Range, Digits, Rate, and Filter 3-9
Filter window — The digital filter uses a window to control filter threshold. As long as the
input signal remains within the selected window, A/D conversions continue to be placed in the
stack. If the signal changes to a value outside the window, the filter resets, and the filter starts
processing again starting with a new initial conversion value from the A/D converter.
The five window selections from the front panel are 0.01%, 0.1%, 1%, 10% of range, and
NONE (no window). For remote operation, the window can be set to any value from 0.01% to
10% or NONE.
For voltage, the filter window is expressed as a percent of range. For example, on the 10V
range, a 10% window means that the filter window is ±1V. For temperature, the filter window is
expressed as a percent of the maximum temperature reading. The maximum temperature
depends on which thermocouple is being used. For example, for a Type J thermocouple, the
maximum reading is 760°C; a 10% window means that the filter window is ±76°C.
Filter type — There are two digital filter types; moving and repeating. The moving average
filter uses a first-in, first-out stack. When the stack becomes full, the measurement conversions
are averaged, yielding a reading. For each subsequent conversion placed in the stack, the oldest
conversion is discarded, and the stack is re-averaged, yielding a new reading. This process is
depicted in Figure 3-2A.
For the repeating filter, the stack is filled and the conversions are averaged to yield a reading.
The stack is then cleared and process starts over (see Figure 3-2B). Choose this filter for
scanning so readings from other channels are not averaged with the present channel.
NOTES The repeating filter cannot be used with Delta measurements. If the repeating filter is
selected when Delta is enabled, the instrument will default to the moving filter. Delta
measurements are covered in Section 5.
The moving filter cannot be used when stepping or scanning. If the moving filter is
selected when a step or scan is enabled, the instrument will default to the repeating
filter. Stepping and scanning are covered in Section 9.
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