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AP P LICATION BULLETIN
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ISOLATION AMPS HIKE ACCURACY AND RELIABILITY
Isolation Amplifiers Protect Critical Circuitry From Damage and Enhance Performance
by Tom Sommerville
Fault tolerance, transient protection, and interference rejec-
tion are valuable features that isolation amplifiers bring to
critical circuitry. Two useful examples of how designers can
Signal
Two-wire
employ isolation amplifiers to improve their systems’ per-
Power Supply
Transmitter
formance and reliability are process temperature controllers
4 to 20mA
and electrocardiogram (ECG) amplifiers.
In the case of process control, isolation amplifiers galvani-
Input
cally isolate both the input channel and the current-loop
output driver from the controller hardware. Consequently,
neither accidental faults from line-powered manufacturing
plant equipment to the control system circuitry nor ground
loop voltages can compromise the process. In ECG amplifi-
ers, a low-capacitance isolation barrier limits 60Hz leakage
current to safe levels, and a high barrier voltage rating
protects the monitoring equipment from defibrillator tran-
sients and electrosurgery (ESU) interference.
Distributed
Control
System
Isolation
Amplifiers
PT100
Transducer
Power Supply
Signal
Heat
Source
Output
Valve
20mA to 0
Process control loops, in particular, illustrate a number of
ways that isolation amplifiers improve performance and
reliability. In unisolated control systems, the long ground
lines can develop error potentials across the common imped-
ance that can cause component failure and/or inaccurate
control. In contrast, isolating the distributed control systems
(DCS) inputs and outputs close to the controller interrupts
the dc path, replacing it with the large impedance of the
isolation amp’s high-voltage barrier (Figure 1).
FIGURE 1. Isolating the Distributed Control System in a
Process Control Loop Interrupts the DC Path,
Blocking Possible Error Potentials that Could
Cause Component Failure and Control Inaccu-
racy.
Similarly, isolation protects the loop from interruption or
damage. In the example loop, which is a temperature con-
troller, isolation protects the circuit if the resistance-tem-
perature detector (RTD), a PT100, is accidentally shorted to
a grounded metal case or high-voltage conductor. Another
possible problem is a fault from the high-potential wire of a
twisted pair to earth ground. In addition, high current tran-
sients from motors and relays sharing an unisolated system
ground can create voltages that exceed the ±1V maximum
rating of the two-wire transmitter (XTR).
more difficult to deal with. An isolation amp, however, can
reduce their effect on system accuracy, depending on the
amplifier’s transient immunity. This parameter is defined as
the greatest dv/dt that can appear between isolated and
unisolated ground before accuracy is lost at the amplifier’s
output. Few data sheets specify transient immunity, which
can vary from 0.1 to 10,000V/µs.
Choosing the best isolation amplifier from process-control
applications isn’t a simple matter. The common use of 4-to-
20mA current loop transmitters and receivers requires an
isolated power supply that can supply at least 25mA for the
loop and any signal conditioning circuitry. Another consid-
eration is the input supply-voltage range. Some isolation
amps need a regulated 15V supply, while others can tolerate
sharing an unregulated system supply. The IMR needed
depends on the 60Hz line voltage encountered. Most isola-
tion amps specify greater than 100dB rejection at 60Hz,
which is adequate for the majority of process-control appli-
cations.
Isolated control loops also benefit from improved rejection
of 60Hz line interference, because low-level transducers are
especially susceptible to inductive-loop coupling to the
60Hz magnetic field and capacitive coupling to the electric
fields. Not only can the isolation amp prevent system dam-
age, but also the component’s isolation-mode rejection (IMR)
can attenuate the effect on the output by over 1 million to
one.
High dv/dt from inductive current and from radiated electro-
magnetic interference (EMI) caused by relay arcing are
©1994 Burr-Brown Corporation
AB-093
1
Printed in U.S.A. May, 1995
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