Advanced Measurement Technology 273A, Руководство пользователя

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Chapter 5—Operating Instructions 45
Fig. 7. Current Interrupt Waveform at
Electrometer Monitor Connector.
Note: The default values give good results in many applications. Do not change these values
using the applicable functions unless you are well grounded in current-interrupt IR
compensation theory. This is particularly true of functions 17, 18, and 19. A prerequisite
to evaluating these parameter selections is that an oscilloscope be connected to the
rear-panel electrometer monitor connector to observe the current-interrupt waveform, as
discussed in the following paragraphs. Even using the oscilloscope it is not easy to make
optimum determinations. The position of the selected points, as well as the path of the
projection through the points, must be estimated.
Figure 7 illustrates the rear-panel ELECTROMETER MONITOR waveform for a typical Current
interrupt cycle. Referring to the figure, note that a fast potential drop equal to iR , occurs at the
U
moment of interrupt. The purpose of the routine is to measure this drop so that R can be
U
calculated and a correction factor developed.
After interrupt, thirty-two samples are taken at 5 µs intervals. Two of these are selected
(Functions 18 and 19) and extrapolated back to the moment of interrupt. The goal is to select two
points that will provide a projection that exactly intersects the lower end of the iR drop. If the
U
intersection occurs at a higher point, the correction factor will be too small. If it occurs at a lower
point, as is the case for the example depicted in the figure, the correction factor will be too large.
Fortunately, Function 17, by allowing user control of the percentage of correction, eases the
point selection task. For example, if the extrapolated line were to intersect exactly halfway down
the drop, the correction factor would be 50% of the required value. The resulting correction factor
could be easily adjusted to its proper value by using Function 17 to set the % Correction to
200%. Similarly, if the intersection occurred below the drop, as in the depicted example,
Function 17 would be used to set a % Correction of less than 100% (90% would be about right
for the example). Once the parameters are set via the required functions and Current Interrupt IR
Compensation established, no other operator intervention is required during the measurement.
Current Interrupt IR Compensation has several advantages with respect to Positive Feedback IR
Compensation:
1. This technique corrects for essentially the entire potential error caused by the
uncompensated resistance.