# Signal formation and sharing in AC-LGADs using the ALTIROC 0 front-end chip

G. D’Amen111corresponding author W. Chen  C. De La Taille  G. Giacomini  D.
Marchand  M. Morenas  C. Munoz Camacho  E. Rossi  N. Seguin-Moreau  L. Serin
A. Tricoli  P.-K. Wang

###### Abstract

The development of detectors that provide high resolution in four dimensions
has attracted wide-spread interest in the scientific community for
applications in high-energy physics, nuclear physics, medical imaging, mass
spectroscopy as well as quantum information. However, finding a technology
capable of fulfilling such aspiration proved to be an arduous task. Among
other silicon-based candidates, the Low-Gain Avalanche Diode (LGAD) has
already shown excellent timing performances but proved to be unsuitable for
fine pixelization. Therefore, the AC-coupled LGAD (AC-LGAD) approach was
introduced to provide high resolution in both time and space, making it a
promising candidate for future 4D detectors. However, appropriate readout
electronics must be developed to match the sensor’s fast-time and fine-pitch
capabilities. This is currently a major technological challenge. In this
paper, we test AC-LGAD prototypes read out by the fast-time ASIC ALTIROC 0,
originally developed for the readout of DC-coupled LGADs for the ATLAS
experiment at the HL-LHC. Signal generated by either betas from a 90Sr source
or a focused infra-red laser were analyzed. This paper details the first
successful readout of an AC-LGAD sensor using a readout chip. This result will
pave the way for the design and construction of a new generation of AC-LGAD-
based 4D detectors.

##  1 Introduction

Low-Gain Avalanche Diodes (LGADs) are silicon sensors engineered for the fast
detection of minimum ionizing particles (mips) [1]. Being built on thin
silicon substrates (order of 20 - 50 $\mu$m) and thanks to the presence of a
gain layer that boosts the signal by a factor of a few tens, LGADs can achieve
a timing resolution below 30 ps [2]. LGADs were prototyped by CNM (Barcelona,
Spain) under the auspices of the RD50 collaboration at CERN. Nowadays, LGADs
sensors are successfully designed and fabricated by several facilities around
the world. Since their introduction, the interests in the High-Energy Physics
(HEP) community has grown, as detectors based on LGADs will constitute the
High Granularity Timing Detector (HGTD) [3] and MIP Timing Detector (MTD) [4]
upgrades of the ATLAS and CMS experiments at CERN (Geneva, Switzerland),
respectively.

Figure 1: Signal formation in LGAD (left) and AC-LGAD (right) sensors. In AC-
LGADs, signals are capacitively induced to metal electrodes separated from the
active area by a thin dielectric film. The AC-coupled signal is shared among
multiple electrodes.

However, it has been observed that mips crossing close by the pad edge in a
LGAD sensor generate signals that are not amplified, as the generated
electrons - in their drift to the n-type pixel - do not go through the high
electric field. To achieve a uniform multiplication in most of the area and a
high fill factor, the pad pitch must by far greater than the substrate
thickness. Pixel or strip pitches in the order of $\sim$100 $\mu$m are
therefore not achievable in LGAD technology [5].

To overcome this severe limitation, the AC-LGAD concept was developed [6, 7,
8, 9], in which the signal is capacitively induced to metal electrodes placed
over the active area but separated from it by a thin dielectric film made of
silicon oxide or nitride, among others (Fig. 1). Since their fabrication
process is similar to that for standard LGADs, many LGAD facilities around the
world started developing AC-LGADs too. Fabrications aiming at process and
design parameters optimization are on-going, as well as intense testing
campaigns. One of the consequences of the AC-coupling of signal in AC-LGADs is
that signal is shared among neighboring electrodes. Such a property was
demonstrated to provide precise spatial resolution with sparse electrodes
[10].

The most significant limitation for the broad use of AC-LGADs in scientific
applications is the lack of readout electronics that match the precise spatial
and time resolution of AC-LGADs. This article demonstrates that AC-LGADs can
be read out by a state-of-the-art ASIC developed for the acquisition of
signals from standard DC-coupled LGADs.

###  1.1 AC-LGAD sensors as 4D detectors

In recent beam tests, AC-LGADs have demonstrated to achieve spatial resolution
better than one tenth of the strip pitch and timing resolution compatible with
DC-coupled LGADs [11]. Such good spatial resolution results from the
interpolation of signals shared among several AC-coupled electrodes. While
this signal-sharing feature may lead to higher readout occupancy, especially
in high-multiplicity environment, it also makes AC-LGADs first-class
candidates for 4D detectors.

The characterization studies of LGAD and AC-LGAD sensors have been performed
so far on custom-made test-boards (see for example Refs. [12][13]), designed
for the evaluation of a small quantity of sensors. These readout test-boards
are based on discrete electronics, by exploiting either low-noise fast trans-
impedance or RF amplifiers that closely reproduce the current pulse generated
by the sensor. However, these boards are not optimized for power dissipation
and can only readout a limited number of channels; in addition, they typically
do not provide on-line analysis capabilities without the aid of an external
fast oscilloscope.

When looking at large-scale, multi-channel systems targeting 4D tracking, it
becomes clear the need to evaluate the AC-LGAD performance when coupled to
higher-complexity readout systems. For example, in the EIC (Electron Ion
Collider) experiments, AC-LGADs are the sensor baseline to be used in the
Roman Pots detector [14]. The pixel size is expected to be 0.5 mm$\times$0.5
mm, almost 7 time smaller than that used in HGTD/MTD. It is foreseen that the
Roman Pots detector at the EIC will be based on a readout chip tailored to
match the features of AC-LGAD signals and will be based upon an existing
readout ASIC designed for fast-timing and high-bandwidth particle physics
experiments. Such ASIC, called the ATLAS LGAD Timing Integrated ReadOut Chip
(ALTIROC), is demonstrated here to preserve the signal characteristics that
are typical of AC-LGADs, for example signal sharing, and allow fast and
precise tracking.

The ALTIROC chip was developed [15] to readout DC-coupled signals generated in
LGAD sensors while satisfying the strict timing requirements of the upcoming
ATLAS HGTD foreseen for the 2028 High-Luminosity LHC upgrade. The ALTIROC chip
was used to characterize the response of an AC-LGAD sensor to either betas
from a 90Sr source or a focused infra-red (IR) laser. Results obtained using
this setup were compared to the characterization of an identical AC-LGAD strip
sensor attained using a 16-channels readout board designed and produced at the
Fermi National Accelerator Laboratory (FNAL, Batavia, IL, USA).

Section 2 details the experimental setup used in this study, with particular
attention to the layout of the characterized AC-LGAD sensor, the readout
capabilities of the ALTIROC chip, and a breakdown of the particle sources
employed in the study.

Section 3 presents the response of the AC-LGAD sensor readout by the ALTIROC
chip to either an uncollimated flux of beta particles and a focused IR laser.
This is followed by a detailed evaluation of the generated signal amplitude,
full-width at half-maximum (FWHM), jitter and signal sharing between strips.
Finally, the differences in response and signal shape with an identical AC-
LGAD sensor read out by a custom-made RF-amplifier based system are also
presented.

Finally in Section 4 this response is compared to that of the fixed-threshold
discriminator integrated into the ALTIROC chip, allowing for the measurement
of the time over threshold (ToT) of the particle signal. The ToT enables the
use of center-of-gravity algorithms to reconstruct the particle position on
the sensor with sub-pitch precision, thanks to the signal sharing capabilities
of AC-LGADs.

##  2 Experimental setup

The device under test is a strip AC-LGAD sensor fabricated at BNL. The sensor
has an active area of 2$\times$2 mm2, with the metallization layer divided in
16 strips with a 100 $\mu$m pitch separated by a gap 44 $\mu$m wide.

Figure 2: BNL AC-LGAD strip sensors wire-bonded to the ALTIROC 0 ASIC (left)
and a Discrete RF board (right). Only the four central AC-LGAD strips are
wirebonded in the ALTIROC 0 setup. All strips are connected to amplifiers in
the Discrete RF board setup.

Four neighbouring strips indicated as Strip A, Strip B, Strip C and Strip D,
chosen to be far from the the device guard-ring in order to minimize border
effects, were wire-bonded to the four Voltage Pre-Amplifier (VPA) channels on
a ALTIROC chip of version 0-2B (referred to as ALTIROC 0 in the following), as
shown in Fig. 2. The strips on the left and right to this set were wire-bonded
to the same ground as the ASIC to minimize the portion of the device that
remains floating. Fig. 3 details the bonding scheme of the strips on the AC-
LGAD setup. A bias voltage of Vb = -190 V was applied to the AC-LGAD via a
Keithley 2410 source-meter to fully deplete the sensor. The ALTIROC 0 is wire-
bonded to a custom testing board, mounting an ALTERA Cyclone-III FPGA for
direct operation control and management of the data taking, and powered using
an Agilent E3631A power supply.

This setup was compared to a twin AC-LGAD strip sensor, wire-bonded to the
16-channel readout board designed at FNAL seen in Fig. 2. This test-board uses
discrete RF components to translate current signals from the AC-LGAD into
voltage. All sixteen strips of the AC-LGAD were wire-bonded to the input pads
of this Discrete RF (DRF) board (Fig. 3). Each channel is equipped with a
2-stage amplifier chain as described in [10].

a) b)

Figure 3: Schematic representation of the four AC-LGAD strips (strip A, B, C
and D) wire-bonded to a) ALTIROC 0 and b) the DRF board. Strips in blue are
wire-bonded to the readout systems while the ones in yellow are left floating.
In a), the strips in grey are wire-bonded to the same ground (GND) as the ASIC
to minimize the portion of the device that remains floating. In b) all strips
are wire-bonded to the readout system but only A, B, C and D are readout. The
red points L, C and R are focusing points for the IR laser used to
characterize the response of the detector.

Signals generated in the AC-LGAD by interactions with either betas or the IR
laser were acquired using a LeCroy Waverunner 9404M-MS
Oscilloscope222https://teledynelecroy.com/oscilloscope/waverunner-9000-oscilloscopes/waverunner-9404m-ms
using the high-sampling speed channels.

###  2.1 The ALTIROC 0 chip

The readout chip used in this study is the prototype ALTIROC 0 (version 2B),
consisting of only the four voltage pre-amplifier and discriminator stages of
the final ALTIROC chip. Two Time-to-Digital Converter (TDC) units can provide
a digital signal encoding the information of the Time Of Arrival (ToA) and
Time-Over-Threshold (TOT) of each input signal (Fig. 4) [15]. The rising edge
of this digital pulse provides a measurement of the particle ToA while its
width that of the ToT of the analog signal at the pre-amplifier stage [16].
The discriminator threshold is set by an external 10-bit DAC, common to all
channels. The ALTIROC 0 chip was developed to achieve a jitter smaller than 20
ps for an input charge of 10 fC, and provide a time measurement for input
signals with charge as low as 4 fC. The ToT is used in the ALTIROC 0 design to
correct for the signal time walk.

Figure 4: Simplified operation scheme of an AC-LGAD sensor read out by the
ALTIROC 0 chip. The AC-signal generated by the interaction of a particle in
silicon is multiplied in the voltage pre-amplifier stage, and then fed to a
built-in discriminator to produce a digital signal. The width of such signal
provides a measurement of the signal ToT.

###  2.2 Particle sources

Measurements involving both an uncollimated flux of beta particles and a
focused IR laser were performed to characterize the response to one or
multiple mips of injected charge. A 7.5 MBq Strontium-90 emitter was used as
beta source and positioned on top of the AC-LGAD sensor, at a distance of
approximately 2 cm. The source was enclosed in a 3D printed shell (made of
Stratasys RGD875 polymer) with a single circular opening (6 mm radius) towards
the sensor.

Figure 5: Picture of the TCT setup for the characterization of the AC-LGAD
sensor using an IR laser. From left to right are shown the ALTIROC testing
board mounted on a 3D stage with 1 $\mu$m precision; a lens for laser
focusing; an IR camera used for alignment. The setup is enclosed in a light-
tight aluminum box.

A focused IR laser was obtained by means of a Particulars Scanning-Transient
Current Technique (TCT)
apparatus333http://www.particulars.si/products.php?prod=scanTCT.html, shown in
Fig. 5. This setup was used to inject the equivalent of several mips of charge
in the AC-LGAD sensor by means of an IR laser with a wavelength of 1064 nm.
The ALTIROC 0 test-board was mounted on a 3-axis computer-controlled
mechanical stage with a position resolution of less than 1 micron. The setup
is enclosed in a light-tight aluminum box. Dedicated supports for the ALTIROC
testing board were custom designed and 3D printed to improve the mechanical
stability of the setup.

##  3 Preamplifier response characterization

The sensor response was studied on all four connected strips using both the
analog output of the ALTIROC VPA channels and the digital output generated by
the on-board discriminators.

###  3.1 Signal generated by a beta source (single mip)

Betas originated by the decay of 90Sr and by its decay byproduct, 90Y, show a
distribution of energy up to 2.2 MeV and can be considered in first
approximation as mips. Both 90Sr and 90Y are almost pure $\beta^{-}$ emitter,
with very little contamination from photon production. When interacting with
silicon, the energy loss of betas is described by a Landau distribution. No
focusing system has been applied to the betas.

a) b)

Figure 6: Distribution of the a) signal amplitude and b) FWHM for beta signals
interacting with the detector. The shapes of the two distributions for the
central strips B and C are different from those observed for the lateral
strips A and D.

As shown in Fig. 6, signals from strips B and C, close to the center of the
device, have a FWHM of about 4 ns that is compatible to published results for
standard (DC-)LGAD sensors readout via ALTIROC [16]. Single mips signals have
an average amplitude of $\sim$8 mV, with a long Landau tail up to amplitudes
higher than 50 mV. The signal amplitude is computed as the maximum voltage (in
absolute value) recorded by the oscilloscope above the baseline output voltage
of the readout system. Lateral strips show slightly wider and more complex
distributions, with multiple peaks in both FWHM and signal amplitudes
distributions; this effect is hypothesised to be a signal sharing effect
generated by the coupling of the lateral strips to the ground pad. Since the
system can read out only one strip at the time, and the beta beam is not
collimated, it is not possible to disentangle signals from betas directly
hitting the readout strip from signals generated by those hitting a
neighbouring strip and detected in the readout strip due to signal sharing. A
different setup proved to be necessary to study the properties of signal
sharing.

###  3.2 Laser response map

A scanning TCT setup was used to evaluate the response of the detector at
different impact positions of the incoming particle. Since the intensity of
the laser can be tuned, it is possible to emulate the charge deposition
equivalent to one or more mips on the sensor, while eliminating the
statistical dispersion effects of the Landau distribution typical of charged
particles thanks to the point-like energy release of photons in matter.

Figure 7:  Map of charge deposited on Strip A as a function of the shining
position of the IR laser, as read out by ALTIROC 0. Areas with higher
collected charge are represented in yellow, while areas with lower collected
charge are dark blue. Signal sharing can be visually identified by the yellow
gradient around the readout strip A.

Having under control the position of the incident photons, this setup allows
us to estimate the signal characteristics and sharing inside the sensor. This
is achieved by moving the sensor with respect to the laser focus-point and
mapping the response of the AC-LGAD. The magnitude of the response is
evaluated from the integral of the charge collected by the sensor as a
function of the shining position of the IR laser. Fig. 7 shows the response
map obtained from the output of Strip A as a function of the horizontal and
vertical positions of the IR laser shining point. Since the metallization
layer of the electrodes blocks the IR laser, the sensor does not collect
charge when the laser is shone on the strip metal. The scan also allows to
visualize the sharing of the signal in the AC-LGAD sensor: the readout strip
senses charge even when the interaction occurs at a non-negligible distance
from it. Signal can be discriminated from noise at a distance of two pitches
(200 $\mu$m) or more. As demonstrated in Ref. [10] the signal from multiple
strips can be used to precisely reconstruct the position of a charged particle
on the sensor. The results in Fig. 7 show that signal sharing in AC-LGADs is
preserved when the sensor are read out by an ALTIROC 0 chip. A complete
characterization of the signal sharing is presented in Section 3.6.

###  3.3 Signals generated by an IR laser (multiple mips)

Using the position information obtained with the pixel map on Fig. 7, the
laser was focused either between strips A and B (point L), between strips C
and D (point R), or approximately at the center of the sensor, between strips
B and C (point C), as detailed in Fig.3. A precise evaluation of the position
of the laser focus-point on the sensor proved to be unachievable with the
current setup, since vibrations in the mechanical stage due to the large board
size caused shifts in the order of $\sim$ 20 $\mu$m. The IR laser deposits
about 11 mips (32 mips) of equivalent charge onto the AC-LGAD sensor,
estimated by comparing the observed signal amplitude to that computed for a
single mip, when accounting for the readout impedance and the sensor gain.

Left (L) point

Center (C) point

Right (R) point

Figure 8: Examples of signals acquired at the VPA stage on four strips when
the IR laser is focused at the left, center, or right of the sensor. The laser
injects 11 (left) or 32 (right) mips onto the sensor. The signal amplitude
decreases linearly with distance due to the resistivity of the n++ layer.

Examples of signals acquired at the VPA stage on the four strips when the IR
laser is focused on each of the three tested positions (L, C and R) are shown
in Fig. 8. When the laser deposits 11(32) mips of charge onto the sensor it
generates signals with an amplitude of $\sim$55 mV ($\sim$200 mV) on strips
closer to the focus-point. The signal amplitude decreases linearly with the
increase of distance due to the resistivity of the n++ layer [17][18]. This
allows to estimate the linearity of the response of the detector and of the
signal sharing when different charges are injected.

Figure 9: Most probable value of the analog signal FWHM for Strip C when the
AC-LGAD sensor is irradiated with beta particles or an IR laser depositing 11
or 32 mips of charge. Value and error bands are extrapolated fitting each FWHM
distribution using a Landau function. The FWHM value is compatible with
published results for ALTIROC 0 using LGAD sensors and is dependant on the
amount of mips of charge injected into the sensor.

Signals acquired using the VPA channels are fast, with a FWHM in the order of
4 ns for the strips near the laser point of incidence, compatible to what was
observed for signals generated by beta particles in Sec.3.1. The FWHM is
dependant on the amount of mips of charge injected into the sensor, as seen in
Fig. 9.

###  3.4 Jitter measurement

Signal jitter is a major contributor to the timing capabilities of any
detector, and must therefore be characterized with precision. Contributions to
signal jitter come from the readout electronic, from external conditions such
as the environmental temperature, or are intrinsic to the detector. The total
jitter is often computed as:

$\textrm{jitter}=\sigma_{noise}\left(\frac{dV}{dt}\right)^{-1}$ (3.1)

where $\sigma_{noise}$ is the recorded signal noise and $dV/dt$ is the slew-
rate of the signal computed between 30% and 90% of the signal maximum
amplitude. The noise is estimated as the standard deviation of the voltage
spread observed in the first 10$\%$ of all signals recorded by the
oscilloscope. Measurements of signal jitter for all four strips are obtained
from the analog VPA channels of ALTIROC 0 when injecting charge using the IR
laser. Fig. 10 shows the jitter values between the four strips when the laser
is shone onto the points L, C and R.

Figure 10: Jitter of analog signals from the ALTIROC 0 VPA channels when IR
laser injects top) 11 or bottom) 32 mips, as a function of the laser shining
position (Left, Center and Right). Signals from strips near the laser focus-
point are fast, but the jitter degrades for farther strips due to the decrease
in signal amplitude. The error bands represents the width of the Landau
distribution obtained from data fitting.

The mean value and overall shape of the computed jitter is consistent for all
four analog channels, when accounting for the asymmetry generated by the setup
mechanical oscillation previously described; signals generated by the 11 mips
(32 mips) IR laser show a jitter of about 10 ps (6 ps) in the strips near the
laser focus-point, compatible with previous observations and the ALTIROC 0
specifications for LGAD sensors. Strips farther from the laser focus-point
show signals with a jitter of 25 to 35 ps (7 to 9 ps) at one pitch of distance
and 66-67 ps (17 to 19 ps) at two pitches of distance. This degradation of the
signal jitter is a direct effect of the observed decrease of signal amplitude
on strips far from the laser focus-point.

###  3.5 Comparison to DRF board

Results obtained using the aforementioned detector were compared to those
measured using a similar system based on a Discrete Radio Frequency (DRF)
amplifier board designed and produced by FermiLab. Such board is widely used
for prototyping and testing of LGAD and AC-LGAD sensors and is considered a
good benchmark for fast-time multi-channel acquisition of silicon sensor
signals. This DRF board, mounting an AC-LGAD sensor identical to the one
tested together with the ALTIROC 0, was installed on the scanning TCT system.
All sixteen strips were connected to the board and four central strips,
labelled again as strips A, B, C, D, are selected to be on the same respective
positions as the those wire-bonded in the ALTIROC 0 chip to allow for a direct
comparison between the two setups as detailed in Fig. 3.

Left (L) point

Center (C) point

Right (R) point

Figure 11: Examples of signals on four strips connected to the DRF board when
the IR laser is focused at the left, center, or right of the sensor. The laser
injected 11 (left) or 32 (right) mips onto the sensor.

Examples of signals acquired from the four strips when the IR laser is focused
on each of the three tested positions L, C and R are shown in Fig. 11. When
the laser deposits 11(32) mips of charge onto the sensor it generates signals
with an amplitude of $\sim$590 mV ($\sim$1700 mV) on strips closer to the
focus-point and, as in the previous case, the signal amplitude decreases
linearly with the increase of distance. The amplitudes of signals acquired
from the DRF board are significantly (by a factor $\sim$8.5) larger than those
readout by the ALTIROC 0 setup using identical sensors and in the same
conditions, due to the different gain of the two readout systems.

###  3.6 Signal sharing

The signal sharing is evaluated by measuring the amplitudes of signals
generated by the IR laser on four neighbouring strips. The percentage of
shared signal is then computed as the ratio between the signal amplitude
measured by each strip and that measured by the strip closest to the focus-
point.

Figure 12: Sharing profile on AC-LGAD strips read out by the ALTIROC 0 chip
and the Fermilab (FNAL) DRF board at different distances from the focus-point
of an IR laser. The laser injects 11 mips (top) or 32 mips (bottom) on the
sensor. The gray area in each window identifies the focus-position of the
laser.

The evaluation of the signal sharing profile on AC-LGAD has been measured for
the sensors mounted on both the ALTIROC 0 chip and the DRF readout board using
the data presented in Sec. 3.3 and 3.5. Fig. 12 details the signal sharing
profile induced on the AC-LGADs as a function of the laser shining position
when injecting either 11 or 32 mips. The sharing profiles are symmetrical with
respect to the sensor central axis within uncertainty. In the case of the
ALTIROC 0 setup, a small decrease of signal sharing is observed when the laser
is shone at the center point compared to that observed in left and right
points at the same distance; this is likely a border effect caused by the
presence of several non-bonded strips on the sensor. This is a small effect in
magnitude. It can be also observed that signal sharing is 2-3 times more
prominent using the DRF setup when accounting for the strip position and laser
intensity. This difference in signal sharing profile in the two readouts is
expected due to the different trans-impedance and capacitance of ALTIROC 0
with respect to the DRF amplifiers.

##  4 Discriminator response characterization

The ability of the ALTIROC 0 TDCs to encode and transmit the signal ToT is
important for the construction of an AC-LGAD based detector: when combined
with the knowledge of the signal sharing profile of the detector, it allows to
precisely reconstruct the position of the incoming particle at a sub-pitch
level [10], as it approximates the value of the deposited charge. To this end,
the chip ToT response has to be mapped univocally to the signal shared on each
strip of the sensor.

Figure 13: Examples of analog and digital output signals of ALTIROC 0
extracted from Strip C, and generated by the interactions of betas (left) and
IR laser (right). The width of the digital signal, encoding the ToT, is
proportional to the amplitude of the analog signal.

Figure 13 shows the digital response of the ALTIROC 0 discriminator compared
to the respective analog pre-amplifier output when the sensor is irradiated
with either betas or an IR laser. The width of this digital signal encodes the
ToT of the analog signal generated in the pre-amplifier stage.

A characterization of the discriminator response can be obtained by
correlating the FWHM of the discriminator digital output to the ToT of the
analog signal. Signals from beta interactions with a wide range of deposited
charge allows to evaluate this response characteristic, as shown in Fig. 14.
Since the amplitude and the ToT of the analog signal are strictly correlated,
the former is used as proxy of the latter for this evaluation.

Since the threshold of the discriminator can be adjusted, different thresholds
have been applied in the two cases to compensate for the different charge
deposited by the betas and the IR laser.

Figure 14: FWHM of the discriminator output of the ALTIROC 0 ASIC as a
function of the respective amplitude of the analog signals, for signals
generated by $\beta$ particles. The FWHM of the digital signal shows a clear
univocal dependence on the analog signal amplitude and therefore on the
deposited energy.

The effect of Landau fluctuations in interactions with betas produces a long
tail in the distribution of analog signal amplitude. This is reflected in the
measurement of the ToT, where a clear univocal dependence of the digital
signal width to the analog signal amplitude can be observed.

##  5 Conclusions

The first characterization of AC-LGAD signals read out by an ASIC is presented
using the ALTIROC 0 readout ASIC. While designed to acquire DC-coupled signals
from LGAD sensors, ALTIROC 0 proved to be a suitable readout for bipolar AC-
coupled signals generated in BNL-made AC-LGADs. The output of the detector
formed by an AC-LGAD sensor coupled with the ALTIROC 0 chip was studied by
injecting one to multiple mips of charge on the sensor using either beta
particles or a focused IR laser. Characteristics of the readout signals such
as amplitude, shape and FWHM are compatible with those measured with test-
boards used in test-beams to read out AC-LGADs and with previous ALTIROC
results obtained for standard, i.e. DC-coupled, LGADs.

The linearity of the ToT measurement provided by the ALTIROC 0 discriminator
over a wide range of injected charges allows to exploit signal sharing and in
turn improve space resolution in future experiments beyond (pitch
size)/$\sqrt{12}$ as in typical discrete detectors that use a binary readout
system. Such experiments may include high granularity detectors for high-
energy and nuclear physics applications, e.g. at the EIC. It has been proven
that the signal sharing profile of the sensor is affected by the trans-
impedance and capacitance of the readout chip; these parameters can be fine
tuned together with the sensor pitch to provide the targeted spatial
resolution while keeping the sensor segmentation sufficiently sparse to
minimise the number of readout channel, and in turn readout bandwidth, cooling
and costs.

These studies can be used as a stepping stone for the development of novel
types of readout chips, based on the ALTIROC technology and tailored to match
AC-LGAD signal properties, aimed at 4D reconstruction of particles by
exploiting their signal sharing capabilities.

## Funding

This material is based upon work supported by the U.S. Department of Energy
under grant DE-SC0012704. This research used resources of the Center for
Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at
Brookhaven National Laboratory under Contract No. DE-SC0012704.

## Acknowledgments

The authors wish to thank their colleagues at Brookhaven National Laboratory:
Ron Angona and Sean Robinson for sensor fabrication; Don Pinelli, Joe Pinz,
Antonio Verderosa and Tim Kersten for board assembly and wire bonding.

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