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

3.3. CIX 0.2 pixel cell concept 37
Current [nA]
14
12
10
8
6
4
2
0
0 100 200 300 400 500 600 700 800
Time [s]
Fig. 3.14: Detuning of the compensation current in static leakage current compensation
as measured with the CIX 0.2 integrator. The black line indicates the average and the gray
ribbon shows the standard deviation for the 64 pixels. The discharge of CA closes the p-MOS
transistor in the right branch of the second differential pair. As a consequence the second
pair draws current from the input node, which is equivalent to an electron signal coming
from the sensor. The inverting output of the integrator then yields a positive current.
error basically prevents the use of the static leakage current compensation in combination
with an X-ray tube as the sampling has to take place when the X-ray tube is switched
off. However, even if the tube is switched on immediately after the sampling phase, the
powering up process of the tube (a few seconds) is long enough for the discharge of Ca
to have a significant impact on the measurement. The feedback deviates for example by
approximately 300 pA from the original current after only 5 s of operation. Previous
measurements based on on-chip injection circuits [1] were not influenced as strongly as
the measurements presented in this work due to the ns or µs time scales on which the
injection circuits can be activated and deactivated.
Feedback mismatch currents
A second type of offset current is introduced by the differential pairs. Process variations on
the chip can cause imbalances in the current drains at the bottom of the differential pairs
in Fig. 3.9, i.e. the two branches do not draw exactly the same current. In this case the
imbalance is forwarded to the preamplifier input node in the form of a constant mismatch
current. Fig. 3.15 shows an example of these mismatch currents in the first differential
pair as a function of the applied feedback current. It is found that the feedback produces
on average a negative current of approximately 2 nA at maximum feedback settings. Since
the integrator can in principle only measure positive currents, this means that any input
current of the integrator has to be larger than 2 nA. Therefore, in order to measure small
input currents an additional positive bias current has to be applied. Apart from the
negative average input current, Fig. 3.15 illustrates that the spread between individual
pixels is very large (roughly 5 nA). Hence, the globally applied IntBiasI current has to
be much larger than 2 nA in order to cancel the negative mismatch currents in all pixels.
Unfortunately this limits the dynamic range of the detector in some pixels as pixels with
a positive mismatch current will be supplied with an offset current of several nA.