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The 100 MΩ range allows for measurements up to 1.05 GΩ (NI 4080/4082, NI 4070/4072, and NI 4065) or 5 GΩ (NI 4081 and NI 4071), because of the methodology of the measurement and the high–impedance design of the device. This range allows 2-wire operation only. Offset Compensated Ohms cannot be enabled.
It is difficult to get a meaningful measurement with resistances above 10 MΩ without good shielding—including shielding of the resistor under test. Enclose the resistor under test in a metallic box, and connect this enclosure to the LO terminal. In the laboratory, even a small box covered with a single, continuous sheet of aluminum foil helps significantly with this kind of measurement.
The following diagram shows an equivalent circuit for all measurements above the 10 MΩ range.
The voltage across the resistor is measured by the DMM and the resistor is calculated from:
Rx = V /(Is—(V/10 MΩ)
Rx = the resistor under test (up to 1.05 GΩ on the NI 4065 and NI 4070/4072, or up to 5 GΩ on the NI 4071)
V = the voltage measured by the DMM with Rx applied
Is = the source current (nominally 0.95 uA)
In the 100 MΩ range, any environmental noise or interference is amplified as the resistance being measured is increased. The requirement for good shielding increases as the resistor value increases.