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DATA COMPUTATIONS C-1
MON2000
JULY 2010 Data Acquisition
CA
APPENDIX C, DATA COMPUTATIONS
C.1 DATA ACQUISITION
The 2350A GC Controller performs a 12-bit
analog to digital (A/D) conversion every 25
milliseconds on channel 0 during an analysis
period. This is the period when the left side of
the display is counting the elapsed time of the
analysis. The fixed number on the right side of
the display is the total cycle time.
Exactly 40 equi-spaced data samples are taken
every second for analysis by the controller. A
sampling frequency of 40 Hertz (Hz) was
chosen to reduce 60 Hz normal mode noise.
After each point on the chromatograph signal is
sampled, the resulting number is stored in a
buffer area in the controller memory for
processing. During the analysis, only the last
256 data points are available for processing.
Because the data analysis is done as the signal
is sampled (in real-time), only a limited
number of past data samples is required to
analyze any signal.
As a part of the data acquisition process,
groups of incoming data samples are averaged
together before storing to the controller
memory for processing. Non-overlapping
groups of N samples are averaged and stored
and thus reduce the effective incoming data
rate to 40/N samples/second. For example, if
N = 5, then a total of 40/5 or 6 (averaged) data
samples are stored every second. The value for
the variable N is determined by the selection of
a Peak Width parameter (PW). The
relationship is:
NPW=