end of long cable runs, using minimum-length cables
between isolator outputs and equipment inputs. Other
models are available for microphone signals and other
applications. The vast majority of commercial hum
eliminators and a few special-purpose ISO-MAX®
models use output-type transformers, which may be
freely located but offer significantly less CMRR
improvement and have essentially no RF attenuation.
Several manufacturers make active (i.e., powered)
ground isolators using some form of the simple differ-
ential amplifier shown in Fig. 32-31. Unfortunately,
these circuits are exquisitely sensitive to the impedance
of the driving source. Fig. 32-46 compares the measured
60 Hz (hum) rejection of a typical active isolator to a
transformer-based isolator. Over the typical range of
consumer output impedances, 100: to 1 k:, the trans-
former has about 80 dB more rejection!
Passive isolators based on input-type transformers
have other advantages, too. They require no power, they
inherently suppress RF interference, and they’re
immune to most overvoltages that can be sudden death
to active circuitry.32.6.3.2 Multiple GroundingWhen a system contains two or more grounded devices,
such as the TV receiver and the subwoofer power
amplifier in our example home theater system, a wired
ground loop is formed as shown in Fig. 32-47.
As discussed in Sections 32.5.3 and 32.5.4, noise
current flowing in the shaded path can couple noise into
the signal as it flows in unbalanced cables or through
the equipment’s internal the ground path. This system
would likely exhibit a loud hum regardless of the input
selected or the setting of the volume control because of
noise current flow in the 20 ft cable. You might be
tempted to break this ground loop by lifting the safety
ground at the subwoofer. Reread Section 32.4.2 and
don’t do it!
One safe solution is to break the ground loop by
installing a ground isolator in the audio path from
preamp to subwoofer as shown in Fig. 32-48. This
isolator could also be installed in the path from TV
receiver to preamp, but it is generally best to isolate the
longest lines since they are more prone to coupling than
shorter ones.
Another safe solution is to break the ground loop by
installing a ground isolator in the CATV signal path at
the TV receiver as shown in Fig. 32-49. These RF isola-
tors generally should be installed where the cable
connects to the local system, usually at a VCR or TV
input. If an RF isolator is used at the input to a splitter,
ground loops may still exist between systems served by
the splitter outputs since the splitter provides no ground
isolation. Although it can be used with a conventional
TV or FM antenna, never install an RF isolator between