Stay Well-Grounded

Ask real-estate agents about key property attributes and you'll hear, “Location, location, location.” Ask engineers about key needs of a measurement system, and they should say, “Ground, ground, ground.” Often, however, a good ground proves elusive. Here are several tips and references that can help set up a ground system that works.

An earth ground and a current-return path are different. An earth ground provides a path for current to flow to, well, ground. So, a short circuit between a voltage source and a grounded metal frame means someone who touches the frame doesn't get a nasty shock. Ordinarily, no current flows in a true earth ground. A return, unfortunately also called a ground, carries current and provides a return path to a power source. If possible, separate return and earth-ground paths.

Often, equipment manufacturers treat return and earth ground as the same and use this all-in-one “ground” to establish a 0-V reference. But, ground connections have impedances that can cause small voltage differences between “ground” connections in an instrumentation setup. These voltages can range from 10s to 100s of millivolts and they cause current to flow in “ground loops.” These potential differences often superimpose an interfering signal on the signals you plan to measure. To reduce or eliminate ground loops, configure equipment so one central point serves as the ground (0-V) reference.

If you suspect a ground loop influences your measurements, draw a schematic that shows all ground connections and their relative positions. You may find loops you can break. If you cannot break loops or consolidate ground connections, use signal conditioners and isolated power supplies. Sensors such as thermocouples and variable resistors “float” and operate without an earth-ground connection. Floating power supplies can produce excitation voltages for some sensors.

Single-ended multiplexers seem particularly prone to the effects of ground loops when one side of each signal connects to an earth ground. Look for single-ended data-acquisition systems that provide a floating ground-reference input that connects only to an internal amplifier's reference, or minus, input. This technique reduces the effects of ground loops.

If you must distribute dc power in a measurement system, use point-to-point wiring to connect power and ground to sensors, amplifiers, buffers and other devices. (See Figure.) Central connections for power and ground provide separate paths for each device. And a supply's sense inputs provide feedback from a central point, not from a location some distance down a distribution path.

“Serial” distribution of power means equipment along the lines can experience fluctuations on the power and ground lines.

	Point-to-point wiring for power and return currents (top) reduces problems caused by current changes as loads and signal amplitudes change. The serial power connections (bottom) let current chagnes at the buffer affect power to the differential amp and the ground reference on cable shields.