VTI Instruments EX1401, User Manual

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EX1401 Instrument Drivers 83
D
ATA
A
CQUISITION
D
ATA
F
LOW
The EX1401 instruments have a dedicated A/D converter at each analog input channel. The A/D
converter samples analog data at fixed frequency specified by a multiple of the
ClockFrequency
( F
C
). The output data rate of the ADC is F
C
. The digitized data by the A/D converter can then be
decimated and filtered by any one of three selectable types of digital filters until the signal is band
limited to the desired frequency span and the sample rate (
F
S
). These band-limited samples are
constantly sent to the FIFO memory buffer located in DRAM.
A
D
C
PLL
CLK=32*2^n*fs
Decimate 2^n
FIR
(Customizable)
(n=1 to 16)
CIC
(/4 to /8192)
CFIR
(/4)
Customizable
Medium Latency
Blind
Downsampling
(/1 to /65536)
2^n*fs
Raw | (Filter = None)
Gain/Offset/
Engineering
Unit Conversion
High Performance
Low Latency
F
IGURE
5-8: DECIMATION FILTER AND MEASUREMENT SPAN
FIFO
The decimated and band-limited samples by decimation filters are temporarily stored in a circular
buffer (or FIFO) until the data is read out and transferred to the host.
Figure 5-9 illustrates the circular data buffer (FIFO). The figure shows three unread data records in
the data buffer. The white
unused space and over-writable space are the areas where new data can
be written until this data buffer becomes full.
Record N
Record
N+1
Record
N+2
Trig N Trig N+1 Trig N+2
From ADC
FIFO Overflow Limit
Overwritable Unused Space New Data
F
IGURE
5-9: C
IRCULAR
FIFO
When the FIFO buffer is full and there is no more space to write new data to the FIFO, the
measurement is aborted. After the last data record in the FIFO is read, a FIFO overflow error will
be issued to indicate why the acquisition was stopped.
D
ATA
S
TREAMING
The Digitizer driver comes with an optimized data-reading interface for high speed and/or low-
latency acquisitions. This interface relies on a separate thread which receives data asynchronously
from the instrument as soon as it is available and acts on it immediately based on user-set
preferences. There are three streaming modes, any of which can be enabled at the same time.