Impact of AC-coupling on the SSD performance.
Report given at ITS meeting at CERN on 5.12.2001.
Vladimir Gromov (vgromov@nikhef.nl)
Department of electronics,
NIKHEF, Amsterdam, the Netherlands.
Analogue multiplexer diagram.
P-side
P-side
to ADC
N-side
N-side
. due to use of double-sided structure, the front-end electronics of SSD on both
sides operate at different potentials (detector bias  55V)
. the front-end chips of the detector modules are readout in series, both the Pand N side of each detector module to one ADC channel with 10MHZ rate.
. AC-coupling is the only way to provide the signal transmission. However it
results in signal distortion inasmuch as the signal gets differentiated according to
F(p)=p/(1+p), where =C 1.2k
. the coupling capacitor value should be kept as low as possible for safety and
space reasons. On the other hand the smaller the capacitor is the greater signal
distortion becomes. That leads to the detector performance deterioration.
What is the problem with AC-coupling?
AC-coupling in front of analog buffer changes the shape of the signal coming out of the front-end chip
(HAL25).
Distortion of the hit charge could leads to wrong position information,
Signal before
AC-coupling
Exp(-t/td)
mistake
Signal after AC
-coupling
mistake
mistake
mistake
Strip#N
Strip#N+6
Strip#N+1
Strip#N+5
Capacitor is bigger.
mistake
Exp(-t/td*)
Strip#N
Strip#N+6
Strip#N+1
Strip#N+5
Exponential tail
AC-coupling causes extra inefficiency.
The signals turn to be
below the threshold
causing inefficiency
Threshold =2000e
Method is Monte-Karlo simulations.
Items taken into account:
1. number of channels (strips) to be read out at every TRIGGER coming in
6*128=768
2. number of hits for a TRIGGER is Poisson statistics with average of occupancy*768
3. charge deposit to the detector by a hit is distributed according to Landau with most probable value
(MIP) of 22000e (41.8)
4. threshold is set at 5*400e(ENC)=2000e (3.7).
5. noise contribution to a signal variation is not taken into account.
6. pitch (strip-to-strip distance) is 100um.
7. .deposit charge is eaten by one strip if an hit interaction point is within 30um from center of the
strip (digital zone).
8. deposit charge is shared between two neighbouring strips if an hit interaction point is within the
area from 30um to 70um from center of a strip ( analog zone).
9. AC coupling is simulated with response of differentiation circuit
where
t - current time,  - circuit time constant,  - delay time, (t)=1, if t>0, (t)=0, if t<0.
10. the signal is sampled with 90ns delay in respect to the leading edge of it.
11. position of the hit is determined by central gravity method.
Example.
Charge deposit
in strip#4
Charge4=89
Input of the AC-circuit.
Hi#1
Position1=4.59
Charge1=43
Hi#2
Position2=4.35
Charge2=75
Hit#3
Position3=10.19
Charge3=40
Charge deposit
in strip#3
Charge3=12
Charge deposit
in strip#5
Charge5=17
Charge deposit
in strip#10
Charge10=40
Hit#4
Position4=15.41
Charge4=51
Charge deposit
in strip#15
Charge16=38
Charge deposit
in strip#16
Charge16=13
Output of the AC-circuit.
Sampled value
in strip#4
S4=85
Sampled value
in strip#3
S3=11
Sampled value
in strip#5
S5=15
Sampled value
in strip#10
S10=37
Sampled value
in strip#15
S15=35
Sampled value
in strip#16
S16=9
ERR n = Position n -
Xkn Skn + Xk+1n Sk+1n
Skn + Sk+1n
ERRQ n = Charge n - [Skn + Sk+1n]
Results:
Position resolution distortion due to AC-coupling.
Runs=100. Occupancy=5%.
Entries
Almost ideal case
 = 1ms , C = 1000nF
occupancy=5%
m
Entries
 = 1us , C = 1000pF
occupancy=5%
m
 = 150ns , C = 150pF
Entries
occupancy=5%
m
Results:
Position resolution distortion due to AC-coupling.
Runs=100. Occupancy=5%, 7.5%, 10%.
Entries
Almost ideal case
 = 1ms , C = 1000nF
occupancy=5%
m
 = 1ms , C = 1000nF
Entries
occupancy=7.5%
m
Entries
 = 1ms , C = 1000nF
occupancy=10%
m
Results:
Amplitude distribution distortion due to AC-coupling.
Runs=100. Occupancy=5%.
Entries
 = 1ms , C = 1000nF
occupancy=5%
Initial Landau
distribution
Arbitrary units
 = 1us , C = 1000pF
Entries
occupancy=5%
Initial Landau
distribution
Arbitrary units
Entries
 = 150ns , C = 150pF
occupancy=5%
Initial Landau
distribution
Arbitrary units
Results:
Amplitude distribution due to AC-coupling.
Runs=100. Occupancy=5%, 7.5%, 10%.
Entries
 = 1ms , C = 1000nF occupancy=5%
Initial Landau
distribution
Arbitrary units
 = 1ms , C = 1000nF occupancy=7.5%
Entries
Initial Landau
distribution
Arbitrary units
Entries
 = 1ms , C = 1000nF occupancy=10%
Initial Landau
distribution
Arbitrary units
Results:
Base line fluctuation due to AC-coupling.
Runs=100. Occupancy=5%.
=100us. C=100nF.Occupancy=5%
Total noise
tot=[el2+bl2]0.5=1.3el=520e
Entries
Base line fluctuation
bl=346e,
mean=533e
Electronic noise
bl=400e, mean=0
Arbitrary units
=500us. C=500nF.Occupancy=5%
Total noise
2
tot=[el +bl2]0.5=1.04el=416e
Base line fluctuation
bl=107e,
mean=110e
Electronic noise
bl=400e, mean=0
Entries
Arbitrary units
=1000us. C=1000nF.Occupancy=5%
Total noise
tot=[el2+bl2]0.5=1.01el=404e
Base line fluctuation
bl=64e, mean=40e
Electronic noise
bl=400e, mean=0
Entries
Arbitrary units
Results:
Base line fluctuation due to AC-coupling.
Runs=100. Occupancy=5%, 7.5%, 10%.
=500us. C=500nF.Occupancy=5%
Total noise
2
tot=[el +bl2]0.5=1.04el=416e
Base line fluctuation
bl=107e,
mean=110e
Electronic noise
bl=400e, mean=0
Entries
Arbitrary units
=500us. C=500nF.Occupancy=7.5%
Total noise
tot=[el2+bl2]0.5=1.08el=432e
Base line fluctuation
bl=160e,
mean=160e
Entries
Electronic noise
bl=400e, mean=0
Arbitrary units
=500us. C=500nF.Occupancy=10%
Total noise
2
tot=[el +bl2]0.5=1.13el=452e
Base line fluctuation
bl=213e, mean=213e
Electronic noise
bl=400e, mean=0
Entries
Arbitrary units
Conclusion.
1. Hit position information is almost not distorted by ACcoupling. Position resolution remains almost the same when the
coupling capacitors are in range down to 150pf (=150ns) used even
under relatively high occupancies of 10%.
2. Amplitude information has been heavily distorted when AC
circuit with small time constant is used (below 1ms, capacitor is
1000nF). No considerable amplitude distortion can be noticed even
under 10% occupancy if capacitor of 1000nF is used.
3. AC-coupling causes base line fluctuation and hence extra noise.
If the coupling capacitor is 500nF (=500us) the total noise increases
by 13% in the worse case when occupancy is 10%.
Results on AC-coupled ALABUF testing.
14.10.2002.
Vladimir Gromov.
NIKHEF, Amsterdam, the Netherlands.
Objectives of the testing.
By doing measurements with a real set of signals I am going to confirm Monte-Karlo
simulation earlier carried out. I will prove that:
a). additional noise (base line fluctuations) occurs due to ac-coupling in front of the
ALABUF chip.
b). the smaller nominal of the coupling capacitor the bigger the additional noise is.
c) the smallest acceptable (additional noise is negligible in comparison with expected
electronic noise of HAL25 chip) value of the capacitor is below 100nF.
Introduction.
According to Monte-Karlo simulations ac-coupling causes signal distortion and base line
fluctuations while fast (10MHz) analog signal read is going on. The fluctuations slightly modulate the
red-out signal giving a deviation from the initial value. Such a deviation can be interpreted as an
additional noise and assigned with statistical parameters (standard deviation  and mean value). Taking
into consideration expected electronic noise of HAL25 chip, we are able to find out operation conditions
under which the additional noise contribution becomes negligible.
The effect we are looking at is very small (0.02*MIP) it makes us to avoid any side distortions
capable to hide the effect. Namely settling of the AWG signal must be better 1% within 100ns and the
ALABUF output signal must fit to the full dynamic range of 12-bit ADC (-1V….+1V). That is why an
attenuators and the second ac-coupling used between the ALABUF and the ADC card. The second accoupling chain does almost nothing to the signal shape as long as its time constant is very large (=10uF
1k=10us).
The experimental set-up.
ALABUF
Gain=5.9
Cin
Output+
Attenuator
20dB
Input+
Output+
Output-
Attenuator
20dB
Input-
Output-
Arbitrary
Waveform
Generator
5V range,
Freq=10MHz
5
100
5
Cin
10uF
ADC card
PCI-DAS 4020/12
Input1
[-1V…+1V]
Input2
Fig. 1. Experimental set-up used for the testing.
10uF
1k
1k
1. Calibration (Cin=10uF).
To start with the real measurements an accurate calibration has been carried out first. To do so I
generated two set of number to cover 5V range . Then I loaded the sets into Arbitrary Waveform
Generator device, which generated stimulus signals for the ac-coupled ALABUF chip (see Fig.1, Fig.2).
The ALABUF chip is self-biased circuit operating at +1.25V on the input pads. Therefore it cannot be
coupled to an external generator but via capacitors. For the calibration purpose I used capacitors as large
as 10uF. In this case signal distortion is surely are below level of interest.
100ns
AGW
positive
output
50mV
5V
100ns
#5
#3
#1
#2
#4
#6
“zeros”
#2
#4
#6
#1
AGW
negative
output
#3
-50mV
#5
-5V
100ns
100ns
#100
Fig. 2. Calibration. Example of AGW output signals.
Due to signal attenuator (0.9) behind of the ALABUF chip its dynamic range seems a little bit
narrower than the actual one (1.05V) (see Fig.3, Fig.4).
Calibration of ADC Channel #0 (positive), AWG Channel#1. Scaling factor is (X-2034)/2048
1
Uout_pos, V
Input
data
(generated
number).
0.9
0.8
Scaled output
data
(positive
output of the
ALABUF after
analog-to-digital
conversion).
0.7
Sg05P
Sg05Pn
0
0.6
k
k
1 0.5
0.4
0.3
0.2
0.1
0
0
0.25
0.5
0.75
1
1.25
(k
5
9) 
200
Fig. 3. Calibration. Positive output of the ALABUF chip.
1.5
1.75
2
2.25
Uin_pos, V
2.5
Calibration of Channel #1(neggative), AWG Channel #2 . Scaling factor is (X-2079)/2048
0
0.1
0.2
0.3
Sg05N
k
Sg05Nn
0
0.4
k
1 0.5
Scaled output
data (negative
output of the
ALABUF after
analog-to-digital
conversion).
0.7
0.8
0.9
Uout_neg, V
1
0
0.25
0.5
0.75
1
1.25
(k
S1
Input
data
(generated
number).
0.6
1.5
1.75
5
9) 
200
1
READPRN( "H:\ASCII_to_WFM\arbascii\signal21" ) 
2.27
2
2.25
2.5
-Uin_neg, V
0
( READPRN( "H:\ASCII_to_WFM\arbascii\sig
S4
2048
Fig. 4. Calibration. Negative output of the ALABUF chip.
Difference between input and output data lies in range (3mV) is a result of imperfection of the
AWG generator. This imperfection restricts sensitivity of the experimental set-up to the effect of base
line fluctuation. Shift of the values corresponding to even samples (zeros) is 4mV. It occurs due settling
process following the AWG pulse. The greater the pulse is the greater its residual becomes giving
shifting values instead of a fixed level (see Fig 5, Fig.6).
0.01
0.01
Uout_pos, V
End of the ALABUF dynamic
range
Imperfection of the AWG
0.0083
0.0067
0.005
0.0033
Sg05P
k
Sg05Pn
0.0017
k
1
0
0
0.0017
0.0033
0.005
Shift of the“zeros”
0.0067
0.0083
0.01
0.01
0
0
0.25
0.5
0.75
1
1.25
(k
5
9) 
200
1.5
1.75
2
2.25
Uin_pos, V
2.5
2.5
Fig. 5. Difference between generated data and the ALABUF chip output data. Positive output.
0.01
0.01
0.0083
Shift of the“zeros”
0.0067
Uout_neg, V
Imperfection of the AWG
0.005
0.0033
Sg05N
k
Sg05Nn
0.0017
k
1
0
0
0.0017
0.0033
0.005
End of the ALABUF dynamic
range
0.0067
0.0083
0.01
0.01
0
0.25
0.5
0.75
1
0
1.25
(k
1.5
5
9) 
200
1.75
2
2.25
2.5
2.5
-Uin_neg, V
Fig. 6. Difference between generated data and the ALABUF chip output data. Negative output.
2. The measurements
MathCad facilities have been used for the data generation. For each pattern there were 768 numbers
generated according to charge left in the strip of the detector. Landau distribution has been taken into
account as well as 5% occupancy on the strips (see “Study of AC-coupling impact on SSD performance
by means of Monte-Karlo simulation”). When running at 10MHz frequency the numbers convert into a
set of 100ns pulses (see Fig.7). In total there were 39 patterns coming with 250us gap in between (see
Fig.8).
Genetated signal
Signal
at
the
ALABUF output.
Fig. 7. An example of the signals to be analyzed.
Time, ns
Pattern#1
768st 5%=38events
1
Pattern#2
Pattern#3
Pattern#4
Pattern#5
1
0.8
768strips 100ns=76.8us
0.6
T
250us
ms
0.4
0.2
0
0
0
0
1 10
2 10
4
3 10
4
4 10
4
4
5 10
4
6 10
4
7 10
8 10
4
4
9 10
9
4
0.001
0.0011
0.0012
0.0013
0.0014
0.0015
0.0016
ms 100 10
0.0017
3
1.635 10
Fig. 8. An example of the signals to be analyzed.
Amplitude distribution of the ALABUF output signals given in Fig.9. A substantial portion of the
small signals is caused by charge division mechanism built-in into event generator. The amplitude
distribution becomes Landau distribution like when signals from adjacent channels (strips) are summed
up. The most significant information on the plot is that MIP is 180 mV therefore expected electronic
noise of HAL25 is
el=180mV 400e/22000e = 3.3mV
MIP=180mV
536
600
Entries
Amplitude distribution
of
450
the ALABUF output signals
H4a
ka
H45a
ka
Amplitude distribution of the
ALABUF output signals when
signals from adjacent channels
(strips) are summed up.
300
150
0
0
0
0
0.1
0.2
0.3
0.4
0.5
inta
ka
Fig. 9. Amplitude distribution of the ALABUF output signals.
0.6
0.7
0.8
0.9
Uout_pos, V
1
1
. 3. Results of the measurements with Cin=10uF (  10uF1k  10ms).
As it was mentioned for the tests and calibrations there were large capacitors (Cin=10uF) used to
couple the ALABUF chip to AWG generator. It this case difference between generated data and the
ALABUF chip output data caused by the AWG generator imperfection and resolution of the ADC card.
As we can see standard deviation of the difference distribution is  =1.3mV whereas that of expected
electronic noise is el=3.3mV (see Fig.10). It means that the experimental set-up is “sensitive” enough
to observe effect of base line fluctuation we are going to see with smaller coupling capacitors. For the
negative output the resolution is slightly worse ( =1.9mV) hence it is more difficult to observe the
effect after all.
440 exp
G1( x)
Entries
Cin=10uF.
Data
from the positive
output
of
the
ALABUF
chip
400
H4
2
( x ( 0.0083 ) )  1
2
2
0.0013
Expected electronic noise
of HAL25 (el=3.3mV).
(=1.3mV).
i
G1( x )
N( x ) 3 200
0
0.03
0.025
0.02
0.015
0.01
0.005
0
0.005
0.01
0.015
0.02
int  x  x
i
0.025
0.03
Uout_pos, V
Fig. 10. Cin=10uF. Difference between generated data and the ALABUF chip output data. Positive
output.
G2( x)
300 exp
2
( x ( 0.0125 ) )  1
2
2
0.0019
Cin=10uF.
Data
from the negative
output
of
the
ALABUF
chip
400
Entries
H5
Expected electronic noise
of HAL25 (el=3.3mV).
(=1.9mV).
i
G2( x ) 200
N( x ) 2
0
0.03
0.025
0.02
0.015
0.01
0.005
0
int  x
i
0.005
0.01
0.015
0.02
0.025
0.03
Uout_neg, V
Fig. 11. Cin=10uF. Difference between generated data and the ALABUF chip output data. Negative
output.
. 4. Results of the measurements with Cin=100nF (  100nF1k  100us).
When capacitors in front of the ALABUF chip are Cin=100nF, the base line fluctuation determine
difference between generated data and the ALABUF chip output data. Distribution of the differences
becomes wider ( = 2.1mV for the positive output and  =2.4mV for the negative one) (see Fig.12,
Fig.13).
300 exp
G3( x)
Cin=10uF. Data from the positive
output of the ALABUF chip
2
( x ( 0.0072 ) )  1
2
2
0.0021
(=1.3mV).
Entries
H4
H7
400
i
i
G3( x ) 200
Expected electronic noise
Cin=100nF. Data
from the positive
output
of
the
ALABUF
chip
of HAL25 (el=3.3mV).
(=2.1mV).
N( x ) 3
0
0.03
0.025
0.02
0.015
0.01
0.005
0
0.005
0.01
0.015
int  int  x  x
i
i
0.02
0.025
0.03
Uout_pos, V
Fig. 12. Cin=100nF. Difference between generated data and the ALABUF chip output data. Positive
output.
250 exp
G4( x)
Entries
2
( x ( 0.0124 ) )  1
2
2
0.0024
Cin=10uF. Data from the negative
output of the ALABUF chip
(=1.9mV).
400
Expected electronic noise
H5
H8
of HAL25 (el=3.3mV).
i
i
Cin=100nF. Data from the
negative output of the
ALABUF
chip
200
G4( x )
(=2.4mV).
N( x ) 2
0
0.03
0.025
0.02
0.015
0.01
0.005
0
0.005
int  int  x  x
i
i
0.01
0.015
0.02
0.025
0.03
Uout_neg, V
Fig. 13. Cin=100nF. Difference between generated data and the ALABUF chip output data. Negative
output.
Conclusion.
The measurements carried out with a real set of signals show that additional noise (base line
fluctuations) occurs due to ac-coupling in front of the ALABUF chip.
The additional noise becomes “visible” by the experimental set-up when coupling capacitor in front
of the ALABUF chip is smaller than 100nF. It is hiding behind finite resolution of the experimental setup (=1.3mV) if the coupling capacitor is much larger than 100nF.
By reconstructing of the amplitude distributions of the ALABUF output signals I determined the
MIP
value=180mV
and
calculated
expected
electronic
noise
of
HAL25
(el=MIP400e/22000e=3.3mV).
When the coupling capacitor is 100nF, standard deviation of the additional noise is =2.1mV (see
Fig.12). Ratio between expected electronic noise and the additional noise is 2.1mV/3.3mV = 0.63. That
is in reasonable agreement with simulation results 346e/400e = 0.86 (see “Study of AC-coupling impact
on SSD performance by means of Monte-Karlo simulation”). The discrepancy is most probably caused
by imperfection at the stage of fitting the data with Gaussian functions.
Set-up.
The electronics
to test
Real time process
containing pulses coming
out of the front-end chip.
ALABUF
Gain=5.9
Cin
Output+
Attenuator
20dB
Input+
Output+
Output-
Attenuator
20dB
Input-
Output-
Arbitrary
Waveform
Generator
5V range,
Freq=10MHz
Real time process
containing pulses
coming out of the
electronics.
5
100
5
Cin
10uF
ADC card
PCI-DAS 4020/12
Input1
[-1V…+1V]
Input2
10uF
1k
1k
The file contains description
of pulses coming out of the
fron-end chip
Detector
Simulator
Restored information
over the events (precise
track
Initial information over
the events (precise track
position in space Xi
Event
Generator
n
position in space X i
position in time Ti)
Ai
n
position in time T i)
MathCad software
Occupancy
1.147
Generated events
1
0.8
Sig
m0
Signals coming out of the front-end chip.
0.6
 0.385
Q
j0  0 41
0.4
0.2
0
0
0
0
5000
4
1 10
4
1.5 10
4
2 10
4
2.5 10
3 10
4
4
4
3.5 10
4 10
100
m 100  Pos
j0  0
4
4.5 10
5 10
4
5.5 10
4
4
6 10
4
6.5 10
7 10
4
7.5 10
4
76800
Signal before the AC-coupling
mistake
Signal after the AC-coupling
mistake
Strip#N
Strip#N+5
Strip#N+1
Strip#N+6
Cin=100nF. Difference between generated data and the ALABUF chip
output data.
Entries
el=3.3mV
bl=2.1mV
Cin=100nF. Data
from the positive
output
of
the
ALABUF
chip
Expected electronic noise
of HAL25 (el=3.3mV).
(bl=2.1mV).
Uout_pos, V
 = (2 + 2el)0.5
∑
bl
 = 1.2∙el
∑
Report on analogue buffer chip (ALABUF) development in 0.25u CMOS technology
for the ALICE Silicon Strip Detector (SSD).
28 March 2002.
V. Gromov (vgromov@nikhef.nl), R. Kluit.
ET NIKHEF, Amsterdam.
Abstract.
For the purpose of driving of analog signals from the on-detector front-end electronics of the ALICE SSD to
the off-detector ADC, an analog buffer chip (ALABUF) has been designed. The design is performed in 0.25
CMOS technology.
Inputs of the design as well as the design goals specification to be met are described along with circuit
optimization procedures and detail chip description.
Results on testing of the chips taken from the experimental batch are presented and compared to the
simulations.
Fig.1. Principal diagram of
the on-detector electronics of ALICE SSD
Read-out rate =10MHz.
P-side of the
SSD detector.
Bias= 55V
ADC
N-side of the
SSD detector.
Bias= 0V
Fig.2. The
AC-coupling effect on the shape of the signals.
Signal before the AC-coupling
mistake
mistake
Signal after the AC-coupling
Strip#N
Strip#N+5
Strip#N+1
Time, ns
Strip#N+6
Fig.2. Principal diagram of the analog buffer board.
.
Output differential
signal,
OutP-OutN
mV
Chip#2, channel 2B
5.66 (InP-InN)
Chip#2, channel 1B

Input differential signal, InP-InN, mV

 
Fig.20. Measurements of linearity and dynamic range of the ALABUF chips.
Settling time 20ns
OutP, Ch4
InP, Ch1
InN, Ch2
OutN, Ch3
Fig.24. Measurements. Differential transient response of the ALABUF chip.
Talk at the werkbespreking.
ALABUF chip for the ALICE SSD detector.
Design and test.
Vladimir Gromov. ET.
9.04.2003.
Content.
1. General information.
2. Function of the ALABUF chip.
3. Principal and schematic diagram.
4. Main specifications of the chip.
5. Detector simulator approach to test AC-coupling in front of the
chip.
6. The experimental set-up.
7. The test results discussion.
8. Conclusion.
Conclusion.
To meet the needs of the read-out of the ALICE SSD a new analog
buffer chip ALABUF has been designed in 0.25u CMOS technology.
In order to test the new-designed chip we have developed a new
method. The method is based on simulation of the operation of the
detector. The output of this simulation is a file that describes signal
process coming out the detector. The file is an input for the software
controlled Arbitrary Waveform Generator. The generator produces
“detector-like” real time signals for the inputs of the tested chip. The
ADC card digitizes and saves signal process at the output of the chip
in a file. By taking note of difference between the input and the
output files we analyze signal distortion caused by chip. Thank to
statistical nature of the analysis tiny effects become visible.
I suggest that a many of other applications can make use of this
method.
1.
1) ALABUF chip for the ALICE SSD detector. Design and test. Is the
subject of my present talk.
2) Here are the issues I would like to cover today.
3) Right after a few general words I am going to tell over the function of
the ALABUF chip in the detector read-out.
4) Then we will have a closer look at the inside of the chip as well as its
performance and its main specifications.
5) Further I will disclose the substance of the approach we have developed
to test the chip.
6) We will examine the experimental set-up and will briefly discuss the
test results.
7) Finally, of course, I will draw some conclusions out this development.
2 . 1) Since two years ago, VLSI group has been taking part in project
ALICE.
2) The group is responsible for development and mass production of two
chips in 0.25u CMOS technology for the Double-Sided Silicon strip detector.
3) Chip I am going to tell is an analog buffer. It is designed for taking
analog signals from the front-end chips on the both sides of the Silicon strip
detector, amplifying and sending them over 25m twin-pair cable to an offdetector ADC.
4) The read-out rate will be 10MHZ.
5) The Front-end chips on P-side and N-side of the detector will operating
at different potentials.
6) So as to couple the chips to the rest of the world we had to break DCpath and insert capacitors in front of the analog buffer chip.
7) However the capacitive or AC-coupling is never for free, it bring a lot of
troubles at the same time. What are they?
8) Passing over the capacitor each signal leave behind a tail of negative
polarity, which actually is the capacitor discharge.
9) The tails overlap each other forming a fluctuating substrate for next
coming signals.
10) By making the capacitor larger we make the effect less significant on
the one hand.
11) But on the other hand then the capacitor will store a huge charge as
long as it holds 55Volts. In the case of accidental break down the will likely
destroy all the electronics around here.
12) Therefore we fall into dilemma. From safety point of view we would
prefer a small capacitor although to minimize signal distortion the capacitor
must be large.
13) You may often come across the same problem in your application,
therefore it makes sense to give you a perspective on the approach we have
developed to resolve it.
3 . 1) First, however, I would like you to have a look at the ALABUF chip
itself.
2) The analog buffer called ALABUF consists of a feed-backed operational
amplifier and a pair of analog multiplexers to be able switch inputs to the bus
on any side of the detector.
3) So as to save power when the chip is out of use a Disenabling option has
been implemented.
4) Schematic of the amplifier is on the plot.
5) It has a fully differential configuration (differential input and differential
output).
6) The two-stage structure with Miller capacitors guarantees high open loop
gain factor (50dB) and good stability (phase Margin is 83.5 degrees).
7) Resistive feedback sets differential gain (5.9).
8) The common mode feedback takes care that outputs and inputs are kept
at the middle of the power supply range.
4.
1) The amplifier stays linear until the output swing does not exceed
1.85V (VDD=2.5V).
2) The Step response signal smoothly settles to the dc level in the course of
20ns.
5 . 1)
The ALABUF chip houses 2 channels of the analog buffers on area
as large as 2mm by 2mm.
6 . 1) Main specifications of the analog buffer have been put together in
the Table. All of them comply with the requirements.
7 . 1). As soon as the electronics has been designed and preliminary tests
are in line with the simulation you may start to think over a functionality test,
when the ALABUF operates as a link of the read-out chain being set under
real conditions.
2). The most common way to provide a real condition tests and to judge
the functionality is to go the real beam of particles. However this way
involves a lot of facilities, time and manpower.
3). At the same time we could think of a signal source capable of
simulating processes taking place in the detector.
4). What, in fact, make the detector on the beam a special signal source,
as long as for the electronics it is just a signal source?
5). First we know that the signals are disorderly spread in the time domain.
6). Second the value of the signal varies in a very wide range.
7). Third a charge will be shared between two neighbouring strips in a
special manner.
8 . 1). If we put all these distribution and dependences into the MathCAD
software we will generate a file that contains description of pulse coming out
of the “Detector”.
2). The Arbitrary waveform generator converts the file into real time
signals and sends them to the AC-coupled ALABUF chip.
3). The ADC card does digitizing of the chip output signals.
4). We estimate signal distortion by taking difference between signals
coming in and out of the chip.
9 . 1). This picture illustrates how the signal patterns look like.
2). Each pattern virtually corresponds to one trigger event.
3). It takes 76.8us to read-out all the strips.
4). After 250us next trigger is coming and the read-out starts again.
10 . 1). Lets have a look at the test results.
2). Here you see statistical distributions of the mistakes.
3). On Y direction are entries, on X direction are the mistake’s value.
4). In the first case the coupling capacitor is huge 10uF, the difference
between input and output signals comes from nonideality of the system itself.
5). When the coupling capacitor is smaller 100nF contribution of the base
line fluctuation prevails in the distribution.
6). All the mistakes are negative. This is due to the fact that the base line is
sinking a little bit.
7). Moreover the distribution has a nonzero width that means that the base
line fluctuates.
8). We may consider the base line fluctuation as additional noise .
9). So as to evaluate it we fit it this distribution with a Gaussian curve.
10). Standard deviation or root mean squire value is a figure to compare
the base line fluctuation to the expected electronic noise that is always present
in here.
11). According to the general rule to sum independent random processes
the total noise goes to 3.9mV.
12). It yield 20% addition the electronic noise. To make this addition
smaller we must choose for the larger capacitor.
11 . Read CONCLUSION.
.
To figure out what the real conditions are we must recall that signals coming out of the detector are of random
amplitude and expected to be a random process in the time domain.
As an engineer I design electronics for particle detectors. I always want to make sure that the electronics is the best to fulfill functions it is designed
for.
What are these functions? In general, the electronics together with the particle detector constitute a device by means of which we are able to obtain
some information over the particles. A Tracking system reconstructs tracks of the particles and a Calorimeter measures energy. In reality the information is
wrapped in pulses coming out of the detector. The function of the electronics is to process the incoming pulses in order to unwrap the information over the
particles in the best way.
How can I judge the functionality of the electronics? The most common way to provide a real condition tests and to judge the
functionality is to go the real beam of particles. However this way involves a lot of facilities, time and manpower. At the
same time we could think of a signal source capable of simulating processes taking place in the detector. Having an Event
Generator inside such a source could generate an array with initial information over the events (precise track position for
instance) and a file, which contains description of pulses coming out of the front-end chip. This file is an input for the
Arbitrary Waveform Generator to produce a real time process to be a stimulus for the AC-coupled ALABUF chip. I could
use signals at the output to restore the initial information. Difference between initial information and restored information
numerically characterizes distortions caused by the tested electronics. I may consider the difference as a figure of merit when
tuning the electronics in order to reach the best functionality.
This approach has been developed to test electronics for the ALICE Silicon Strip Detector. A software signal source
plays the key role in the set-up. It is made on the basis of MathCad software and involves:
1) random distribution of the events in time according to Poisson statistics
2) Landau statistics of the charge deposition in each event
3) mechanism of charge sharing between adjacent strips of the detector.
Besides the software signal source the experimental set-up includes an Arbitrary waveform generator (12 bit in range
50mV…5V, clock frequency up to 250MHz), an ADC card (12 bit in range –1V…+1V). The electronics to test is an analog
buffer chip ALABUF and AC –coupling network in front of it.
What in particular we are going to look at. It is to AC or capacitive coupling that each signal is followed by a long tail
of negative polarity, which in fact is discharge the capacitor. The tails pile up each other thus forming a base line fluctuation.
The fluctuation is actually noise that spoils performance of the detector. For safety and space reasons it is preferable to keep
values of the capacitors as low as possible. On the other hand the smaller the capacitor is the more substantial the base line
fluctuation noise becomes. Our goal is to determine a minimum value where the base line fluctuation noise gives a not yet
noticeable addition to the intrinsic electronic noise.
Results of the measurements are given here. Difference between primary (generated) data and data restored after
passing through the electronics to test is a histogram with a fitting curve on top of it. As for the fitting function it as a normal
Gaussian distribution. The standard deviation parameters of the distribution are the values to compare with those of expected
electronic noise.
When the capacitor is enormously big (10uF), difference between primary and restored data is not caused by the tested
items but the finite resolution of the set-up. This is the ideal case. If the capacitors are becoming smaller the base line noise is
approaching the expected electronic noise. However at the value of 100nF the standard deviation of the expected electronic
noise is still 1.5 times as large that for the base line fluctuation noise.