Emerson Daniel Danalyzer 1000, Hardware Reference Manual

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MODEL 1000
SEP 2005
DESCRIPTION
1-13
1.6.2. Data Acquisition
Every second, exactly 40 equi-spaced data samples are taken for analysis by the GC Controller (i.e.,
once every 25 milliseconds). Each data sample, after having been precision-amplified, is subjected
to a twelve bit analog to digital (A/D) conversion. The 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 GC Controller’s 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 the result is stored to the Controller’s 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:
N = PW seconds,
where PW is given in seconds. All the various details in the analysis process are independent of the
value of N. Allowable values of N are 1 to 63, which corresponds to values of PW from 2 to 63
seconds.
The variable N is known as the integration factor. This term is used because N determines how
many points are averaged or integrated to form a single value. The integration of data upon input,
before storing, serves two purposes. First, the statistical noise on the input signal is reduced by the
square root of N. In the case of N = 4, a noise reduction of two would be realized. Secondly, the
integration factor controls the bandwidth of the chromatograph signal. It is necessary to match the
bandwidth of the input signal to that of the analysis algorithms in the GC Controller. This prevents
small, short duration perturbations from being recognized as true peaks by the program. It is
therefore important to choose a Peak Width corresponding to the narrowest peak in a group under
consideration.