Examining the important
aspects of signal isolation; what it does, why we need it, and how to test for
it.
Acromag is in the business of
signal conditioning. We manufacture circuits that amplify, isolate, filter, and
convert one signal form to another. Most of these circuits also provide
electrical isolation. However, added isolation has a cost. Sometimes customers
question their need for isolation or fail to recognize the need for adding
isolation in their application. This paper covers the basic aspects of
electrical isolation.
Briefly, electric current
refers to the conceptual flow of atomic particles or electrons through wires
and electrical devices. Conductive materials like metal and water allow
electric current to easily pass through them. The force that drives electric
current to flow through a conductive medium is potential difference or voltage.
The opposing force that curbs or limits this current flow is resistance.
Comprised of 60% water, the human body is an excellent conductor; except that
electric current from a source allowed to pass through the body can induce
injury via electric shock. Materials that are weak conductors of electricity
have high resistance to current flow; these materials are often used to add
isolation or insulate circuits. In general, greater force (voltage) along a
conductive path will drive higher current flow; if poorly controlled in the
absence of resistance (insulation/isolation), it may result in circuit damage,
personal injury, or even death.
Table of Contents
1. What
is Isolation?
2. Common
Methods of Signal Isolation
·
Transformer or Inductive Coupling
·
Galvanic Isolation vs Galvanic Isolator
·
Optical Isolator, Optical Coupler, or Fiber
Optic Link
·
Capacitor
·
Magnetoresistance
3. Why Do
I Need Isolation?
·
Block High or Hazardous Voltage
·
Protection from Electric Shock
·
Reject High Common-Mode Voltages
4. More
Resources
Click here to download the complete white paper Why Do
I Need Signal Isolation?
What is Isolation?
With respect to electric
circuits and electronic instruments; “isolation” means the deliberate
introduction of a non-conductive separation to inhibit current flow. Galvanic
Isolation is the process of blocking current flow to prevent a direct
conduction path between circuits is called. This term sometimes causes
confusion because “galvanic” refers to metal and the electrochemical process in
which one metal corrodes to another when both metals are in electrical contact
and in the presence of an electrolyte. But galvanic isolation refers to the
absence of metal or a conduction path.
Galvanic isolation is
accomplished by physically adding distance, clearance, or insulating material
around a circuit to block unwanted current flow. But how do we preserve a
circuit signal and allow it to be transmitted across an isolation barrier? We
can additionally isolate the signal by transmitting it magnetically using
transformers or magnetoresistance. We could transmit it optically using optical
couplers, optical isolators, or fiber-optic media. Or we could capacitively
couple the signal across an isolation barrier using capacitive isolators.
Signal isolation is usually
accomplished by a combination of actions; physical separation and insulating
material, combined with a method of isolated signal transmission (magnetic,
optical, or capacitive). The important thing is that regardless of our
isolation method, isolation prevents the electrical conduction of unwanted
current between circuits, while still allowing our wanted signal to cross an
isolation barrier without providing a conductive metal path.
Acromag offers the industry’s best selection of
process signal isolators. Click
here to view them.
Common Methods of Signal Isolation
So, the real trick in
isolation is not how to add insulation or separation to a circuit. The trick is
adding electrical isolation to block unwanted signals, while still allowing the
wanted signal to transmit through the circuit; and without providing a direct
(galvanic) path for signal conduction. Below are some common ways to isolate a
signal between two points without providing a direct conduction path between
them.
Transformer or Inductive Coupling
The most common example of a
galvanic isolator would be the transformer. The primary and secondary windings
of a transformer are insulated from one another. They don’t connect to each
other electrically, so there’s no metal to metal contact. Instead, they use
magnetic field flux, generated by coils of wire overlapping a ferromagnetic
material; signals are inductively coupled to/from the ferro-magnetic material
using a varying magnetic field.
Transformers buffer or change
voltages by stepping them up or down. They’re also used for isolating signals
for safety, as well as isolating a circuit from AC line voltage. A transformer
allows its secondary windings to be offset from a ground reference on the
primary side. Thus, breaking potential ground loops between the primary and
secondary circuits. Because it involves the mutual inductance of magnetic
fields from coils, it can be more susceptible to magnetic interference.
Further, unless properly shielded, it can also be a source of magnetic
interference to adjacent circuitry (inductive and radiated emissions).
Transformers are traditionally bulkier than optical or capacitive isolators.
However, there’s newer technology that uses chip-scale transformers, encased in
integrated circuit style packages, to magnetically isolate signals. (For one
example of this technology, see Analog
Devices isoPower® and iCoupler® technology.)
(Isolated
Transmitters: Acromag 650Ts: Acromag
651T/652T with Transformer Isolation)
Galvanic Isolation vs Galvanic Isolator
“Galvanic isola-tion” should
not be confused with “galvanic isola-tors.” A galvanic isolator is used to
block low voltage DC currents from coming on board boats, via shore power
ground wires. These DC currents can accelerate galvanic corrosion on underwater
metals of boats and cause extensive damage; metal in hulls, zinc anodes, prop,
drive-shaft, etc. Galvanic isolators are used because boats plugged into shore
power at marinas each act like giant batteries; contributing DC voltage to the
power signals via the ground wires. This produces corrosive electric currents
through all the metals that contact the water. The metal and water form a giant
battery, causing the metals to corrode in galvanic fashion; the way terminals
and plates of a battery corrode as current passes through them. Zinc anode is a
sacrificial metal added to a boat’s conductive metal surface; concentrating the
resultant corrosion to itself.
How Galvanic Isolators are Used
Galvanic isolators are
inserted in-line with the green safety ground as they enter the boat, between
the shore-power inlet and the boat’s electrical panel. It allows AC fault
current to pass through it while blocking DC current. Thus, AC faults are
transmitted back to the power source, where they can safely trip a breaker or
open a fuse. Simultaneously, destructive galvanic DC battery currents are
blocked/minimized to reduce galvanic corrosion. This enables the zinc anodes of
your boat to help protect its underwater metals and not those of other vessels
that surround it; as they act to control the corrosion of the metal attached to
your own boat. Most galvanic isolators are designed to be fail-safe; meaning
that if they fail, they do not also open the path to ground for fault current.
Your first instinct might
suggest, “Why not just remove the ground-wire?” However, this would be
dangerous. The ground wire must be present to carry fault current back to the
dock power source or transformer. Otherwise, if you accidentally contacted the
shore power AC line by some type of wiring fault, you could become the medium
to carry fault current back to the transformer; this could be fatal.
Learn more: Why
You Need USB Isolation for Industrial I/O?
Optical Isolator, Optical Coupler, or Fiber Optic Link
Optical devices transmit
information through their medium or across their barriers using varying levels
of light intensity; with no direct electrical conduction path. A light source
(transmitter, typically an LED) sends light waves to a photo-sensitive device
(receiver, typically a photo-transistor). The combination is often held in
place with insulating plastic, like that of an integrated circuit IC.
Alternatively, transmit and receive functions are separated using a transmitter
linked to a remote receiver via fiber optic cable. One major benefit of optical
isolation is its inherent immunity to EMI (Electro-Magnetic Interference or
electrical and magnetic noise).
Some comparative disadvantages to optical isolation are
its:
- Generally higher power dissipation
- Susceptibility to temperature effects
- Traditionally slower speed (specifically
optical couplers, not fiber optic links)
- Finite life of its transmitter (LEDs degrade
over time)
(Acromag 612T DC
Voltage/Current Input Dual-Channel DC-Powered Transmitter Drawing with Optical
Isolation)
Capacitor
Remember that capacitors
generally allow AC current to flow, but block DC current. Thus, they efficiently
couple AC signals between circuits, at different DC voltages, via a varying
electric field. There are many capacitive isolation devices available, and it
is a common technology of digital isolators. Many modern devices will even use
isolation-rated capacitors to connect between grounds on each side of an
isolation barrier. This provides a conduction path for transient signals;
perhaps to earth ground (also helpful in quelling radiated emissions).
Capacitive isolation is faster than optical isolation.
Unfortunately, capacitors are
more prone to failure when stressed by voltages above their voltage rating. And
for some capacitors, this failure mode can result in a short circuit condition;
abruptly ending its isolation-ability, as well as possibly rendering its
circuit unsafe or hazardous. Safety rated Y-type capacitors are used in line to
ground applications and are designed to fail open; while X-types are used in
line-to-line filtering applications and may fail short. Also bothersome when
used to isolate digital signals; often the first bit transmitted after power-up
using capacitive digital isolators is used to setup the data stream, and must
be ignored (only the trailing bits contain useful data).
Magnetoresistance
Magnetocouplers use Giant
Magneto Resistance (GMR) to couple from AC to DC. An explanation of GMR
isolation is beyond the scope of this paper. Briefly: GMR refers to an
isolation scheme that relies on the property of a material to change the value
of its electrical resistance, when an external magnetic field is applied to it.
It’s important to remember that GMR operates like a transformer; it uses the
variable magnetic field of an AC coil. However, it does this to linearly alter
the DC resistance of a physically isolated sensing element.
Click here to download Why Do
I Need Electrical Isolation?
Why Do I Need Electrical Isolation?
We have two principal reasons for introducing isolation
into an electric circuit:
- To
block the transfer of high or hazardous voltagesTo
break ground loops
- Block High or Hazardous Voltage
We use isolation to prevent
the transfer of high or hazardous voltages between circuits. We typically block
these voltages using isolation for safety reasons and protection from electric
shock; but also to block high common mode voltage present in our signals, which
can prevent its measurement and damage equipment. Isolation can also block
transient voltages for the same reasons. High voltage may drive injury via
electric shock and the unintended flow of electric current through the body.
Additionally, it may also drive damage to an electrical circuit because of
unintended electric current flowing between conductive circuits.
Protection from Electric Shock
One reason we isolate a
circuit is to help prevent electrical shock. That is, by introducing isolation
between conductive bodies, we minimize or eliminate the potential for unintended
current flow. With no shared common reference or conductive path between two
conductors or circuits you cannot complete a circuit for current to flow. This
is because of potential differences between them sufficient to produce electric
shock; the sudden and rapid flow of electricity between potentials when crossed
with a conductor. Shock currents in the body can be felt at about 0.5mA; they
can drive an erratic heartbeat and potentially be fatal above 10mA; and they
can stop a human heart at 2A. Isolation blocks voltage potentials that could
drive dangerous current levels through a body if contacted/crossed.
Learn more: How to
Select the Right Isolator?
Reject High Common-Mode Voltages
Isolation blocks the dangerous
transmission of high voltages between circuits which can drive electric shock
to personnel or equipment. Another key use of isolation is to enable the measurement
of a signal with a high common-mode voltage that prevents valid measurement and
could damage equipment. The reality is that most instruments will have a
common-mode input range inside of ±10V; unless specifically designed to reject
high common-mode voltage. Thus, signals with an offset from measurement ground
greater than 10V cannot be converted properly and could damage the instrument.
Isolation rejects the unwanted high common-mode voltage present in some
signals, allowing the real signal of interest to be discerned.
Remember that electromagnetic
noise is ever-present in most environments because of nearby machinery and
electric motors, relays, fluorescent lighting, etc. As a result, common-mode
noise can be capacitive-coupled, inductively coupled, or radiated into the
measurement system. And it will typically take the form of a DC offset,
combined with a continuously variable 50-60Hz component (and even higher
frequency harmonics of 50-60Hz) that can mix with and obscure your measurement.
Isolation blocks the transmission of this error through our system (see Ground
Loops below). But some applications will naturally contain a greater offset
voltage than this.
Example
If an input is restricted to
voltage potentials in the ±10V range; how would you measure one cell of a large
array of solar cells connected in series? Or measure the individual cell of a
large hybrid battery? Since these signals are offset from circuit common by
larger amounts (they have high common-mode voltage potentials), certainly
greater than ±10V; this makes their measurement difficult and potentially
dangerous to your equipment. Note: the common-mode portion of an input signal
is normally computed as the sum of the voltage potential of the positive lead,
with respect to measurement return or common; and the voltage potential at the
negative lead, with respect to measurement return or common, divided by two
(Vcm = [Vin+ + Vin-]/2). Signal isolation blocks the high common-mode portion
of input signals like these; which otherwise make our measurement difficult and
can damage our equipment.
(Courtesy of Acromag)