Common Mode Voltages and Non-isolated Instruments
You’ve arrived at part six of this series because you have noise on one or more acquired analog channels. Using procedures described in Part 1 of the series you’ve determined that the noise is a significant percentage of the instrument’s full scale range and is not attributable to the instrument. Further, you have determined that your instrument is not isolated, or at least does not provide both input-to-output and channel-to-channel isolation. Given this last fact, it is almost certain that your noise is caused by a common mode voltage.
Whenever the ground reference of a non-isolated instrument is connected to the ground reference of a non-isolated signal source and the grounds are at different potentials, current will flow through the grounds and manifests itself as noise in the acquired signal. Some points to ponder:
- Common mode voltage can be defined as the potential difference between two grounds.
- The ground reference of the signal and the instrument are almost never at the same potential in industrial settings.
- The magnitude of noise will be directly proportional to the magnitude of the common mode voltage.
- The sum of common mode voltage plus signal amplitudes can exceed the maximum that your instrument will tolerate and cause permanent damage.
- If the signal source is isolated, then the instrument does not need to be and vice versa.
It should be clear from the above that the elimination of common mode voltages leads directly to the elimination of noise. How you do that will be different in almost every situation, but realizing that the term “ground” is strictly relative, and that any given set of grounds are almost always at different potentials is a great starting point. Follow these tips to eliminate noise-inducing common mode voltages:
- Examine the power supplies of your signal source and instrument. Are they plugged into the same outlet or terminal strip? If not then try that as a first remedy.
- Have you connected your instrument to the signal source using shielded cable? Is the shield connected to ground on both ends? If so, then remove the connection on one end to prevent current from flowing through your shield, destroying its effectiveness.
- If you’re using thermocouples, are the junctions in direct contact with a grounded, metallic object? If so, then consider using an insulating material between the junction and the metallic object that removes the CMV, and yet provides good thermal conductivity.
- Are you measuring across a shunt resistor, like a 250Ω resistor in a 4-20 mA process current application? If so, then ensure that the resistor is in stalled in the low side of the loop. Placing the resistor in the high side of the loop exposes your instrument to a common mode voltage equal to the loop power supply voltage.
- Even if you’ve placed the shunt resistor in the low side of the 4-20 mA current loop, make sure that the loop power supply’s ground is floating (isolated) from input power common. If it isn’t, common mode voltages that most likely exist between loop and instrument grounds will cause trouble.
If you’ve examined your field wiring for all of the above and still cannot solve the problem, then you may have to toss in the towel and move to an isolated instrument that is largely immune to the problems caused by a CMV. Depending upon your application, you may be surprised at how little isolation costs, how much time it saves you, and much your measurement improves.