Minimize Measurement Errors in Component Test Fixtures

By carefully selecting - and maintaining - fixture components, you

Robert M. Brown, QuadTech, Maynard, MA -- Test & Measurement World, 11/1/2000

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. When designing a test fixture, you must minimize stray capacitance, extraneous resistance, and unwanted EMI. 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. A floating measurement instrument is described as “guarded” when it has an additional conductive guard between its low terminal and ground. 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.

Figure 1. 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. When measuring resistivity, you measure resistance from top to bottom through the sample material. Place one electrode on top of the material and place the other electrode on the bottom. With two electrodes, a meter measures the resistance as a parallel combination of the resistance through the sample and the surface resistance across the top surface, down the side, and back across the bottom surface. The parallel combination produces a low resistivity measurement. You can effectively eliminate the surface resistance path by placing a guard ring (Fig. 1 ) around one of the electrodes.

You can also shield your measuring circuit against noise from unwanted EMI by constructing a metal enclosure around the measuring instrument and component under test. I recommend using a metal fixture with a hood to completely enclose the DUT. Connect the enclosure to earth ground.

Pay Attention to Cables

Shielded cables can also reduce noise from unwanted EMI. Remember that braided shields in coaxial cables don’t provide 100% coverage against EMI. The higher the percentage of braid coverage, the more effective the shield. Connect cable shields to ground, but connect each cable shield to ground at one point only. Otherwise, you could introduce ground loops, which would add noise to your system.

Cable length also affects measurements. If your meter manufacturer requires a specific length of cable, then use cables of that length only. If your meter manufacturer doesn’t specify cable length, then keep cables as short as possible.

Keep your cables organized to save significant time in troubleshooting. Separate and, if necessary, shield cables that carry different signal types. Running all cables from the fixture to the instrument in one piece of conduit can introduce signal errors. Large analog signals can easily interfere with digital control lines, and digital lines can interfere with low-level analog signals.

When cables move, triboelectric noise develops that can produce erratic measurements on high impedance DUTs. You can minimize triboelectric noise by using cable ties to keep the cables stationary.

Use the Right Components

Proper use of guards, shields, and cables can reduce measurement errors, but you can improve your measurements even further. Relays are still a mainstay in switching analog signals, both low-level and high-level voltages and currents. When choosing a relay, you must check the manufacturers’ specs for contact resistance, insulation resistance (especially over a humidity range), and thermal EMF.

Contact resistance in relays, pogo pins, and other contacts will affect your measurements, especially when you measure low-impedance components. Specify components with the lowest possible contact resistance compared to the impedance measurements you’re taking. Mercury wetted reed relays, which have low contact resistance, work well in low-impedance, low-signal-level applications.

Unfortunately, as relays, pins, and contacts age from use, your fixture’s contact resistance will change. Use four-terminal Kelvin connections between a DUT and a meter. Regularly replace your fixture’s components to minimize contact resistance.

A relay’s insulation resistance affects high-impedance measurements. Most relays have an insulation resistance of 1 GV or slightly higher, but insulation-resistance measurements using a megohmmeter often exceed 100 GV. That makes relays extremely difficult to use in such applications because the meter’s excitation signal will leak through the relay, causing a low measurement value. Fortunately, you can compensate for the relay’s low insulation resistance with your meter as long as the insulation resistance doesn’t change over time.

Most LCR meters have an “open compensation” or offset adjustment that compensates for insulation resistance. To perform the adjustment, first measure open circuit resistance and capacitance. Then, use the measured resistance and stray capacitance to correct future measurements. Remember, though, that the meter bases future measurements upon a previous “compensation” measurement. If the resistance or capacitance in the fixture changes, then an error will occur even in the compensated measurement. If the relays’ insulation resistance changes because of a change in humidity, your compensation will no longer work. You can also use the offset adjustment to compensate for contact resistance.

If your fixture will operate on a production line where humidity levels could exceed 85% RH, you should select relays rated for high humidity. Otherwise, you’ll get measurements that change by the hour. High humidity levels also will reduce the overall life expectancy of the relay. A combination of an electrical arcing and humid conditions can produce nitric acid in environmentally sealed relays. The acid can corrode the contacts, resulting in a relay that fails too early in its life.

Where dissimilar metals meet and a temperature gradient exists, you’ll get unwanted thermal EMF voltages in your system. The voltage level depends upon the metals and temperature of their junctions. When measuring low-level voltages with a milliohmmeter or an LCR meter, you must compensate for these thermal EMFs with your meter. You also can minimize the errors by using low-thermal-EMF relays.

Because a relay’s characteristics change with age, temperature, and amount of use, you’ll have to replace them regularly to maintain consistent measurements. Therefore, select relays that you can get easily, and always keep spares in stock. T&MW

FOR FURTHER READING

A Guide to LCR Measurements (P/N 035010), QuadTech, Maynard, MA, June 2000. www.quadtech.com/resources/appnoteindex.html.

Bennett, Emeric, “Applications Dictate Capacitance Measurements,” Test & Measurement World, September 2000. p. 25.   

LCR Measurement Primer, QuadTech. www.quadtech.com/primer.

Strassberg, Dan, “Using Relays to Switch Analog Signals Is Neither Silly Nor Trivial,” EDN, July 20, 1992. p. 118.

Van Der Burgt, Marty, Solving Signal Problems. Belden, Richmond, IN. www.belden.com/products/tpbroad.htm.

Robert M. Brown is an applications engineer with QuadTech. He has more than 13 years of experience in instrumentation and holds a B.S.E.E. from University of Maine at Orono. E-mail: rbrown@quadtech.com.