Wet Labs ac-s, Руководство пользователя

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ac-s User’s Guide (ac-s) Revision F 9 Feb. 2006 11
3. Data Processing
Data from the ac-s is acquired and processed through the following steps.
3.1 Analog Acquisition
Optical radiation at the reference and signal channels is continuously monitored by
operational amplifiers operating in a current to voltage configuration. The amplifiers are
configured for a gain of approximately 10
7
for the reference channels and approximately
2 * 10
6
for the signal channels.
3.2 Analog-to-Digital Conversion
The A/D used in the ac-s supports six input channels that are triggered pair wise. Four
channels are dedicated to the attenuation beam reference and signal channels and the
absorption beam reference and signal channels. Another channel is dedicated to the
output of the optional pressure sensor amplifier. The sixth channel is preceded by an 8
channel analog mux, and is used to sample ancillary analog signals including the internal
and external temperatures via thermistors within the unit. During a single filter wheel
rotation, the A/D concurrently samples all 4 signal and reference detectors for both
beams. Ancillary signals are sampled during the “dark” periods when the beams are not
passing through the filters.
3.3 Digital Processing and Data Output
The CPU takes multiple samples of both signal and reference channels, accumulates them
through the sampling period, and then averages the values at the end of the sampling
period. Once averaged, light and dark values are collected for each channel. The CPU
takes the difference of these values to derive its output value. Once signal data is
accumulated over a given filter wheel rotation its output is transmitted through the RS-
232/RS-485 port. For a more detailed explanation, see Section 6.4, Signal Processing.
3.4 Data Format
The ac-s outputs data in binary format. If you will not be collecting the data using
WETView, your collection system will need to be able to handle binary data. Beware that
some serial port terminal programs such as HyperTerminal are not capable of capturing
true binary data streams. HyperTerminal, for example, fails to capture ASCII NULL
characters (hex 00). WETView is capable of processing collected binary files in addition
to serial data. If you prefer to use your own software to process the ac-s output data,
please refer to Appendix A for a description of the ac-s data packet formatting.
3.5 Post Processing
Once accumulated by a host computer the data must be post processed to yield
meaningful scientific data.
The primary transfer equation for yielding the attenuation coefficient,
c,
or the absorption
coefficient,
a,
for a given wavelength is: