Keithley 6487, Справочное руководство

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Model 6487 Reference Manual Measurements and Sourcing Voltage 3-21
Alternating voltage ohms mode
Overview
Ohms can be measured in one of two ways: DC (normal) or alternating voltage (A-V). The
alternating voltage ohms method is especially useful when the resistance or device being
measured exhibits high background currents or high noise currents. These are typical
problems seen when measuring high resistances, devices with moderate to high capaci-
tance, or when adequate shielding is unavailable. By measuring current differences caused
by a change in applied voltage, the alternating voltage method greatly reduces effects of
currents that are not caused by the applied voltage, i.e., not resistive current. The A-V
mode consists of switching the source level between 0V and a user-selected value
(
Figure 3-5 ). During each phase, one or several readings are collected into separate buffers
for that phase, designated V-High and V-Zero. A third buffer is created by subtracting the
n-th reading of the V-Zero buffer from its counterpart in the V-High buffer and storing
these differences in a buffer designated V-Delta. Both from the front panel and via remote,
A-V ohms readings always come from the V-Delta buffer.
The purpose of the alternating voltage ohms mode is to improve the accuracy and
repeatability of very high resistance measurements, which are subject to errors from
background currents in the test setup. By taking two current measurements, one at a
specific step voltage and a second at 0V, these background currents can be largely nulled
out and the resistance calculated from the source voltage and measured current is closer to
the actual DUT resistance. Data stored in the buffer can also be averaged to improve
repeatability.
Key test parameters for A-V ohms include the step voltage, measurement time, and the
number of test cycles. The optimum step voltage value depends on the measured resis-
tance and desired current. The measurement time must be carefully chosen to assure ade-
quate settling during both the step-voltage (V-High) and 0V (V-Zero) phases of the
measurement. The number of cycles to measure and average is often a compromise
between improvement in repeatability and the overall measurement time.
Figure 3-5
Alternating voltage ohms
V-High
V-Zero (0V)
One Cycle
Time Time