Keithley 2750, User Manual

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3-12 Basic DMM Operation Model 2750 Multimeter/Switch System User’s Manual
Crest factor
AC voltage (and current) accuracies are affected by the crest factor of the waveform, the
ratio of the peak value to the RMS value.
Table 3-2 lists the fundamental frequencies at
which the corresponding crest factor must be taken into account for accurate calculations.
Low level considerations
For sensitive measurements, external considerations beyond the Model 2750 affect the
accuracy. Effects not noticeable when working with higher voltages are significant in
microvolt signals. The Model 2750 reads only the signal received at its input; therefore, it
is important that this signal be properly transmitted from the source. The following para-
graphs indicate factors that affect accuracy, including stray signal pick-up and thermal
offsets.
Shielding
AC voltages that are extremely large compared with the DC signal to be measured may
produce an erroneous output. Therefore, to minimize AC interference, the circuit should
be shielded with the shield connected to the Model 2750 input low (particularly for low
level sources). Improper shielding can cause the Model 2750 to behave in one or more of
the following ways:
• Unexpected offset voltages.
• Inconsistent readings between ranges.
• Sudden shifts in reading.
To minimize pick-up, keep the voltage source and the Model 2750 away from strong AC
magnetic sources. The voltage induced due to magnetic flux is proportional to the area of
the loop formed by the input leads. Therefore, minimize the loop area of the input leads
and connect each signal at only one point.
Table 3-2
Crest factor limitations
Crest factor Fundamental frequency
2
3
4-5
50kHz
3kHz
1kHz
2750-900-01.book Page 12 Wednesday, August 3, 2011 7:56 AM