Filters; Digital Filter Types; Digital Filter Modes - Keithley 2001 Operator's Manual

Front Panel Operation
3.9

Filters

Filtering stabilizes noisy measurements. The Model 2001
uses a digital filter and an analog filter.
The digital filter is based on reading conversions. The
displayed, stored or transmitted reading is simply an average
of a number of reading conversions. When the digital filter is
enabled, the selected digital filter configuration for that
measurement function is in effect. Digital filtering is
performed only on primary display measurements; it has no
effect on multiple displays. Paragraphs
explain how to configure and control the digital filter.
The analog filter is simply an RC network for the DCV func-
tion that filters out high frequency noise (>10kHz) seen at
the input of the instrument. Detailed information on using
the analog filter is contained in
3.9.1

Digital filter types

The Model 2001 has two types of digital filters: averaging
and advanced. Both types are a simple average of one to 100
reading conversions. The difference between them is the
user-programmable noise "window" of the advanced filter.
The noise window, which is expressed as a percentage of
range (0-100%), allows a faster response time to large signal
step changes (e.g., 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 percentage of range,
the reading is based on an average of reading conversions. In
this case, the advanced filter works the same as the averaging
filter. If the noise does exceed the selected percentage, the
reading is a single reading conversion, and new averaging
starts from this point. The two filter types are compared in
Figure 3-40, part A.
3.9.2

Digital filter modes

An additional filter parameter is the mode, either moving or
repeating. A moving filter is a first-in, first-out stack, where
the newest reading conversion replaces the oldest. An
average of the stacked reading conversions yields a reading.
Therefore, after a selected number of conversions, a moving
filter gives a new reading for every new conversion.
A repeating filter takes a selected number of reading
conversions, averages them, and yields a reading. It then
3-86
3.9.1 through 3.9.6
paragraph 3.9.7.
flushes its stack and starts over. This characteristic is useful
when scanning channels.
If burst mode is enabled with filtering, the post-processing
time increases. A filter mode setting of repeating is ignored
in burst mode.
Filter modes are compared in Figures 3-40B and 3-40C.
2001-900-01 Rev. K/ August 2010