Optical isolation and ground loops
What is Optical Isolation and When Do You Need It?
You have probably noticed that optical isolation is a popular feature on data communication products. But what is optical isolation? And why is this feature in demand? This article will present a brief overview of optical isolation and surge protection.
The Basic Theory.
The concept behind optical isolation is simple and solves the vast majority of data transmission isolation problems presented by transformers and surge suppressors.
Data is brought into an isolation module and converted into a light signal using a tiny light emitting diode (LED). This signal is picked up by a photo diode receptor within the same isolation module and converted back into its original form. The physical gap between the light transmitter and receptor provide the needed isolation from transient current.
In data communications, the primary application for optical isolation is in a point-to-point data circuit that covers a distance of several hundred feet or more. Because the connected devices are presumably on different power circuits, a ground potential difference likely exists between them. When such a condition exists, the voltage of "ground" can be different, sometimes by several hundred volts.
Where a ground potential difference exists, a phenomenon called ground looping can occur. In this phenomenon, current will flow along the data line in an effort to equalize the ground potential between the connected devices. Ground looping can, at the very least, severely garble communications--if not damage hardware!
Optical isolation solves the problem of ground looping by effectively lifting the connection between the data line and "ground" at either end of the line. If an optically coupled connection exists at each end, the data traffic "floats" above the volatility of ground potential differences.
If the voltage potential is large enough, your equipment will not be able to handle the excess voltage, and one of your ports will be damaged. Even small ground loop voltages cause transmission errors with data signals riding on top of the ground loop current. At worst, ground loops are a long-term condition that can slowly heat, and even cook your circuits.
Optical Isolation vs. Transformer Isolation.
A common belief is that optical isolation is superior to transformer isolation in every case. Theoretically this is true, because optical isolation provides a "true" physical barrier, whereas transformer isolation is a coupling designed to merely "absorb" unwanted frequencies. However, in practice optical isolation is a less efficient transmitter of energy than transformer isolation--an important consideration when signal strength is an issue. Therefore transformer isolation is sometimes the best choice for very long-distance applications. And optical isolation also becomes a prohibitively expensive solution at higher data rates. So in the real world, transformer isolation still has its place.
Optical Isolation vs. Surge Protection.
Another common belief is that optical isolation takes the place of surge protection. After all, if optical isolation provides a barrier against ground loops, won't it provide a barrier against transients as well? This belief fails to account for the fundamental difference between ground
loops and transients.
Ground loops tend to be of long duration and relatively low voltage. Transients, on the other hand, tend to be of short duration and very high voltage. Consequently, the amount of current instantly presented by a transient must be directed safely to ground. An optocoupler will be destroyed by a high voltage transient exceeding its rating. True, the transient will not get past the barrier, and components on the other side of the optocoupler will be spared. But components on the side receiving the "hit" (usually the analog line side) can be damaged. In any case, the unit will no longer pass data.
What's needed, therefore, is surge protection placed ahead of the optocoupler--right where the line enters from the outside world. Surge protectors respond instantly and shunt relatively large amounts of current quickly to chassis ground. This dangerous current is not permitted to roam around and damage components (including the optocoupler). Then the optocoupler can do its job of providing a constant barrier to low voltage ground loops.
You can't test for ground loops. You don't know you have one until a vital component fails. Only prevention works—use optical isolation or fiber optic cable on all your long data lines.
A benefit of optical isolation is that it is not dependent on installation quality. Typical surge suppressors used in data line protection use special diodes to shunt excess energy to ground. The installer must provide an extremely low impedance ground connection to handle this energy, which can be thousands of amps at frequencies into the tens of megahertz. A small impedance in the ground connection, such as in 1.8 meters (6 feet) of 18 gauge wire, can cause a voltage drop of hundreds of volts - enough voltage to damage most equipment. Isolation, on the other hand, does not require an additional ground connection, making it insensitive to installation quality.
Isolation is not a perfect solution. An additional isolated power supply is required to support the circuitry. This supply may be built in as an isolated DC-DC converter or external. Simple surge suppressors require no power source. Isolation voltages are limited as well, usually ranging from 500V to 4000V. In some cases, applying both surge suppression and isolation is an effective solution.
When choosing data line protection for a system it is important to consider all available options. There are pros and cons to both surge suppression and optical isolation, however isolation is a more effective solution for most systems. If in doubt, choose isolation.
Here are the most commonly used surge protection options:
- Surge Suppressors
- Mechanical Isolation (transformers)
- Optical Isolation
- Triple Isolation
Transient voltage, surges and ground loops primarily have two paths they follow: data lines and ports and power supply lines. Both of these routes provide a "path of least resistance" for surges and must be protected.
Here is a summary of the four main protection components available:
A surge suppressor suppresses the voltage, simply – it suppresses a surge and dissipates it before it reaches and destroys critical components.
When used properly, or in conjunction with other protection measures, suppressors can be very effective. Suppressors are available to protect power lines and data lines, but are most effective when utilized as a first line of defense on power supply lines. Commonly used products has surge protection ratings of up to 39kA and less than 1ns response time.
Shunting harmful currents to ground before they reach the data port is the function of components such as Transient Voltage Suppressors (TVS), Metal Oxide Varistors (MOV) or gas discharge tubes. These devices all work by turning on at a set voltage. Once the clamp voltage has been exceeded, the devices provide a low impedance connection between terminals. These shunting devices are most often installed from each data line to the local earth ground, and should be selected to begin conducting current at a voltage as close as possible above the system's normal communications level. For RS-422 and RS-485 systems, the voltage rating selected is typically 5 - 7 volts, in RS-232 systems 12 - 15 volt devices are appropriate.
Installation of surge protectors.
Surge suppressors must be installed as close to the port to be protected as possible, and must have an extremely low resistance connection to the local earth ground of the unit being protected. This ground connection is crucial for proper operation, providing a shunt path for excess energy as well as a ground reference at the same potential as the host. In most cases, this means the ground connection should be made from the surge suppressor directly to the chassis of the host device.
The resistance of the ground connection is critical. The voltage presented to the data port is equal to the clamping voltage of the surge suppression device plus the voltage drop in the suppressor's ground path to the node being protected. Any voltage drop in the ground connection will effectively increase the clamping voltage seen at the data port. Transient currents can be very large, with magnitudes measured in thousands of amps. At these current levels, the DC voltage drop (I X R) can be very large. For example, 6 feet of 18AWG wire has approximately 0.039 ohms resistance. Although this initially appears to be a good ground connection, calculating I X R with a 3000 amp transient yields a voltage of 117 volts across the ground wire - enough to destroy any data port. The ground connection should be made with heavy gauge wire and kept as short as possible. If the cable must be longer than one meter, braided cable intended for grounding purposes must be used.
Selecting a Surge Suppression Device.
Two basic types of surge suppression products are available as illustrated below. In either case, the system designer should consider the clamping voltage of the unit as well as its physical attributes, such as connector type and method of making the grounding connection.
Single Stage Devices.
The most common device uses a single TVS or MOV for each protected line. This type of unit is usually small and inexpensive. If a proper ground connection is made, they should offer protection against most transients. A disadvantage of this device is that if a large transient damages one or more of the components there often is no indication that the unit has failed, leaving the node unprotected against future transients.
Three Stage Devices.
More advanced units use three components on each protected line to handle much larger surge currents and to provide internal self protection, reducing the risk of undetected failures. The first stage is a gas discharge tube; this stage can shunt very large currents, but is slow reacting and requires a relatively large voltage before conduction begins. The second stage is series impedance; this stage limits the current flowing into the final stage of the circuit. Finally, a TVS device clamps at a voltage acceptable for the data port and maintains the clamp until the gas discharge tube begins conduction.