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Figure 45. Settling time characteristic for resistance measurements assuming <200pF shunt capacitance in the circuit tested. For small values of resistance, there is no real advantage to setting the delay to less than the default values. Resistance above 100 kW require longer settling times to reach final values: hence settling delay times for these values may save measurement time at the expense of measurement accuracy.
Another feature of the 3458A is OffsetCompensated Ohms. Very much like auto zero in concept, offset-compensated Ohms makes a measurement of the input resistance without the current applied to measure any thermally generated DCV offsets. As shown in Figure 46, the current is applied, the offset voltage is subtracted from the measurement of the unknown resistance and the result is presented to the display. Like auto zero, it takes two measurements to make a final determination of the unknown resistance. In reality, offsets like this are only encountered in lower values of resistance. The 3458A offers a 10 mA current source that will, at least, mask the effect of the thermally generated offset. Hence, in many cases Offset-Compensated Ohms may not be needed for lower resistance measurements.
Figure 46. Offset compensated ohms removes the effect of small series voltage sources such as thermocouple effects in the circuit. By measuring the voltage across the unknown resistance, Ve, with the current source off and then measuring the voltage across the unknown resistance with the current source on, the effect of Ve on the measurement is eliminated.
