MAROC, a generic photomultiplier readout chip - Omega

MAROC, a generic photomultiplier readout chip
Sylvie Blin, Pierre Barrillon and Christophe de La Taille, member IEEE
Abstract– The MAROC ASICs family is dedicated to the
readout of 64-channel Multi Anode PMT and similar detectors.
Its main roles are to correct the gain spread of MAPMT channels
thanks to an individual variable gain preamplifier and to
discriminate the input signals (from 50fC i.e 1/3 photo-electron)
in order to produce 64 trigger outputs. A multiplexed analog
charge output is also available with a dynamic range around 10
pe (~1.6 pC) and a 12 bit Wilkinson ADC is embedded. Three
versions of this chip have been submitted. MAROC 2 is the
production version for the ATLAS luminometer and MAROC3 is
a version with lower dissipation and significant improvements
concerning the charge (30 pe: ~5 pC) and trigger (discrimination
from 10fC). This third version showed very good characteristics
that are presented here.
Front end electronic
24 MAPMTs
Fibers connectors
Scintillating fibers
×10
I. INTRODUCTION
MAROC stands for Multi-Anode ReadOut Chip. It has been
designed to readout 64 channels photomultipliers. The main
application that will use MAROC is the ATLAS luminometer
[1] which is made of 8 Roman Pots (RP). They consist in
0.5mm2 scintillating fibers arranged in 10 planes in the U and
V directions. Each plane is composed by 64 fibers. The light
produced is then collected by multi-anode photomultiplier
tubes H7546 from Hamamatsu [2], that will run at 800 to 950
V which corresponds to a gain of 3.105 to 2.106. These PMTs
have an important non-uniformity that cannot be corrected by
applying a different high voltage to each channel. Therefore
one has to amplify the output signal with different factors for
each channel. Figure 1 shows a schematic of a Roman Pot and
the front end electronic. In total eight RPs, equipped with
around 200 chips MAROC2 [3] installed on dedicated PCB
called PMF [4], have been installed at 240m from the ATLAS
interaction point.
Other applications using PMTs or silicon PMs, like medical
imaging and neutrino experiments, are using second and third
versions of MAROC.
S. Blin, P. Barrillon, C. de la Taille are members of the
CNRS/IN2P3/LAL/OMEGA, UPS 11 - Bat 200, Orsay, FRANCE (email:blin@lal.in2p3.fr)
Roman Pot
Fig. 1. Schematic of a Roman Pot and the front-end electronics.
II. ASIC OVERVIEW
A. Main features
The third version of this ASIC has been submitted in order
to fix few bugs and improve performances. Similarly to
previous versions it has been designed in AMS Si-Ge 0.35 µm
technology and developed with a CQFP240 package. It has an
area of 16 mm2 (4 mm × 4 mm) and operates with 3.5V power
supply. Its power consumption is around 220 mW
(3.5mW/ch).
Fig. 2. Layout of the third version of MAROC
The block diagram of the ASIC is given in Figure 3. For
each of the 64 channels, the PM signal is first amplified thanks
to a variable gain preamplifier which has low noise and low
input impedance (about 50 ohms) to minimise crosstalk (lower
than 0.25%). It allows compensating for the PM gain
dispersion up to a factor 4 to an accuracy of 1.5% with 8 bits.
The amplified current then feeds a slow shaper combined with
two Sample and Hold buffers to store the charge in 2pF and
provide (5 MHz) an analog and digital multiplexed charge
output up to 5pC. A second S&H has been added in order to
allow the measurement of both the baseline and the maximum
of the signal. The digital charge output is provided by a 8, 10
or12 bit ADC Wilkinson.
In parallel, 64 trigger outputs are produced via fast
channels: two fast shapers (one unipolar and another bipolar)
followed by one discriminator are dedicated for the photon
electron counting and one bipolar fast shaper with lower gain
followed by its discriminator has been added to provide a
trigger signal at higher input charge. The 2 discriminator
outputs are multiplexed to provide only 64 trigger outputs.
The thresholds are loaded by 2 internal 10-bit DACs common
for the 64 channels.
Two outputs (OR1 and OR2) are implemented to make a
OR of 64 triggers obtained from the first and the second
discriminators. This new feature can be use to perform a
charge measurement in auto trigger mode. Like MAROC2 the
sum of up to eight preamplifier outputs is produced.
8 SUMs
- A charge measurement up to 30 photoelectrons with
a linearity of 2% or better
- A cross talk of 1%
- A noise of 2 fC
The results of the characterization tests carried out to check
these specifications are presented in the next section.
III. CHARACTERISATION TESTS
A. Laboratory test set-up
The test board developed at LAL in order to perform the
characterization tests is showed on Figure 4. This top view
exhibits the principal elements: the MAROC chip in its
package, the control FPGA (Altera), the USB port and the 64
channels PM socket.
Hold 1
Hold 2
(pedestal) (pulse max)
CHANNEL 63
in_ADC
CHANNEL 0
EN_ADC=1 and
H1H2_choice=1
Fig. 4. Picture of the test board used for characterization tests.
S&H
SUM of 8
channels
gnd
3
4
MUX
S&H
Variable
slow shaper
RC
cmd_sum
EN_ADC=1 and
H1H2_choice=0
Multiplex
charge output
gnd
cmd_ss
5
cmd_fsu
in63
cmd_fsb_fsu
Unipolar
Fast shaper
Preamplifier
in0
Gain correction
(8 bits)
4
cmd_fsb
cmd_fsb_fsu
Bipolar
Fast shaper
Vth 0
Half Bipolar
Fast shaper
Vth 1
MAROC3
Vbandgap
Bias
Hit_ch0
Hit_ch63
mask_2
d1[63..0]
LVDS/CMOS
Bandgap
MUX
d2
Discri
Common to the 64 channels
Clk_40M
Clkb_40M
d1
Discri
mask_1
4
d2[63..0]
DAC 0
Vth 0
DAC 1
Vth 1
10 bits
OR1
OR2
Wilkinson ADC
in_ADC
8bits, 10bits or 12 bits
ADC_output
10 bits
Fig. 3. Block diagram of MAROC3
B. Requirements
The main requirements concerning MAROC are the
following:
- A 0-4 variable gain preamplifier in order to correct
for the PM non uniformity.
- The trigger efficiency must reach 100% for a signal
larger than 1/3 of photoelectron (pe), which
corresponds to a charge of 50 fC for a PM
functioning at gain 106 (900 V).
In addition to the test board and the control PC, other
equipments were used: a pulse generator to provide the input
signal through a 10 pF capacitor, a voltmeter and an
oscilloscope in order to visualize the charge or trigger outputs.
The board and the other elements of the set-up were
controlled through LabVIEW software [5] via USB and GPIB
respectively. A complete set of tests was available to check
MAROC performances [6]. The data analysis was performed
mainly with Igor, PAW and ROOT software [7].
B. Performances
1) Trigger outputs
The trigger signal appears when the fast shaper output
signal (Bipolar fast shaper: negative pulse for example)
crosses the threshold value set by a 10-bit DAC which is
common to all channels. DAC0 and DAC1 are respectively
associated to the first and second discriminator. A slow
control bit allows to improve the accuracy of DAC0 (small
DAC) while reducing its ranges. The figure 5 shows the
linearity of these DACs which consists in measuring the
amplitude (Vdac) obtained for different DAC register values.
By fitting this line in the region without saturation, we
obtained a nice linearity of ± 0.2 % on a large range.
The evolution of the 50% trigger efficiency input charge as
a function of the channel number without and with gain
adjustment gives a mean value is about 461 uDAC and rms
decrease from 8 to 5 uDAC. Similarly good performances
were reached scanning the injected charge while the threshold
was fixed.
The effect of the threshold on these s-curves was studied by
looking at them on selected range of the DAC. S-curves were
recorded for a single channel for 100 different DAC values
(Figure 8).
Fig. 5. DAC linearity
Well known S-curves were also studied. They correspond to
the measurement of the trigger efficiency during a scan of the
input charge or the threshold while the other parameters, like
the preamplifier gain, are kept constant.
Figure 6 and 7 represent the trigger efficiency, obtained
with FSB1, as a function of the threshold for the 64 channels
of a single chip with 50fC (which corresponds 1/3pe.). This
measurement has been performed with all channels set at unity
gain or after an adjustment of gain showing nice uniformity.
Fig. 8. Trigger efficiency of the one channel as a function of the injected
charge at different threshold value
Figure 9 represents the evolution of the 50 % trigger
efficiency input charge as a function of the applied threshold.
The gain of the fast shaper is about 2.3V/pC and the minimal
input charge for which we can trig is 5fC.
Fig. 6.Trigger efficiency of the 64 channels as a function of the DAC value for
50fC injected charge and the same preamplifier gain
Fig. 9. 50% trigger efficiency as function of the threshold
2) Charge output
The charge is digitized by a 8, 10 or 12-bit ramp ADC. The
pedestals of all channels have been measured (Figure 10) and
show a nice homogeneity and a dispersion equivalent of 3fC at
unity gain.
Fig. 7. Trigger efficiency of the 64 channels as a function of the DAC value
for 50fC injected charge and adjustment preamplifier gain
charge data output. If the preamplifier gain is lower than 8 the
charge measurement can reach 30pC. The trigger and charge
crosstalk have been decreased. All the characteristics tested so
far have matched the requirements [8]. This will allow the use
of MAROC3 chips for all experiments using MaPMTs.
REFERENCES
[1]
Fig. 10. Pedestals as function of channel via 12-bit ADC
Fig. 11. Histogram of pedestals for 3 channels
The linearity of the charge measurement has been checked
at different preamplifier gain (Figure 12). A linear fit was
performed on a limited range (e.g: 0 to 3pC for unity gain and
0 to 30pC for gain 1/16) to get 2% linearity. The gain of the
charge measurement is from 636 uadc/pC (159 mV/pC) for the
unity gain to 35 uadc/pC (9 mV/pC) for the gain 1/16.
Fig. 12. Charge linearity for different preamplifier gain.
IV. CONCLUSION
Most of the tests carried out have showed improved
performances with respect to the other versions of MAROC.
With MAROC3 it is easy to obtain triggers for an input charge
down to 10fC and to adjust the preamplifier gain to have good
homogeneity. In this ASIC the internal ADC permits digitized
ATLAS Collaboration, ATLAS Forward Detectors for Measurement
of Elastic Scattering and Luminosity Determination, Technical Design
Report, CERN/LHCC/2007-xxx
[2] Hamamatsu web site, PM H7546B datasheet.
[3] P. Barrillon et al., MAROC: Multi-Anode ReadOut Chip for MAPMTs,
proceedings of 2007 IEEE conference.
[4] P Barrillon et al., PMF: the front end electronic of the ALFA detector,
Nuclear Inst. and Methods in Physics Research, A 623 (2010), pp.
463-465
[5] LabVIEW web page: http://www.ni.com/labview/
[6] P. Barrillon et al., MAROC3 Labview software manual – USB version
1, April 2010.
[7] Igor Pro, web site: http://www.wavemetrics.com/
Paw software web site: http://paw.web.cern.ch/paw/
ROOT software web site: http://root.cern.ch/
[8] P. Barrillon et al., 64-channel Front-End readout chip – MAROC3
datasheet – http://omega.in2p3.fr