When high-resistance measurements are too low
Dale Cigoy, Applications Engineer, Keithley Instruments, Cleveland, OH -- Test & Measurement World, 4/1/2005 2:00:00 AM
When you make high-resistance measurements, 1 GΩ and up, your instrument setup itself can introduce problems. Here are some steps you can take to prevent them:
• Low-insulation resistance. The text fixture's insulation resistance could be in parallel with the device under test.
| For a description of guarding, see: "How does that work?: Femtoamp (fA) measurements" Test & Measurement World, May 2002. |
Remedy: Use a test fixture and connecting cables with higher insulation resistance. You can also use a driven guard to effectively increase the shunt resistance.
• Low input resistance in the meter. The input resistance of the voltmeter (Zin) may be too low for the measurement (see figure).
|
| A meter’s input impedance (Zin) can influence high-resistance measurements. |
Remedy: Use the force-voltage-and-measure-current technique to measure the high resistance. Put the unknown resistance in series with a picoammeter or electrometer ammeter, and apply a constant voltage. This voltage can come from the picoammeter's internal source function or from an external source. Because an ammeter's low input resistance produces a voltage drop of less than 200 µV, essentially all the voltage drop appears across the unknown resistance. The ammeter measures the resulting current and calculates the DUT's resistance.
• Offset current. Offset currents may be caused by charge stored in the material (dielectric absorption), static or triboelectric charge, or piezoelectric effects, and can be equal to or greater than the current stimulated by the applied voltage.
Remedies: You have two options here. In the first, you adjust the meter's zero baseline to compensate for offset current. Meters often have some offset in the front-end circuitry. After power-up, allow the meter to reach thermal equilibrium, then cancel the offset by selecting the correct reading range and using the zero function of the meter.
The second option is to use the alternate polarity technique. Make the measurement once and record the result. Then, reverse the applied voltage's polarity and measure again. Taking the average of the readings should give an accurate result. You can repeat the polarity reversal process any number of times and base your final measurement on a weighted average of the most recent measurements.
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