4-14
6517B-900-01 Rev. A / Jun 2008
Section 4: Measurement Options
Model 6517B Electrometer User’s Manual
Filter types
The Model 6517B has two types of digital filters: averaging and advanced. Both types are a simple
average of 1 to 100 reading conversions. The difference is a user-programmed “noise window” for
the advanced filter. The noise window (expressed as a percentage of range) allows a faster
response time to large signal step changes (see
). See Section 9 of the Reference
Manual for complete information on filter types.
Figure 4-6
Filter response/noise window
Filter modes
There are two filter modes: moving or repeating. The moving 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, the new conversion is placed into the stack, the oldest conversion is
discarded, and a new reading is averaged.
For the repeating filter, the stack is filled and the conversions are averaged to yield a reading. The
stack is then cleared and the process starts over. See Section 9 of the Reference Manual for
complete information on filter modes.
Median filter
Median filtering uses the middlemost reading from a group of sample readings arranged in
ascending order. For example, assume the following readings:
20V, 1V, 3V
They are then arranged in ascending order as follows:
1V, 3V, 20V
The median (middlemost) reading from the above sample group is 3V.
The number of sample readings is determined by the specified rank as follows:
Sample readings = (2
×
R) + 1
Where: R is the selected rank (1 to 5)
For example, a rank of 4 will use the last nine readings to determine the median;
(2
×
4) + 1 = 9.
Each new reading replaces the oldest reading, and the median is then determined from the
updated sample group of readings.
Value displayed
with filtering
Response with the noise
window turned on shifts
when the input signal moves
beyond the specified range
Response with
no noise window
Limits of the
user-specified
noise window
Input signal
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