Download Folie 1 - Indico

Drain Current Digitizer Chip
(DCD)
Status and Future Plans
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DCDBv1 Measurements
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DCDBv1 has been tested with and without sensor
The chip is working well
There are two smaller problems:
A) Noise problem:
At the designed full clock frequency, 350MHz, that corresponds to 20s frame readout
time - the electronics noise is higher than expected. Noise is 150nA, that corresponds
to ~ 300 ENC in DEPFET (gq=500pA/e) and SNR of 13 (assumption 4000 e signal).
 The noise at a lower clock frequency – e.g. 200MHz (32s frame readout time) is as
expected – 35nA (~70 ENC). SNR would be 53.
 B) Yield problem:
 There are too many non functional ADC channels on the chip, typically 6 out of 256.
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DCDBv1 Measurements
 Both problems have been extensively studied.
 The noise problem is most probable related to one wrongly dimensioned component on
the chip – the bias diode. This component generates the bias voltage for certain logic
delay elements. The delay time is too long, which leads to malfunction of ADCs at
higher clock rate. The result is excessive noise.
 In order to examine the yield problem we tried to localize the damage in the non
working channels. We have found out that the long digital lines (5mm long and 0.3 m
wide) that connect the analog ADC part and digital data processor (see page 4) are
broken. The layout of these lines is not optimal. Each of them has at least 5 single-“via”
connections between layers and no ESD protecting devices (antenna diodes) that
should protect the digital gates lines are connected to. This can lead to the damage of
a few per cent of ~1000 lines on the chip. (2 per ADC, 512 ADCs on the chip).
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DCD test chip and DCDBv2
 In order to test the assumed cause of the noise problem we have designed and
submitted a test chip with adjusted bias diode.
 The remaining circuits are identical as on the big DCD, just the size of the chip is
smaller. The test chip contains two out of 16 channel columns (full size DCD). (Layouts
of both chips are shown in page 4.) The test chip works excellent even at the clock
frequency that is higher than the designed one. At the clock frequency of 400MHz (~
16s frame readout time) we measure electronic noise between 30nA and 52nA,
depending of the adjusted gain of the input amplifier. (Higher gain leads to lower noise,
but also to a reduced dynamic range.) SNR of DEPFET would be from 38 up to 66. The
success of the test chip is a good sign that we have localized the source of the speed
problem.
 We have submitted a new version of the DCD chip – DCDBv2 this March.
 The chip has the adjusted bias diode and the improved layout of the long digital lines
for better yield. We are quite confident that the new chip will work as good as the test
chip and have much lass non working channels than DCDBv1.
 We expect the first measurements on DCDBv2 in October.
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Test chip- and full scale DCDB layout
3mm
Test chip
3mm
2 columns = 32 channels
Long digital lines
Full-scale DCDB
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Test chip measurements
Gain: 37nA/LSB
Noise 1.3 LSBs or 48nA
ADC characteristic measured with DCD
test chip at 400MHz clock frequency (80ns
sampling rate). ADC gain is 37nA/LSB
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ADC noise for each input current. The
average value is 1.3 LSBs that
corresponds to RMS 48nA.
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Pedestal dispersion
 As expected, we have measured that the readout noise can be decreased by using
higher gain settings for the DCD input amplifier.
 Typical measured values (test chip at 400MHz clock frequency) are summarized in the
following table.
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Gain
Noise
Dynamic
range
1
52 nA
16 A
2
48 nA
8 A
4
30 nA
4 A
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Pedestal dispersion
 Unfortunately, a higher gain setting leads not only to a better noise than also to a reduced
dynamic range
 The required dynamic range must cover the DEPFET pedestal current dispersion and
leave some room for signal.
 There are hints that DEPFET pedestal dispersion after irradiation can be as large as
150A!
 Clearly, auxiliary circuits are needed to extend the dynamic range of the ADC.
 The present strategy is to use an extra 2-bit DAC. This can typically reduce pedestal
dispersion by factor of ~ 4. Our ADC must able to cope with the remaining pedestal
dispersion and the signal
 This would require a gain setting of ~ 0.3. The price of such a low gain would be
excessive input referred noise.
 We have designed a circuit that can automatically compensate for the radiation induced
part of the pedestal dispersion in very high range. The circuit will allow us to keep high
gain settings for DCD amplifier even after many months of irradiation and keep the noise
low.
 The circuit have been implemented as an extra feature in the new DCDBv2.
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SWITCHER Chip
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SWITCHER Status
 The preset version of SWITCHER chip has been tested on a dedicated PCB and used
to build DEPFET modules
 The stand alone chip works excellent. However, only one was tested so far.
 Some of the chips that were mounted onto the DEPFET hybrids was broken after
assembly.
 The mounting yield issue is still under investigation.
 Currently a diploma student in Heidelberg is developing a probe station based test
setup for SWITCHER and DCD.
 The test setup will allow us to test SWITCHERs and DCDs before they are mounted.
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SWITCHER Plans
 Presently a student in Heidelberg is working within her practice semester on the
translation of the SWITCHER circuits and layouts to the newer technology - AMS HV
0.18m.
 A SWITCHER in 0.18m would have certain advantages, such as:
 Smaller chip width (the present chip extends over the active DEPFET area)
 Better radiation hardness -> no need for the use of non-standard annular transistors ->
standard logic cells / HV transistors can be used -> safer and easier design
 Digital supply voltage compatible to the one of DCD and DHP, less power lines on
DEPFET module, no level shifter, simpler design
 Less (digital) power consumption, faster switching
 Possibility to buy the chip with solder bumps
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