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Aperture and
Integration Time
Chapter 4
The aperture time or integration time is the time which the multimeter
samples the input signal. Aperture time is expressed in seconds and
integration time is expressed in power line cycles. Integrating multimeters,
like the E1326B, may be programmed to integrate an integer number of
power line cycles (PLC). These have a common mode rejection ratio. The
common mode rejection ratio is increased by the normal mode rejection
ratio. This is known as the effective common mode and is shown as follows:
[AC common mode rejection ratio] + [Normal mode rejection ratio]
The effective common mode rejection is only for power line frequencies.
However, this is the most common noise that needs to be rejected.
Therefore, the effective common mode rejection for DC and resistance
measurements is as follows:
110 db + 60 db = 170 db
when the voltmeter aperture is set for one power line cycle (PLC).
When setting an aperture or integration time, consider the following:
1. Normal mode rejection of 50 Hz or 60 Hz noise is only achieved with
aperture times 16.7 ms (60 Hz) ( 20 ms (50 Hz)), or with
integration times 1 power line cycle.
2. The longer the aperture or integration time, the greater the normal
mode noise rejection, but the lower the reading rate.
3. Setting the aperture time or integration time sets the other and the
resolution. Of these three parameters, the settings of the other two
are based on the one most recently set.
Understanding the HP E1326B/E1411B Multimeter
97
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