Fluke 8842a Making Accurate HIGH-RESISTANCE Measurements, Shielding for Low Voltage Measurements

Measurement Tutorial 4

MAKING ACCURATE HIGH-RESISTANCE MEASUREMENTS

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Figure 4-10. Shielding for Low Voltage Measurements

For low-level resistance measurements, connect the test lead shielding as shown in Figure 4-11. Use the 4-wire ohms function to minimize the error caused by the resistance of the test leads.

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Figure 4-11. Shielding for Low Resistance Measurements

Errors due to thermal voltages should also be considered when making low-level voltage or resistance measurements. Techniques for reducing thermal voltages are presented earlier in this section.

4-24. MAKING ACCURATE HIGH-RESISTANCE MEASUREMENTS

When high resistances are measured (typically 1 MΩ or greater), leakage resistance at the test circuit can provide enough shunt resistance to degrade the accuracy of the measurement (see Figure 4-12). To minimize leakage resistance, watch out for contamination, high humidity, poor-quality interconnections, and poor-quality insulation of the stand-offs on which test resistors are mounted.

High-resistance measurements are also susceptible to error from electrical noise pickup. For accurate measurements, use short test leads and enclose the test leads and test circuit in a proper shield that is connected to the 8842A’s INPUT LO terminal. Pickup of slowly fluctuating noise can also be reduced by using the OFFSET feature as described in paragraph 4-7.

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