Digital Filter Window; Figure 4-3: Repeating Average Filter - Keithley 3700 series Reference Manual

Section 4: Range, Digits, Rate, Bandwidth, and Filter

Figure 4-3: Repeating average filter

Digital filter window

The digital filter uses a ―noise‖ 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 starts processing again,
starting with a new initial conversion value from the A/D converter.
The noise window, which is expressed as a percentage of range (or maximum temperature
reading), allows a faster response time to large signal step changes (for example, scanned
readings). A reading conversion outside the plus or minus noise window fills the filter stack
immediately.
If the noise does not exceed the selected window, the reading is based on the average of the
reading conversions. If the noise does exceed the selected window, the reading is a single
reading conversion and new averaging starts from this point. The noise window for the two filter
types are compared in the following filter window.
For both front panel and remote programming, the window can be set to any value from 0.0% to
10%, where 0.0% represents no window being applied.
For voltage, current, and resistance, 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. For temperatures below 0°C, the overflow point is -200ºC, so a 10%
filter window is ±20ºC. If using ºF units, a 20% filter window is calculated as follows: 9/5 x 20 =
36. The filter window for the 20% window is ±36ºC.
4-8
Series 3700 System Switch/Multimeter Reference Manual
Document Number: 3700S-901-01 Rev. A / August 2007