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Theory of Operation
Firmware Description
Subtract Dark Current
Raw data coming from the photodiode array through the signal conditioning
electronics (which includes a variable gain amplifier) are read from the A/D
converter, see on page 37. This raw data are intensity values of each
photodiode of the array.
During a blank measurement a dark current and electronic offset
measurement is performed for every diode on the photodiode array. This
offset spectrum is stored and subtracted from all subsequent intensity
spectra to give dark-corrected intensity spectra.
Gain Correction
Also during a blank measurement every photodiode is assigned a gain value,
depending of the amount of light falling on the diode array. These gain values
are stored in a table. They are used to adjust the amplification factor in the
signal conditioning electronics, see “Spectrophotometer Data Acquisition
(SDA) Board” on page 37. This process is used to adjust the signal level to the
optimum range of the A/D converter on the SDA board.
The DSP firmware uses the gain table to adjust the dark-corrected intensity
spectra.
Temperature Correction
The quantum efficiency of the photodiode array depends on the temperature
and is different for each wavelength. The temperature dependency follows an
e-function over the wavelength scale, for example, it increases with the
longer wavelengths.
To correct the intensity spectrum for temperature effects, the temperature on
the photodiode array is averaged over 5–10 s. With the help of a table which
stores correction coefficients for each wavelength, correction factors for the
current temperature are calculated and used for the temperature correction
of the intensity spectrum.
Temperature Filter
The temperature filter determines if temperature correction has to be
applied. If temperature changes of more than 0.004 K occur, update is
performed at a minimum interval of 5–10 s.
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