Most differential field bus technologies, like CAN and RS-485, specify a parameter called common-mode voltage. Common-mode voltage refers to the range of bus input voltages, with respect to the transceiver’s ground, within which the transceiver maintains consistent input receiver thresholds. In other words, when voltages present on the bus are within this range, the receiver maintains normal operation.
There are two frequent causes of common-mode voltage in industrial networks:
One source of common-mode voltage is the potential coupling of electromagnetic noise present in the vicinity of the network onto the bus wires. This noise could be generated from motors, fluorescent lights, relays, transformers, parallel cabling, and other RF sources. These noise sources only couple equally onto bus wires as a common-mode voltage, if twisted-pair cabling is used. In the event that non-twisted pair cabling is used, noise will not couple symmetrically and differential noise will be introduced into the system, reducing receiver margins.
Another frequent cause of common-mode voltage is the difference in ground potential that can occur between different nodes on the network. Substantial ground potential differences are common in industrial networks because electrical installations can often reach tens if not hundreds of meters. Figure 1 shows a simple two-node network with 50 meters of cabling between the two nodes, each of which draws 5 amps of current. Even with just 5 mΩ of resistance per meter on standard 12 AWG cabling, this results in a 1.25 V ground potential difference between the two nodes.
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| Figure 1. | Effects of wire resistance on long electrical installations. |
This difference in ground potential causes the receiving nodes on the network to “see” the voltages at their bus input terminals at a different voltage potential. Taking the example in Figure 1 and assuming an RS-485 network where Node 2 transmitted a bit stream, Node 1 would receive that bit stream with a 1.25 V higher DC offset (Figure 2).
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| Figure 2. | Ground potential differences to a receiving transceiver. |
It is easy to see how this ground potential difference could become even larger when additional loads are added to the network and the total length of cabling expands. Current from each additional load is superimposed on top of the existing current in the cable, and the effects of the cable resistance become proportionately worse. Imagine an elevator in a 50-story building with a node on every floor and five meters of cabling per floor. The ground potential difference could easily reach tens of volts, exceeding the common-mode range of most transceivers (for example, –7 to +12 V).
However, by implementing a resistor divider and biasing network on the front-end of the receiver block (Figure 3), differential transceivers are able to handle common-mode voltages. Resistors both attenuate the input signal seen on the bus pins and bias it towards VCC/2 so that the comparator inputs are not saturated. By increasing the attenuation ratio of these resistors, larger voltages can be present on the bus pins before exceeding the input voltage range of the comparator.
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| Figure 3. | Common differential receiver topology. |
The tradeoff is that a higher attenuation ratios result in smaller differential voltages seen by the comparator. This places a lot of burden on the comparator’s front-end in terms resolution, signal-to-noise ratio (SNR), DC gain, and bandwidth. Therefore, as long as the bus voltages remain within the allowable range for a given receiver design, common-mode voltages can be attenuated and adjusted, so that only the remaining differential voltage affects the output state of the comparator.
In summary, common-mode voltages occur frequently in industrial applications. By doing your due diligence and identifying what possible common-mode voltages may exist in your system, you can then choose an appropriate transceiver that is capable of handling the identified range. This allows you to alleviate potential communication failures caused by common-mode voltages before they even occur.
