Princeton PIXIS-XB System User Manual page 45

Chapter 5 Operation 45
ADC Offset (Bias)
With the camera completely blocked, the CCD will collect a dark charge pattern,
dependent on the exposure time and camera temperature. The longer the exposure time
and the warmer the camera, the larger this background will appear. To minimize the
amount of this signal that gets digitized, the baseline has been offset by adding a voltage
to the signal to bring the A/D output to a non-zero value, typically 500-600 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 500-600 counts lower.
Note: It is important to note that the bias level is not noise. It is a fully subtractable
readout pattern. Every device has been thoroughly tested to ensure its compliance with
Princeton Instruments' demanding specifications.
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