DI-145 Data Acquisition Starter Kit Developer's Diary
2/4/2011 Note #13 Weekend Reading: Tracking the missing grounds
Those who have looked closely at the block diagram on page 3 of the DI-145 data sheet may have noticed something odd: Missing analog and digital ground connections on the DI-145’s terminal strip. If you thought that the block diagram omitted them for the sake of clarity or simplicity, you’d be incorrect. What gives?
Although both grounds exist inside the unit, they are not exposed for a very good reason: Common mode voltages (CMV). There’s that phrase again. What is it, and why does DATAQ Instruments keep harping about it? A CMV is simply defined as a difference in potential (read voltage) between the signal’s ground reference and the instrument’s. I know what you’re thinking: How can ground have a voltage? Isn’t ground zero volts? Isn’t it always safe by definition? The answer may surprise you, especially if you use instruments in an industrial setting with a lot of electrical machinery. In truth, ground is almost NEVER zero volts relative to another ground. There is almost ALWAYS a voltage difference between the two and many things, from the presence of electrical machinery to roundabout electrical wiring, can affect the magnitude of the difference. How big can the difference be? Here’s a real life example:
I called on a DI-720 customer once who was in the habit of blowing up the front end of their instrument. They had returned it for service at least three times in three months, and the resolution was always the same: Replace the front end multiplexers, wash rinse, repeat. I asked them to show me how they use the 720, and the setup was pretty simple. They had a bank of amplifiers in a rack-mounted system. On an adjacent desk were a PC and the DI-720 that was powered and connected to the PC, but not yet connected to any signals. I asked for a voltmeter and proceeded to make a measurement that surprised me almost as much as the customer. On the ACV range, with one lead connected to the ground of the signal source and the other connected to the DI-720’s chassis (which is the same as the 720’s signal ground since it isn’t an isolated device) I measured an incredible 65 VAC. This is the most extreme example of an unintentional CMV that I had ever seen, or since seen. We determined that the amplifier chassis was plugged into an outlet at one side of the room, and the PC and 720 to one at a different location. This was an old facility that had seen several remodels and the two outlets were obviously taking entirely different paths back to the breaker box, perhaps even to two different boxes. The result was that there was absolutely no relationship between their respective grounds.
Circling back to the DI-145, if we exposed the analog common you’d be tempted to do what our DI-720 customer had done…connect it to signal ground. Upon making that connection in the presence of a CMV, current immediately flows through the two grounds. If the voltage and current are large enough you put the instrument and even the PC it’s connected to at risk. Earlier starter kits ‘solved’ this problem by placing a 10-Ohm resistor between signal ground and the instrument’s real ground, which is the same and the PC’s. In the presence of high enough CMVs and currents, the resistor opens, essentially sacrificing itself to save the remainder of the instrument and especially the PC’s USB port. Of course after the event the instrument’s ground is simply floating and not connected to anything, which renders it useless as a measurement tool, but replacing the resistor restores normal operation until the next event. We even had a customer complain once that our design was faulty because the resistor kept blowing! Since the DI-145 doesn’t expose analog and digital grounds the prospect of a CMV causing a failure is completely removed, especially since the instrument’s differential inputs are designed to handle well in excess of line voltage.
DI-145 Product Page: http://www.dataq.com/products/di-145/index.html.