Appendix D Optimizing Throughout and Reading Rate 327
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 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.
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.
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