Why Do I Need Signal Isolation?
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
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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.)
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)
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.
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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 voltages
- To 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.
