52
34405A User’s and Service Guide
3
Measurement Tutorial
Errors in High Resistance Measurements
When you are measuring large resistances, significant errors can occur
due to insulation resistance and surface cleanliness. You should take the
necessary precautions to maintain a "clean" high–resistance system. Test
leads and fixtures are susceptible to leakage due to moisture absorption in
insulating materials and "dirty" surface films. Nylon and PVC are relatively
poor insulators (10
9
W
) when compared to PTFE insulators
(10
13
W
). Leakage from nylon or PVC insulators can easily contribute a
0.1% error when measuring a 1 M
W
resistance in humid conditions.
AC Measurements
Each single ACV or ACI measurement is computed based upon a RMS
(root- mean- square) value calculated on an array of 25 sequential A/D
converter samples acquired with constant sample- to- sample timing.
Samples are acquired at a rate very close to the maximum trigger- settle
rate for the A/D converter as shown below.
When configured for an ACV or ACI measurement, the multimeter acquires
an array of 25 sequential samples which comprise the AC reading data
set. The final AC reading result is computed from the acquired data set
as shown by the equation below:
10 M
Ω
205 nA
420 nW
100 M
Ω
205 nA ||10 M
Ω
35 nW
AC Reading =
√
Average [Data (1:25)]
2
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