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Chapter 5 Operation 53
53
Example: The following descriptions assume that the actual incoming light level is
identical in all three instances. The numbers used illustrate the effect of changing an
analog gain setting and do not reflect actual performance: gain at the Low, Medium,
and High settings depends on the CCD installed and the amplifier selected.
Low requires four electrons to generate one ADU. Strong signals can be acquired
without flooding the CCD array. If the gain is set to Low and the images or spectra
appear weak, you may want to change the gain setting to Medium or High.
Medium requires two electrons to generate one ADU. If the gain is set to Medium
and the images or spectra do not appear to take up the full dynamic range of the CCD
array, you may want to change the gain setting to High. If the CCD array appears to
be flooded with light, you may want to change the setting to Low.
High requires one electron to generate one ADU and some noise sources are reduced.
Because fewer electrons are needed to generate an ADU, weaker signals can be more
readily detected. Lower noise further enhances the ability to acquire weak signals. If
the CCD array appears to be flooded with light, you may want to change the setting
to Medium or Low.
Note: The baseline level may require adjustment if you change the analog gain. See the
"ADC Offset" section on page 54 for more information.
Digitization
Introduction
After gain has been applied to the signal, the Analog-to-Digital Converter (ADC)
converts that analog information (continuous amplitudes) into a digital data (quantified,
discrete steps) that can be read, displayed, and stored by the application software. The
number of bits per pixel is based on both the hardware and the settings programmed into
the camera through the software (see "Readout", page 49).
Factors associated with digitization include the digitization rate and baseline signal.
Depending on the camera model, you may be able change the speed at which digitization
occurs and/or offset the baseline. These factors are discussed in the following paragraphs.
Digitization Rate
Depending on the PI-SCX system, the camera may contain single (1 MHz) or dual
digitizers (100 kHz/1 MHz or 50kHz/1 MHz). When a camera has a single digitizer, the
data can only be digitized at that speed. However, for cameras with dual digitization, you
have a choice of how quickly the data will be digitized (with optimum signal-to-noise
ratios at both readout speeds). Because the readout noise of CCD arrays increases with
the readout rate, it is sometimes necessary to trade off readout speed for high dynamic
range. The 1 MHz conversion speed is used for the fastest possible data collection and the
100 kHz or 50 kHz conversion speed is used where noise performance is the paramount
concern. Switching between the conversion speeds is completely under software control
for total experiment automation.
Note: If the camera has dual digitization, the ADC rate can be changed on the
Experiment Setup|ADC tab.