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54 PI-SCX System Manual Version 2.E
54
ADC Offset
ADC offset (also known as baseline offset) provides another way of dealing with dark
charge (see "Dark Charge", page 48). 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.
Notes:
1. Do not be concerned about either the DC level of the baseline signal or its shape
unless it is very high (i.e., > 1000 counts with 16-bit ADC).
2. The ADC Offset can be adjusted by using the F and S Zero pots located on the rear
panel of the controller. If these pots are not present, the ADC Offset may be
software-adjustable on the Experiment Setup|ADC tab.
3. 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 you may have excessive humidity
in the camera's vacuum enclosure. TURN OFF THE SYSTEM IMMEDIATELY.
Contact Princeton Instruments Customer Support for information on how to refresh the
vacuum. See page 108 for contact information.
WARNING!