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

More documents

Recommendations

Info

32 3. CIX 0.2 detector Equally the largest current is registered if the charge pump is triggered every CKInt clock cycle, i.e. the pump is working at its maximum rate Nmax: IMax = QP ump · NMax · tF rame = QP ump · tF rame · fCKInt · tF rame In general the precision of one such integrator measurement is given by: 1 tF rame · fCKInt (3.5) (3.6) This means that a change in the time difference between TimeFirst and TimeLast by only one clock cycle already translates into a different current result. So in theory, neglecting the electronic noise and assuming the integrator is set to a clock rate of 10 MHz and a 1 ms frame duration, a change in the input current of only 1/1000 can be detected. A further consequence of this is that the system has an approximately constant measurement precision over its dynamic range. Approximately refers to the fact that the precision is slightly reduced at the lowest input currents (close to IMin) as in that case the charge on CInt at the beginning of a frame introduces some measurement uncertainty. For a more detailed treatment of the system’s measurement precision refer to [1]. The basically constant measurement precision has to be compared to a current measurement based on an ADC 5 . Such devices usually determine the input signal by step-wise approximating the input with a reference signal. The minimum step-width of the ADC then determines the minimum input current as well as the precision. This results in a relative measurement precision, which depends on the size of the input signal. In the case that CIX 0.2 has to measure currents smaller than IMin, the chip is equipped with an internal IntBiasI bias current source. By using this IntBiasI current, it is possible to effectively shift the dynamic range of the integrator to lower input currents. At the same time, the upper limit of the dynamic range is also slightly lowered because the sum of signal and bias current still has to stay below the IMax condition. So, by selecting a particular fCKInt integrator clock frequency, tF rame frame duration and QP ump charge pump size, the integrator’s precision and its dynamic range with the IMin and IMax limits can be tuned according to the application. Integrator charge pumps Fig. 3.8 shows a schematic of one of the three charge pump types available on the ASIC. This capacitive charge pump works by switching the potentials across the capacitor CP ump Int. VPumpMean Pump CPump Reset VPumpHi VPumpLo Fig. 3.8: Schematic of the CIX 0.2 capacitive charge pump. During the reset/recharging of the pump the switches at the nodes VP umpMean and VP umpLow are closed while the other two remain open. When the state of the switches is inverted a charge packet of CP ump · (VP umpHi − VP umpLo) is injected. CP ump has a size of 25 fF. 5 ”Analog to digital converter”
3.3. CIX 0.2 pixel cell concept 33 and thus injects a configurable amount of charge into the Int node. The size of the charge packet is determined by the potential difference between VP umpHi and VP umpLo. The VP umpMean connection guarantees that the action of the switches themselves does not inject additional charges. Apart from the capacitive charge pump, the system also features a switched constant current source. This circuit discharges CInt by enabling the IP ump current source for half a CKInt clock cycle. The size of the individual QP ump packets is controlled by the current source’s DAC setting. More details on the charge pumps can be found in [1]. 3.3.3 Feedback The feedback circuit of CIX 0.2 is the heart of the simultaneously counting and integrating concept. Its task is to discharge the counter preamplifier and at the same time to reproduce the input current for the integrator. These at first glimpse conflicting requirements are met by using a number of differential pairs as illustrated in Fig. 3.9. Due to the high significance of the feedback for the simultaneous counting and integrating concept, the following paragraph will give a detailed description of the operation of the feedback circuit. Assume that the sensor produces an electron signal, i.e. it draws current from the preamplifier input node. This current signal is integrated on the feedback capacitor CF b and produces an increase of the voltage of the inverting preamplifier. Since the preamplifier output is connected to the left p-MOS of the first differential pair, this transistor will close and hence allow less current from IF b to flow into the left branch of the pair. However, as the current sources situated at the bottom of the two branches both drain 50 % of the IF b current, the imbalance in the p-MOS switches will also cause the current source in the left branch to draw current from node 1. Similarly, the right branch will be supplied with too large a current and hence it will deliver a net current to the input node of the preamplifier. This current cancels the electron signal of the sensor and thus steers the preamplifier into a dynamic equilibrium. In other words, a constant current like a leakage current, which flows into the input of the preamplifier, causes a constant offset at the preamplifier output Out. This deviation from the VCountRef reference potential shifts current from one branch of the first differential pair to the other such that the original constant current is canceled and the integrated charge on CF b reaches a constant value. At the same time this imbalance also causes a net current flow into or out of node 1 thereby achieving the replication of the input current. It is worth mentioning, that the baseline shift, i.e. the offset at the preamplifier output, that has to be maintained in order to cancel a constant current B VCountRef D ILeakComp 2 2 nd Ca A C E E E 1 st 1 IFb VCount- Baseline In sensor input node VCountRef CFb Out bypass transitor integrator Fig. 3.9: Schematic view of the CIX 0.2 feedback. The sensor connection to the preamplifier is indicated by the arrow on the right side. A complex switching network allows different leakage current compensation mechanism to be applied.
Page 1: . . UNIVERSIT AT BONN Physikalische
Page 5 and 6: Contents 1. Introduction . . . . .
Page 7: B. CIX 0.2 readout . . . . . . . .
Page 10 and 11: 2 1. Introduction This work is stru
Page 12 and 13: 4 2. X-ray imaging 2.1.1 Photoelect
Page 14 and 15: 6 2. X-ray imaging described by the
Page 16 and 17: 8 2. X-ray imaging (a) Fig. 2.5: (a
Page 18 and 19: 10 2. X-ray imaging electrons back
Page 20 and 21: 12 2. X-ray imaging and were used i
Page 22 and 23: 14 2. X-ray imaging component and a
Page 24 and 25: 16 2. X-ray imaging a) c) b) d) Fig
Page 26 and 27: 18 2. X-ray imaging a) b) Pt CdTe:C
Page 28 and 29: 20 2. X-ray imaging the following d
Page 30 and 31: 22 2. X-ray imaging sensors [45]. L
Page 32 and 33: 24 2. X-ray imaging C o u n ts [a .
Page 34 and 35: 26 3. CIX 0.2 detector CIX 0.1. For
Page 36 and 37: 28 3. CIX 0.2 detector 3.3 CIX 0.2
Page 38 and 39: 30 3. CIX 0.2 detector distributed
Page 42 and 43: 34 3. CIX 0.2 detector flowing into
Page 44 and 45: 36 3. CIX 0.2 detector closed and s
Page 46 and 47: 38 3. CIX 0.2 detector O ffs e t c
Page 48 and 49: 40 3. CIX 0.2 detector a) 0um 1000u
Page 50 and 51: 42 3. CIX 0.2 detector Alternativel
Page 52 and 53: 44 3. CIX 0.2 detector • Dual cha
Page 54 and 55: 46 4. ASIC performance - Electrical
Page 72 and 73: 64 5. CIX X-ray test setup • Anod
Page 74 and 75: 66 5. CIX X-ray test setup
Page 76 and 77: 68 6. Sensor material characterizat
Page 90 and 91:
82 6. Sensor material characterizat
Page 92 and 93:
Page 94 and 95:
Page 96 and 97:
Page 98 and 99:
Page 100 and 101:
Page 102 and 103:
94 7. CIX module performance under
Page 104 and 105:
Page 106 and 107:
Page 108 and 109:
Page 110 and 111:
Page 112 and 113:
Page 114 and 115:
Page 116 and 117:
Page 118 and 119:
Page 120 and 121:
Page 122 and 123:
114 8. X-ray images images were tak
Page 124 and 125:
116 8. X-ray images a) Average ener
Page 126 and 127:
118 8. X-ray images By placing a di
Page 128 and 129:
120 8. X-ray images C o n tr a s t
Page 130 and 131:
122 8. X-ray images
Page 132 and 133:
124 9. Conclusion and outlook • S
Page 134 and 135:
126 9. Conclusion and outlook princ
Page 137 and 138:
A. Differential current logic The d
Page 139 and 140:
B. CIX 0.2 readout The readout sche
Page 141 and 142:
C. Threshold scans and tuning A thr
Page 143 and 144:
Bibliography [1] E. Kraft, Counting
Page 145 and 146:
Bibliography 137 [32] A. E. Bolotni
Page 147:
Acknowledgements Looking back at my
show all

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

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?