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APPENDIX B
57
Pipeline Delay and Latency
For high speed data recording, data transfer speed is key. For applications that require real-time data
monitoring or processing, on the other hand, the data update rate becomes more important. A real-
time closed loop control is an example. It is necessary to understand that there are delays at almost
every stage of the data acquisition process. Some are within the instrument while others occurs
outside of the instrument, such as at the transducer or in the user’s application.
The analog signal conditioning circuit introduces some delay before the signal reaches the A/D
converter. This is further discussed in 0. Usually the delay in the analog section is small and
negligible, but it must be noted that the AC coupling filter introduces a non-linear delay, especially
in the low frequency range.
In the digital section, the digital filters in the A/D converter and decimation filters introduce a group
delay. It is about 45 µs at 625,000 Hz and increases at lower frequency spans. Please refer to Table
A-2 for more info. The filtered data is transferred to FIFO buffer through a “ping pong” buffer at a
specified internal (see Table 4-5). Once a trigger is detected, the data in the FIFO can be read out to
the user’s application at the chunk of
RecordSize
samples.
Analog
AAF
ADC
Transfer
Buffer
FIFO
Analog
Input
Range
Selection
Decimation
Filters
AC
HPF
Host
Analog Delay
Digital Filter Group Delay
Block
Transfer
FIFO
Accumulation
Block
Transfer
Figure 4-15: Delays in Data Path
Model
EMX-4350/4380
EMX-4250/4251
Clock Freq. (F
CLK
)
409600 Hz
524288 Hz
625000 Hz
131072 Hz
204800 Hz
F
CLK
0.0025
0.001953125
0.0016384
0.0078125
0.005
1/2 F
CLK
0.005
0.00390625
0.0032768
0.015625
0.01
1/4 F
CLK
0.010
0.0078125
0.0065536
0.03125
0.02
1/8 F
CLK
0.020
0.015625
0.0131072
Table 4-5: FIFO Buffer Update Rate (Seconds)
E
VENTS
Events
are an optional feature allowing the EMX-4250/4350/4380 to send notifications to the user,
or other instruments, when a specified event occurs. The notification can be via LAN message or a
PXI trigger line. The user can specify an event when the measurement state is armed, triggered, or
finished. See digitizer/DSA driver’s online help for more information.
