Common Mode Voltages and Isolated Instruments
In the premier edition of this series I talked about how you should initially approach noise elimination. The first step is to ensure that noise is a significant percentage of the full scale range of the analog channel, and then I explained a simple test to ensure that the instrument is not the noise source. If you’ve gotten this far I can assume that signal noise is significant, and the instrument is not defective.
Your next step is to look up the isolation specification of the instrument. Note that if you are under the impression that an instrument with differential analog inputs means that it is also isolated, you need to read this application note before proceeding. Differential and isolated analog inputs are not synonymous, and unless the specifications for the instrument state clearly that its inputs are isolated it most likely is not, regardless of its single-ended and/or differential configuration options. This edition of the Noise Elimination series assumes that you have an isolated instrument. I’ll deal with non-isolated instruments in later editions, since the subject becomes more complex in this situation.
How To Measure Common Mode Voltage Magnitude
So, you have read its specifications and determined that you have an isolated instrument. That’s great because noise-inducing common mode voltages (CMVs) can be largely, but not completely ignored. To absolutely verify that a common mode voltage is not a problem start by answering these questions:
- Is your field measurement one that virtually guarantees that a CMV is present? Examples are exposed thermocouple junctions that are attached directly to a conductor (powered or not), or the measurement of AC or DC current using a current shunt.
- If you answered YES to (1), then determine the magnitude of the CMV. With all field wiring disconnected except power and/or computer interfaces, use a battery-powered multimeter to measure the voltage from the field-side shunt or exposed conductive surface to the instrument’s chassis ground (not its low-side signal input.) Do this on the meter’s AC and DC ranges. Jot the value down and compare it with the isolation breakdown specification for the instrument.
If the measured value exceeds this spec then you have found the source of your noise, and you’re fortunate that the instrument still functions. Your two options are to lower the CMV if possible, or replace the instrument with one that provides a higher isolation spec.
In the next edition I’ll discuss CMV frequency and how this can destroy the best instrument isolation barriers.