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50 PI-MTE System Manual Version 4.C
The ST-133 100 kHz/1 MHz Controller has a single A/D converter that can be run at
either 100 kHz or 1 MHz. The choice of digitization speed is made via the application
software.
The ST-133 2 MHz controller provides six digitization rates (from 50 kHz up to 2 MHz),
providing optimum signal-to-noise ratios at all 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 2 MHz conversion speed permits the fastest
possible data collection and the 100 kHz or 50 kHz conversion speed is suitable for use
where noise performance is the paramount concern. Switching between the conversion
speeds is completely under software control for total experiment automation.
Note: For the 2 MHz controller, ADC baseline offset levels are preset at the factory and
are not user-adjustable.
ADC Offset
ADC offset (also known as baseline offset) provides another way of dealing with dark
charge (refer to "Dark Charge", page 44). By offsetting the baseline signal, much of the
background is ignored during conversion.
Offsetting the baseline is accomplished by adding a voltage to the signal to bring the A/D
output to a non-zero value, typically 50-100 counts. This offset value ensures that all the
true variation in the signal can really be seen and not lost below the A/D “0” value. Since
the offset is added to the signal, these counts only minimally reduce the range of the
signal from 65535 (16-bit A/D) to a value in the range of 50-100 counts lower.
The ability to shift the baseline depends on the controller used in your system. For
100 kHz/1 MHz controllers, the offset values for the Fast (F) and Slow (S) A/D
converters are preadjusted at the factory but may be adjusted via the Zero Adjustment
potentiometers on the back of the controller. In the case of 2 MHz controllers, the
potentiometers are not present and you cannot change the factory set baseline offsets for
the six digitization speeds: changes to the baseline offsets must be performed at the
factory.
Once you have entered the acquisition parameters, wait for temperature lock to occur and
then wait 30 minutes for the camera temperature to completely stabilize. Then try taking
a few dark charge readings.
Notes:
1. Do not be concerned about either the DC level of this background or its shape unless
it is very high, i.e., >400 counts. What you see is not noise. It is a fully subtractable
readout pattern. Each CCD has its own dark charge pattern, unique to that particular
device. Every device has been thoroughly tested to ensure its compliance with
Princeton Instruments' demanding specifications.
2. Do not adjust the offset values to zero or some low-level data will be missed.
If you observe a sudden change in the baseline signal, TURN OFF THE SYSTEM
IMMEDIATELY and contact the factory Customer Support Department. Refer to page
102 for contact information.
CAUTION