VTI Instruments Corp.
APPENDIX B
48
D
ATA
A
CQUISITION
D
ATA
F
LOW
The EMX-4250/4350/4380 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
is decimated and filtered by multiple stages 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.
1
5
1
2
1
2
1
2
Fc
Fc/5
Fs = Fc or Fc/5
Fs = Fc/2 or Fc/10
Fs = Fc/4 or Fc/20
1/5
Decimation
Filter
1/2
Decimation
Filter
1/2
Decimation
Filter
1/2
Decimation
Filter
Highest Span
FIFO
1/2 Span FIFO
1/4 Span FIFO
Lowest Span
FIFO
A-to-D Converter
Figure 4-11: 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 4-12 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.
