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VTI Instruments Corp.
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 Section 5. 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.
AC
HPF
Analog
Input
Range
Selection
Analog
AAF
Analog Delay
ADC
Decimation
Filters
Digital Filter Group Delay
Transfer
Buffer
Block
Transfer
Host
FIFO
FIFO
Accumulation
Block
Transfer
Figure 4-15: Delays in Data Path
Model
Clock Freq. (FCLK)
FCLK
1/2 FCLK
1/4 FCLK
1/8 FCLK
EMX-4350/4380
409600 Hz
0.0025
0.005
0.010
0.020
524288 Hz
0.001953125
0.00390625
0.0078125
0.015625
EMX-4250/4251
625000 Hz
0.0016384
0.0032768
0.0065536
0.0131072
131072 Hz
0.0078125
0.015625
0.03125
204800 Hz
0.005
0.01
0.02
Table 4-5: FIFO Buffer Update Rate (Seconds)
EVENTS
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.
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Using the EMX-4250/4350/4380