Series 3700A System Switch/Multimeter Reference Manual
Section 8: Theory of operation
3700AS-901-01 Rev. D/June 2018
8-7
The Series 3700A is an AC-coupled RMS meter. For an AC waveform with DC content, the DC
component is removed before the RMS is calculated. This affects the crest factor because the peak
value for the DC-coupled waveform is different than the peak value for the AC-coupled waveform. In
an AC-coupled waveform, the peak value is measured from the original DC average value, not DC
zero. For example, if a voltage pulse is measured on the AC function of the Series 3700A with a peak
voltage of VP and a low voltage of zero volts, the AC-coupled peak value will be calculated as
follows:
ACPEAK = VP • (1 - duty cycle)
Therefore, the AC-coupled crest factor will differ from the DC-coupled waveform. The RMS function
will calculate the RMS value based on the pulsed waveform with an average value of zero.
The reason to consider crest factor in accuracy of RMS measurements is because the meter has a
limited bandwidth. Theoretically, a sine wave can be measured with a finite bandwidth because all of
its energy is contained in a single frequency. Most other common waveforms have a number of
spectral components requiring an almost infinite bandwidth above the fundamental frequency to
measure the signal exactly. Because the amount of energy contained in the harmonics becomes
smaller with increasing frequency, very accurate measurements can be made with a limited
bandwidth meter, as long as enough spectral components are captured to produce an acceptable
error.
Crest factor is a relative measurement of the harmonic content of a particular waveform and reflects
the accuracy of the measurement. For a rectangular pulse train, the higher the crest factor, the higher
the harmonic content of the waveform. This is not always true when making spectral comparisons
between different types of waveforms. A sine wave, for example, has a crest factor of 1.414, and a
square wave has a crest factor of 1. The sine wave has a single spectral component and the square
wave has components at all odd harmonics of the fundamental.
The Series 3700A RMS AC volts and AC amps accuracies are specified for sine waves of different
frequency ranges.
Additional error uncertainties are also specified for non-sinusoidal waveforms of specific crest factors
and frequencies. The Series 3700A has capabilities of measuring AC waveforms of crest factors up
to 5.
DMM resistance measurement methods
The method that the Series 3700A uses to measure resistance depends on the resistance range. For
resistance ranges from 1
Ω to 1 MΩ
the Series 3700A uses the constant-current method to measure
resistance. For resistance ranges from 10
MΩ to 100 MΩ, the ratiometric method is used.
When the constant-current method is used, the Series 3700A sources a constant current (I) to the
device under test and measures the voltage (V). Resistance (R) is then calculated and displayed
using the known current and measured voltage (R = V/I).
When the ratiometric method is used, test current is generated by a 6.4 V reference through a 10
MΩ
reference resistance (R
REF
).
For more detail on these methods, see
Constant-current source method
(on page 8-8) and
(on page 8-9).
The Series 3700A uses four methods to detect open leads. For detail, see
