34410A/11A User’s Guide 117
Measurement Tutorial 4
Measurements Below Full Scale
You can make the most accurate ac measurements when the multimeter is
at or near the full scale of the selected range. Autoranging occurs at 10%
(down–range) and 120% (up–range) of full scale. This enables you to
measure some inputs at full scale on one range and 10% of full scale on
the next higher range. In general, the accuracy will be better on the lower
range; for the highest accuracy, select the lowest manual range possible
for the measurement.
High–Voltage Self–Heating Errors
If you apply more than 300 V
rms
, self–heating will occur in the
multimeter's internal signal–conditioning components. These errors are
included in the multimeter's specifications.
Temperature changes inside the multimeter due to self–heating may cause
additional error on other ac voltage ranges. The additional error will be
less than 0.02% and will dissipate in a few minutes.
AC Current Measurement Errors (Burden Voltage)
Burden voltage errors, which apply to dc current, also apply to ac current
measurements. However, the burden voltage for ac current is larger due to
the multimeter's series inductance and your measurement connections. The
burden voltage increases as the input frequency increases. Some circuits
may oscillate when performing current measurements due to the
multimeter's series inductance and your measurement connections.
Low–Level Measurement Errors
When measuring ac voltages less than 100 mV, be aware that these
measurements are especially susceptible to errors introduced by
extraneous noise sources. An exposed test lead will act as an antenna and
a properly functioning multimeter will measure the signals received. The
entire measurement path, including the power line, act as a loop antenna.
Circulating currents in the loop will create error voltages across any
impedances in series with the multimeter's input. For this reason, you
should apply low–level ac voltages to the multimeter through shielded
cables. You should connect the shield to the input LO terminal.
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