Next: Optical Blocking Filter Performance
Up: Status of Model Development
Previous: Known Physics Lacking in
An unexplained bias distortion invariably beginning at the start
of a science run and enduring for hundreds of readout frame times
has resulted in much consternation over worthless data and the current
implementation of the prophylactic regimen with the charming
sobriquet "jitter-dacs" which has become a permanent feature of the
flight software. The distortion is clearly produced by an excess of
charge which is only accrues when the parallel clock low level
does not go below some near 0 volt level for a period of time equal
to or greater than a single frame readout cycle. It is possible to
believe that the sources of the charge are surface states lying
directly under the gates, or at the gate-silicon junctures. Bringing
the gate voltage low (or more precisely, jittering the voltage many
times between high and low) depletes these states once and for all
time. While this may be difficult to believe it is uncontested that
without jitter-dacs neither useful data or bias can be acquired for
long times after start of a science run.
Mark Bautz
11/20/1997