VTI Instruments EX1401, User Manual

Page: 92 / 114

VTI Instruments Corp.
92 EX1401 Instrument Drivers
D
ATA
R
ETRIEVAL
After a measurement is triggered and at least one record of data (the number of samples specified
by
RecordSize property in Sampling ) is available in FIFO buffer, it can be retrieved using the Read
method in Measurement interface or via the Streaming method described in the Data Acquisition
section in this manual. At each trigger event, the number of contiguous records specified by
NumRecordsPerTrigger are acquired. Multiple records can be read separately or all in once if they
are available in the instrument’s FIFO buffer. The
Read method in Measurement returns at least one
record from all enabled channels.
When the user wishes to acquire continuous samples indefinitely after a single trigger event, set the
NumRecordsPerTrigger property to 0 (infinite) and set FIFO mode to Stop . The data acquisition
stops when the user aborted using the Abort method or when the FIFO buffer becomes full (FIFO
overflow). In this setup, the user must retrieve data faster than the ADC data filling into FIFO in
order to avoid FIFO buffer from overflowing.
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 occur
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. Usually the delay in the analog section is small and negligible.
In the digital section, the digital filters in the A/D converter and decimation filters introduce a group
delay. With the
LowLatency filter, it is 24.75 ms at 100 Sa/s and increases at lower frequency spans.
The filtered data is transferred to the FIFO buffer at a specified interval (enough sample periods to
cover at least 3.2 ms). Once a trigger is detected, the data in the FIFO can be read out to the user’s
application in blocks of
RecordSize samples.
F
IGURE
5-12: D
ELAYS IN
D
ATA
P
ATH
E
VENTS
Events are an optional feature allowing the EX1401 to send notifications to the user, or other
instruments, when a specified event occurs. The notification can be via LAN message, EXT trigger
line, or DIO output. The user can specify an event when the measurement state is armed, triggered,
finished, or when an overload condition is detected on one or more channels. See the Digitizer
driver’s online help for more information.
ADC
Transfer
Buffer
FIFO
Analog
Input
Range
Selection
Host PC
Analog Delay Digital Filter Group Delay
Block
Transfer
FIFO
Accumulation
Block
Transfer
Analog Anti-
Alias Filter
Decimation
Filters
ADC