By Robert M. Brown, QuadTech, Maynard, MA Buying a more accurate instrument to measure resistance and impedance won't always improve the accuracy of your measurements. Your problems could be caused by measurement errors that originate in your test fixture. Cables, relays, and contacts all contribute to measurement errors. Fortunately, you can minimize those errors by applying simple techniques. You can guard your DUT to minimize stray resistances and common-mode signals if your LCR meter, milliohmmeter, or megohmmeter has a guard terminal or connection.
Guards take the form of PCB traces, ground planes, and rings.
When properly connected to an instrument's guard terminal, the guard shunts common mode currents away from the measuring circuit. Guards are popular in high-resistance measurements made with a megohmmeter. A metal guard plane can effectively negate alternate resistance paths in parallel with the resistance you're measuring. A guard ring eliminates parallel resistance paths that can cause errors in materials resistance measurements. Guard rings also improve resistivity measurements of materials and are common in measurements made by sensor manufacturers.