Keithley 6482, Справочное руководство

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6482-901-01 Rev. A / August 2012 Return to Section Topics 4-7
Model 6482 Dual-Channel Picoammeter / Voltage Source Reference Manual Section 4: Range, Digits, Speed, and Filters
Median filter
The median filter is used to pass the median, or “middle-most,” reading from a group of readings
that are arranged according to size. For example, assume the following readings:
2 mA, 1 nA, 3 nA
The readings are placed in a stack, rearranged in ascending order as follows:
1 nA, 3 nA, 2 mA
From the above readings, it can be seen that 3 nA is the median reading. Therefore, the 3 nA
reading is allowed to pass and the other two readings are discarded (filtered out). The median filter
provides a good method to reject noise.
The number of reading samples (stack size) for the median filter is determined by the selected
rank (0 to 5), as follows:
Sample readings = 2n + 1
Where: n is the selected rank (0 to 5)
From the above equation, it can be seen that the minimum number of sample readings is 1 (n = 0)
and the maximum number is 11 (n = 5). The following table shows the number of sample readings
for each rank setting.
The first-in, first-out stack for the median filter operates as a moving type after it fills. For example,
if the median filter is configured to sample 11 readings (rank setting 5), as shown in
Figure 4-4 , the
first filtered reading will be calculated (and displayed) after 11 readings are acquired and placed in
the filter stack. Each subsequent reading will then be added to the stack (oldest reading discarded)
and another median filter reading will be calculated and displayed.
NOTE
With autorange enabled, a range change cannot occur until a reading is yielded by the
median filter process. Therefore, autoranging could be very slow when the median filter is
enabled.
If the repeat filter is enabled, the median filter operation will not start until after the repeat
filter operation yields a reading. In other words, after a repeat filter reading is yielded, that
reading will then be sent to the median filter stack.
Figure 4-4
Median filter (rank 5)
Rank
setting Number of sample
readings
0
1
2
3
4
5 1
3
5
7
9
11
Reading #11
#10
#9
#8
#7
#6
#5
#4
#3
#2
Reading #1
Median
reading
#1
•
•
•
Reading #12
#11
#10
#9
#8
#7
#6
#5
#4
#3
Reading #2
Median
reading
#2
•
•
•
Reading #13
#12
#11
#10
#9
#8
#7
#6
#5
#4
Reading #3
Median
reading
#3
•
•
•