Keithley SourceMeter 2400, User Manual, Page: 196 / 594

2400 Series SourceMeter
®
User’s Manual Range, Digits, Speed, and Filters 7-9
Speed programming example
Use any NPLC command to set the speed. For example, send the following command to
set the amps speed to 10 PLC:
:SENS:CURR:NPLC 10
Filters
NOTE
For the Pulse Mode of the Model 2430, filtering is not used, and therefore, can-
not be enabled. Pressing the FILTER key results in the “Invalid in Pulse Mode”
message.
Filter lets you set the filter response to stabilize noisy measurements. The SourceMeter
uses a digital filter, which is based on reading conversions. The displayed, stored, or trans-
mitted reading is an average of a number of reading conversions (from 1 to 100).
There are two averaging filter types to choose from: repeating and moving ( Figure 7-2 ).
For the repeating filter (which is the power-on default), the stack (filter count) is filled, and
the conversions are averaged to yield a reading. The stack is then cleared, and the process
starts over. Choose this filter for sweeping so readings for other source levels are not aver-
aged with the present source level.
The moving average filter uses a first-in, first-out stack. When the stack (filter count)
becomes full, the measurement conversions are averaged, yielding a reading. For each
subsequent conversion placed into the stack, the oldest conversion is discarded. The stack
is re-averaged, yielding a new reading.
When the filter is first enabled, the stack is empty. Keep in mind that a filtered reading is
not yielded until the stack is full. The first reading conversion is placed in the stack and is
then copied to the other stack locations in order to fill it. Thus, the first filtered reading is
the same as the first reading conversion. Now the normal moving average filter process can
continue. Note that a true average is not yielded until the stack is filled with new reading
conversions (no copies in stack). For example, in
Figure 7-2A , it takes ten filtered readings
to fill the stack with new reading conversions. The first nine filtered readings are calcu-
lated using copied reading conversions.
Response time considerations
The filter averaging mode and count affect the overall reading speed. The moving averag-
ing filter is much faster than the repeat averaging filter because the unit does not have to
refill the filter stack for each reading. Also, the number of readings averaged will affect
reading speed; as the number of readings averaged increases, the reading speed decreases.
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