UNIVERSIT . . AT BONN Physikalisches Institut - Prof. Dr. Norbert ...

38 3. CIX 0.2 detector
O ffs e t c u r r e n t [n A ]
4
2
0
-2
-4
-6
-8
0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0
IF b fe e d b a c k c u r r e n t [n A ]
Fig. 3.15: Average mismatch currents of the first differential feedback pair as a function
of the feedback current measured with the CIX 0.2 integrator. The black line indicates the
module-wide average and the gray ribbon shows the standard deviation of the 64 pixels.
The leakage current compensation is deactivated in this measurement.
Workaround and baseline shift
The only solution for these problems is to disable the leakage current compensation for the
X-ray measurements. This means that the second differential pair has to be disconnected
and its current source ILeakComp has to be deactivated (see Fig. 3.16). In this way the whole
feedback and input signal replication is handled by the first differential pair. Constant
offset currents, like the sensor leakage current and the feedback mismatch current, are
forwarded to the integrator and have to be corrected offline by sampling the current
prior to a measurement and storing the pixel values on the PC. Still, the offset currents
cause a baseline shift at the preamplifier output and as a consequence they alter the
relative threshold of the pixels (see section 3.3.3). A so-called threshold tuning algorithm,
1 st
1
IFb
VCount-
Baseline
In
sensor
input node
VCountRef
CFb
Out
bypass
transitor
integrator
Fig. 3.16: Feedback configuration under X-ray imaging conditions. The fast discharge of
the sampling capacitor CA on CIX 0.2 prevents the use of a more complex leakage current
compensation mechanism when a stability over several seconds, e.g. typical turn-on time
of an X-ray tube, is required.