Agilent Technologies Infiniium 8000A, Programmer'S Reference Manual

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Histogram Commands
18-3
The database continues to build until the oscilloscope stops acquiring
data or all both features (color grade persistence and histograms) are
turned off. You can clear the database by turning off all three features
that use the database.
The database does not differentiate waveforms from different
channels or functions. If three channels are on and the waveform from
each channel happens to light the same pixel at the same time, the
counter is incremented by three. However, it is not possible to tell
how many hits came from each waveform. To separate waveforms,
you can position the waveforms vertically with the channel offset. By
separating the waveforms, you can avoid overlapping data in the
database caused by multiple waveforms. Even if the display is set to
show only the most recent acquisition, the database keeps track of all
pixel hits while the database is building.
Remember that color grade persistence, mask testing, and histograms
all use the same database. Suppose that the database is building
because color grade persistence is ON; when mask testing or
histograms are turned on, they can use the information already
established in the database as though they had been turned on the
entire time.
To avoid erroneous data, clear the display after you change
oscilloscope setup conditions or DUT conditions and acquire new data
before extracting measurement results.