0.88 0.9 0.92 0.94 0.96 0.98 1
Threshold Charge [e]
1000 1200 1400 1600 1800 2000 2200
Figure 6.15:Calibrated threshold charge of different external threshold voltages, as-suming a linear relation.
in equivalent chargeQTh is
QTh=Th−BL VFe QFe.
Figure6.15shows the resulting threshold charge as a function of the external threshold voltage. This calibration assumes a linear behaviour of the charge as a function of the voltage, which is not correct as described in Section6.2.2. Therefore, this calibration can only be used as a rough estimate of the charge. In the following, the threshold will be given in units of the injection voltage (ThEV).
The external threshold voltage is the parameter that influences the comparator threshold the most. Figure6.16shows the results of differentThreshold Scans CCPDfor external threshold voltages between 0.89 V and 1 V. For voltages below 0.89 V no measurement was possible, because for some pixels the threshold was so low that only noise hits were registered. The ThEVincreases with increasing external threshold voltages. The dispersion of the threshold increases with the external threshold voltage. In addition, the ThEVis measured for three values of theThResDAC. This parameter was not found to have the expected influence on the ThEV.
As a next step the influence of the feedback parameterVNFBwas studied. The measure-ment of the injection voltage at the comparator threshold for different external voltages and threeVNFBDAC settings is shown in Figure6.17. VNFBhas a stronger influence on ThEVthan the external threshold voltage. For large feedback currents the amplitude of the amplifier output signal is reduced, because part of the signal charge is compensated.
This effect is known as ballistic deficit and leads to an increase of the effective comparator
6 H V 2 F E I 4 C H A R A C T E R I S AT I O N
External Threshold Voltage [V]
0.88 0.9 0.92 0.94 0.96 0.98 1
Threshold Equivalent Voltage [V]
0.2 0.3 0.4 0.5 0.6 0.7 0.8
ThRes 20 40 60
Figure 6.16:Injection voltage at the comparator threshold of the HV2FEI4v2 for dif-ferent external threshold voltages andThResDACs values. The error bars show the pixel dispersion.
External Threshold Voltage [V]
0.88 0.9 0.92 0.94 0.96 0.98 1
Threshold Equivalent Voltage [V]
0 0.2 0.4 0.6 0.8 1 1.2 1.4
VNFB 1 30 60
Figure 6.17:Injection voltage at the comparator threshold of the HV2FEI4v2 for differ-ent external threshold voltages andVNFBDAC settings. The error bars show the pixel dispersion.
68
6 . 2 PA R A M E T E R S T U D I E S
threshold at high values ofVNFB.
Furthermore, the dispersion of the distribution increases withVNFB. This effect hints at the fact that the feedback transistors suffer more from process fluctuations than e.g. the comparator transistors.
Because theVNOut1/2/3DACs will play an important role in the following studies, their influence on the comparator threshold was investigated. Figure6.18shows the measure-ment of the injection voltage at the threshold as function of theVNOutDAC values for all three subpixels. The error bars again indicate the pixel dispersion. The external threshold
VNOut [DAC]
0 10 20 30 40 50 60
Threshold Equivalent Voltage [V]
0.2 0.25 0.3 0.35 0.4 0.45 0.5
0.55
VNOut1
VNOut2 VNOut3
Figure 6.18:Injection voltage at the comparator threshold of the HV2FEI4v2 for dif-ferentVNOutDAC settings. The error bars show the pixel dispersion.
voltage during the measurement was 0.89 V. In the measurement forVNOut1only the subpixel type 1 was enabled, as for the measurements withVNOut2andVNOut3only the subpixel types 2 and 3, respectively.
TheVNOut1/2/3DACs influence the threshold equivalent voltage. This effect was not expected, because theVNOut1/2/3parameters are supposed to only influence the signal amplitude after the comparator (see Figure6.3). No difference among the three subpixels was measured.
So far, only global parameters were studied for an influence on the comparator threshold.
Using STcontrol, the InDAC, which adjusts the threshold per pixel, can be set for every pixel individually.
Figure6.19shows the result of the measurement of ThEVfor differentInDACvalues. The error bars show again the pixel dispersion. The external threshold voltage is 1.0 V. Two independent measurements (A and B) produce the same results.
For the measurement all pixels were set to the sameInDACvalue and aThreshold Scan
6 H V 2 F E I 4 C H A R A C T E R I S AT I O N
InDAC [DAC]
0 2 4 6 8 10 12 14
Threshold Equivalent Voltage [V]
0.5 0.52 0.54 0.56 0.58 0.6 0.62 0.64 0.66
0.68 Measurement
A B
Figure 6.19:Injection voltage at the comparator threshold of the HV2FEI4v2 for dif-ferent InDACs settings. The error bars show the pixel dispersion. Two independent measurements (A and B) produce the same results.
CCPD was performed. The measurement shows that theInDAC does not influence the threshold in a well-defined way, which makes in-pixel tuning very difficult. With this result it was decided not to perform an in-pixel adjustment for the comparator thresh-old, which was supposed to reduce the dispersion, since no generally applicable tuning procedure could be defined.
6.2.4 ToT Studies towards Subpixel Decoding
For a successful subpixel decoding, theToTresponse of the FE-I4 needs to be adjusted to the amplitude of the sensor signal. Before tuning the signals of the subpixel types 1, 2 and 3, the impact of theVNOut1/2/3DACs on theToTis investigated.
Only one subpixel type was enabled at a time and theToT response to differentVNOut settings was measured. The same was repeated for all three subpixel types. Figure6.20 shows theToTresponse as a function ofVNOut1/2/3DAC values. TheToTCodesaturates forVNOutDAC settings above 12 and hence the output voltage of the HV2FEI4v2. Conse-quently, the parameter range is small, in which a significant change of the output voltage is possible. In this range the response of the different subpixel types varies up to 2ToT units. VNOut2 has the highest ToT response. For VNOut1/2/3 DAC settings above 12 theToT response is at its maximum. Consequently, the available range to separate the subpixels is small.
Considering the result above, the target value of the FDAC Tune CCPD was chosen to beToT=3 for a signal withVNOut1=3 to get the lowest possibleToT response. During the tuning only the subpixel type 1 was enabled. TheFDACdistribution, which is the result of the tuning, is shown in Figure6.21. TheFDACdistribution (see Figure6.21b) is broad, which means that the differences among the pixels are large and theFDACsetting
70
6 . 2 PA R A M E T E R S T U D I E S
Figure 6.20:ToTresponse as a function ofVNOut1/2/3DACs for the three subpixel types for a given feedback tuning of the FE pixels. The error bars show the standard deviation of the distribution.
FDAC_TUNE_CCPD.
Tuned Fdacs chip 0 6 8
Figure 6.21:Result of theFDAC Tune CCPD. The distribution of the FE-I4FDACvalues is shown as a pixel map (top) and a histogram (bottom).