National Instruments M Series User Manual page 145

Assume that an input terminal has been low for a long time. The input terminal then changes
from low to high, but glitches several times. When the filter clock has sampled the signal high
on N consecutive edges, the low to high transition is propagated to the rest of the circuit. The
value of N depends on the filter setting; refer to Table 9-2.
Filter Setting
125 ns
6.425 µs
2.56 ms
Disabled
The filter setting for each input can be configured independently. On power up, the filters are
disabled. Figure 9-3 shows an example of a low to high transition on an input that has its filter
set to 125 ns (N = 5).
RTSI, PFI, or
PXI_STAR Terminal
Filter Clock
(40 MHz)
Filtered Input
Enabling filters introduces jitter on the input signal. For the 125 ns and 6.425 µs filter settings,
the jitter is up to 25 ns. On the 2.56 ms setting, the jitter is up to 10.025 µs.
When a PFI input is routed directly to RTSI, or a RTSI input is routed directly to PFI, the
M Series device does not use the filtered version of the input signal.
Refer to the KnowledgeBase document, Digital Filtering with M Series and CompactDAQ,
for more information about digital filters and counters. To access this KnowledgeBase, go to
and enter the Info Code
ni.com/info
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Table 9-2. Filters
N (Filter Clocks Pulse Width
Needed to Guaranteed to
Pass Signal) Pass Filter
5 125 ns
257 6.425 µs
~101,800 2.56 ms
— —
Figure 9-3. Filter Example
1 1 2 3 4 1 2
.
rddfms
M Series User Manual
Pulse Width
Guaranteed to
Not Pass Filter
100 ns
6.400 µs
2.54 ms
—
Filtered input goes
high when terminal
3 4 5 is sampled high on
five consecutive filter
clocks.
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