Filter; Filter Characteristics - Keithley 2750 User Manual

4-14 Range, Digits, Rate, Bandwidth, and Filter

Filter

The digital filter is used to stabilize noisy measurements. The displayed, stored, or trans-
mitted reading is a windowed-average of a number of reading conversions (from 1 to 100).
The filter setup is "remembered" and can be unique for each measurement function (DCV,
ACV, DCI, ACI, Ω2, Ω4, and TEMP). When you select a function, the instrument will
return to the last filter setup for that function.
NOTE

Filter characteristics

In general, the digital filter places a specified number of A/D conversions (Filter Count)
into a memory stack. These A/D conversions must occur consecutively within a selected
reading window (Filter Window). The readings in the stack are then averaged to yield a
single filtered reading. The stack can be filled in two ways (Filter Type): moving or repeat-
ing. The moving filter keeps adding and removing a single A/D conversion from the stack
before taking the average, while the repeating filter only averages a stack that is filled with
new A/D conversions.
Details on digital filter characteristics are provided as follows:
Filter count — The filter count specifies how many consecutive A/D conversions (within
the filter window) to place in the memory stack. When the stack is full, the A/D conver-
sions are averaged to calculate the final filtered reading. The filter count can be set from 1
to 100. Note that with a filter count of 1, no averaging is done. However, only readings
within the filter window will be displayed, stored or transmitted.
NOTE
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 programming, the window can be set to any value from
0.01% to 10% or NONE.
The various instrument operations, including Filter, are performed on the input
signal in a sequential manner. See
details. It includes flowcharts showing where in the processing sequence that fil-
tering is performed.
While the filter processes readings, the FILT annunciator blinks. Readings that
are being displayed while the FILT annunciator blinks are not final filtered
readings. When the FILT annunciator stops blinking, the filter has settled.
Model 2750 Multimeter/Switch System User's Manual
"Signal processing sequence," page D-2, for