How Sample Rate/Interval is Determined
The DMM always attempts to make the sample interval greater than the expected time required to take the
measurements, whether in continuous, data log, or digitize mode. A number of settings go into the cal-
culation of the minimum allowable sample interval. These settings include the measurement function,
NPLC, aperture, autorange, autozero, offset compensation, AC filter, TC open check and gate time. For
example, when autorange is on, the assumption is that no more than one range change will be required. If
more than one level of change occurs, the measurement may be delayed and an error will be issued.
When you change any of these settings, the sample interval (sample timer in continuous mode, sample
interval in data log or digitize mode) is increased to be greater than the calculated measurement time. In
the continuous and data log modes, attempting to reduce the sample interval below the calculated value
results in an error message. You must then choose among the various ways you can achieve a smaller
measurement time to achieve the smaller sample interval, such as decreasing the NPLC setting. For digit-
izing, the emphasis is on high sample rates and all settings except aperture/NPLC are fixed to achieve min-
imum measurement time. NPLC and aperture are automatically set to their minimum values upon entering
digitize mode. If you set NPLC or aperture to greater values, the sample interval will be increased (or the
sample rate decreased) to accommodate the longer measurement. However, in digitize mode, unlike con-
tinuous and data log modes, if you subsequently decrease the sample interval (or increase the sample
rate), the NPLC or aperture will be reduced as needed.
Another way to put it is that for continuous and data log modes, sample timer/interval is always con-
trolled by the other measurement settings. In digitize mode, the sample timer/interval, NPLC, and aper-
ture have equal control; changing any one setting can change the other settings.
Keysight True
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Series Operating and Service Guide
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Measurement Tutorial
