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\vspace{6cm}
{\par\centering \textbf{\Huge The ALMY Sensor Modules}\Huge \par}
\vspace{2cm}

{\par\centering {\LARGE F.Bauer\( ^{1} \), V.Danielyan\( ^{1,2} \), S.Horvat\( ^{1} \)}\footnote{%
on leave of absence from Ru\dj er Bo\v skovi\' c Institute, Zagreb, Croatia
}{\LARGE , H.Kroha \( ^{1} \)} \LARGE \par}
\vspace{1cm}

{\par\centering {\large 1. Max Planck Institute for Physics, Munich, Germany}\large \par}

{\par\centering {\large 2. Yerevan Physics Institute, Yerevan, Armenia}\large \par}

\newpage
\pagestyle{headings}
\setcounter{page}{1}
\bigskip{}
INTRODUCTION
\bigskip{}

A desired performance of the modern large area tracking detectors can only be
achieved by a precise monitoring of the relative positions of the detector components.
For example in the muon spectrometer of the ATLAS experiment at the LHC, an
optical alignment system has to provide the misalignment corrections to the
track curvature with an accuracy of 30~\( \mu  \)m (RMS).

A novel optical laser alignment monitoring system (``ALMY System''), developed
for such applications, monitors the alignment of several semi-transparent optical
position sensors along a collimated laser beam. The principle is shown in Fig.1.

\vspace{0.5cm}
{\par\centering \resizebox*{0.8\textwidth}{0.3\textheight}{\includegraphics{faser.eps}} \par}

{\par\noindent \centering {\footnotesize Fig.1. Principle of the ALMY laser alignment
monitoring system with semi-transparent position sensors and a light distribution
system with single-mode optical fibers (SMF).}\footnotesize \par}
\vspace{0.5cm}

\vspace{0.5cm}
The GaAlAs laser diodes emit the visible light at 690~nm and 780~nm wavelength.
The beams are collimated via single-mode optical fibers and have a Gaussian
beam profile with 2-5~mm diameter over a 15~m distance. The laboratory tests
and the tests at the TESLA Free Electron Laser facility demonstrate that more
than 10 sensors can be aligned along a 780~nm laser beam.

The layout of an ALMY sensor is shown in Fig.2. The sensor consists of a 0.3-1~\( \mu m \)
thick layer of hydrogenated amorphous silicon (a-Si:H) between the two orthogonally
segmented indium-tin-oxide (ITO) electrode layers which cover the sensitive
area of 20x20~mm\( ^{2} \). The top 64 strip electrodes act as Schottky contacts,
with 64 ground electrodes at the bottom. Such a layout allows for a two-dimensional
position measurement. The active sensor layer is deposited on the 0.5-1~mm thick
glass substrate, with an additional anti-reflective coating on the back side
of the glass (10 cm diameter glass wafers are cut into 7 sensor substrates).
A transmittance of 90\%(75\%) has been achieved with the 780~nm (690~nm) lasers.
In the last several years extensive tests have been performed on 150 ALMY I
and ALMY II sensor modules with integrated readout electronics (see Fig.3.).
The results are described in the first section of this note. The system with
ALMY II modules has been tested for the performance in the ATLAS end-cap muon
spectrometer (section II). Section III describes the performance of the new,
recently developed, readout electronics (ALMY III modules).

The recent changes also include the improvements in the production of the glass
substrates. Deposition of the active a-Si:H cell onto the new substrates is
under development, the thicknesses of various layers still to be optimized.
The most recent results and the performance of the new sensor cells with ALMY
III electronics is outlined in the section IV. Section V describes the influence
of the long-term laser illumination of a sensor on the changes in its sensitivity
and the position resolution. The results of the radiation hardness tests are
outlined in the section VI.

\begin{figure}[!htbp] 

\begin{picture}(80,80)(30,80)

\vspace{0.3cm}
{\par\centering \resizebox*{0.6\columnwidth}{0.25\textheight}{\includegraphics{asi_sd3.eps}}  \par}
\vspace{0.3cm}

\put(-180,-20){\footnotesize Fig.2. Layout of an amorphous}

\put(-160,-30){\footnotesize silicon strip sensor.}

\end{picture}

\begin{picture}(80,80)(-180,0)

\vspace{0.3cm}
{\par\centering \resizebox*{0.5\columnwidth}{0.25\textheight}{\includegraphics{almy_asic_electronics.eps}} \par}
\vspace{0.3cm}

\put(-180,-20){\footnotesize Fig.3. ALMY sensor module with integrated}

\put(-160,-30){\footnotesize readout electronics.}

\end{picture}

\end{figure}

\vspace{1.5cm}
I. ALMY II SENSOR PERFORMANCE
\vspace{0.5cm}

Fig.4. shows the typical current-voltage characteristics of the ALMY II sensor
diodes, characterized by a low dark current of 10\( ^{-9} \) to 10\( ^{-7} \)A/cm\( ^{2} \).

\vspace{-0.5cm}
{\par\centering \resizebox*{1.05\textwidth}{0.45\textheight}{\includegraphics{almyii-iv.eps}} \par}

\vspace{-2cm}
\noindent {\footnotesize Fig.4. Current-voltage characteristics of the ALMY
II Schottky diodes. (a) Measurements on the illuminated cell (light) and non-illuminated
cell (dark) are shown in a logarithmic scale. (b) Measurement on a non-illuminated
cell is also presented in a linear scale.}{\footnotesize \par}
\vspace{0.2cm}

The anti-reflective coating on the back side of the glass substrate reduces
the reflectivity of the glass surface to less than 0.5\%, thus making the interference
effects negligible. In this way the variations in the sensors position measurement
are avoided. A uniform position resolution of 1~\( \mu  \)m has been achieved
over the whole active surface (see Fig.5.)

{\par\raggedright \resizebox*{1.1\textwidth}{0.60\textheight}{\includegraphics{sensor_ns030.eps}} \par}

\vspace{-1.0cm}
\noindent {\footnotesize Fig.5. Position resolution (\( \Delta x, \) \( \Delta y \))
of an a-Si:H sensor with anti-reflective coating in the two coordinates (x\( _{pos} \),
y\( _{pos} \)).}{\footnotesize \par}
\vspace{0.5cm}

The position resolution is insensitive to high magnetic fields due to the low
Hall conductivity of amorphous silicon (\( \mu _{n}^{H}=10^{-2}-10^{-3} \)
cm\( ^{2} \)/Vs). 

The finite wedge angle and the curvature of the glass substrate can introduce
the uncertainties in the refraction of the traversing laser beam. However, the
beam refraction angle can be measured in a scan of the sensor surface and used
as a senor parameter. The uncertainties are then reduced to less than 3 \( \mu  \)rad
(see Fig.6.). 
\vspace{0.5cm}

%{\par\centering \resizebox*{1\textwidth}{0.45\textheight}{\includegraphics{ref%r-ns028.eps}} \par}
%\vspace{-1cm}
%\noindent {\footnotesize Fig.6. Laser beam refraction angle for a sensor with
%the glass substrate of a finite wedge angle (a) \( \vartheta _{x} \) before
%and (b) \( \vartheta _{x} \)\( ^{corr} \)after the calibration parameters
%are applied. Similar result is also obtained for the y-component.}{\footnotesi%ze \par}
%\vspace{0.5cm}

\vspace{1.5cm}
II. PERFORMANCE IN ATLAS END-CAP MUON SPECTROMETER
\vspace{0.5cm}

Fig.7. shows the polar alignment rays connecting the MDT chambers in three to
four different stations of the ATLAS end-cap muon spectrometer.

%\vspace{0.3cm}
%{\par\centering \resizebox*{0.55\textwidth}{0.3\textheight}{\rotatebox{270}{\i%ncludegraphics{ECALN1.ps}}} \par}
%\vspace{0.3cm}
%
%\noindent {\footnotesize Fig.7. Polar alignment rays in one octant of the ATLAS
%end-cap muon spectrometer, connecting the MDT chambers of all three (four) muon
%stations.}{\footnotesize \par}

The rays have to provide the misalignment corrections of \( \sim  \)30 \( \mu m \)
to muon track sagitta, as well as to measure the angular displacements with
the precision of \( \sim 50 \) \( \mu rad. \) The performance of an ALMY system
along one of such alignment rays (the polar monitor) has been extensively tested. 

{\par\centering \resizebox*{1\textwidth}{0.6\textheight}{\includegraphics{refr-ns028.eps}} \par}
\vspace{-1cm}
\noindent {\footnotesize Fig.6. Laser beam refraction angle for a sensor with
the glass substrate of a finite wedge angle (a) \( \vartheta _{x} \) before
and (b) \( \vartheta _{x} \)\( ^{corr} \)after the calibration parameters
are applied. Similar result is also obtained for the y-component.}{\footnotesize \par}
\vspace{0.5cm}


\vspace{0.3cm}
{\par\centering \resizebox*{0.75\textwidth}{0.3\textheight}{\rotatebox{270}{\includegraphics{ECALN1.ps}}} \par}
\vspace{0.3cm}

\noindent {\footnotesize Fig.7. Polar alignment rays in one octant of the ATLAS
end-cap muon spectrometer, connecting the MDT chambers of all three (four) muon
stations.}{\footnotesize \par}


The polar monitor consists of three ALMY telescopes along one laser beam, one
for each of the muon stations. ALMY telescope (see Fig.8.) is assembled of two
ALMY modules mounted parallel to each other at distance of 10 cm. It measures
the laser beam position and the angular direction of the beam.

\vspace{0.5cm}
{\par\centering \resizebox*{0.6\textwidth}{0.3\textheight}{\includegraphics{freiburg_almytest_4.ps}} \par}
\vspace{0.5cm}

\noindent {\footnotesize Fig.8. ALMY telescope for the measurement of the laser
beam position and direction. It consists of two ALMY sensors mounted parallel
to each other at the distance of 10 cm.}{\footnotesize \par}
\vspace{1.0cm}

Fig.9 shows the setup for the performance tests. 
\bigskip{}

\resizebox*{0.9\textwidth}{0.3\textheight}{\includegraphics{freiburg-4m.ps}} 

\vspace{0.5cm}
\noindent {\footnotesize Fig.9. Setup for the absolute performance test of an
ALMY polar monitoring system for ATLAS end-cap muon spectrometer.}{\footnotesize \par}
\vspace{1.0cm}

Two independent measurements are made: the local laser beam position is measured
by the ALMY sensors, and the global sensor coordinates are determined using
the 3D Coordinate Measuring Machine. The comparison of these two measurements
gives the information about the absolute sensor performance. The middle telescope
can be tilted around the x- and y-axis, allowing to test the angular resolution
of the telescope. Eight different angular orientations are tested, each using
nine different laser beam directions. Fig.10. shows the outcome for the sagitta
resolution and an angular resolution of a described setup.

{\par\centering \resizebox*{1.1\textwidth}{0.65\textheight}{\includegraphics{sagitta.eps}} \par}

\vspace{0.5cm}
\noindent {\footnotesize Fig.10. Sagitta resolution (\( \mu m \)) and the angular
resolution \( \Delta \alpha _{x} \), \( \Delta \alpha _{y} \) (\( \mu rad \))
of an ALMY polar monitor in the ATLAS end-cap muon spectrometer.}{\footnotesize \par}
\vspace{0.5cm}

The false sagitta resolution is \( \sim  \)20 \( \mu m \), which is well within
the specified requirements. The angular resolution of 60-80 \( \mu rad \) still
has to be improved (50 \( \mu rad \) r.m.s. required) by using a twice larger
telescope lever arm. The test has also demonstrated the efficiency of the calibration
procedure (i.e. corrections due to the the laser beam refraction).

Additional test performed with a 14m long laser beam demonstrated the stability
and the straightness of the beam, independent of the air turbulence around it.

\newpage
\vspace{0.5cm}
III. ALMY III ELECTRONIC BOARDS
\vspace{0.5cm}

The new ALMY III sensor electronics (see Fig.11.) is currently in the last stages
of development. It has an advantage of smaller outline dimensions (60x60~mm\( ^{2} \)).
The new design is more reliable - no trimming potentiometers are needed, there
is no need for a regulated power supply and the power dissipation is decreased
by a factor of 6. The more detailed description can be found in the Appendix
to this note.
\medskip{}

{\par\centering \resizebox*{0.65\textwidth}{0.25\textheight}{\includegraphics{new_almy_elec_4.ps}} \par}

\bigskip{}
\noindent {\footnotesize Fig.11. New ALMY electronic boards (analog board on
the lefthand and the digital one on the righthand side).}{\footnotesize \par}
\bigskip{}

Fig.12. shows the position resolution distribution obtained using the ALMY~III
readout electronics. No cross-talk effects are observed and the distribution
is uniform over the whole sensor surface.

{\par\centering \resizebox*{1.1\textwidth}{0.4\textheight}{\includegraphics{newelc-olds.eps}} \par}

\vspace{0.5cm}
\noindent {\footnotesize Fig.12. Position resolution \( \Delta x \) (\( \mu m \))
of an old a-Si-H sensor with the new ALMY III readout electronics. Similar distribution
is obtained for y-direction as well.}{\footnotesize \par}
\vspace{0.5cm}

\vspace{0.5cm}
IV. ALMY III MODULES WITH NEW SENSOR CELLS
\vspace{0.5cm}

As from recently the calibration procedure (needed for reducing the uncertainties
in the laser beam refraction) can be avoided by using the high-quality polished
parallel glass wafers for sensor fabrication. Fig.13. shows the refraction angle
distribution for a typical 10 cm diameter glass wafer. The average beam refraction
and the beam refraction error are both below the 3~\( \mu  \)rad.

\vspace{-2cm}
{\par\centering \resizebox*{1.1\textwidth}{0.47\textheight}{\includegraphics{gl23-coat.eps}} \par}

\vspace{-1cm}
\noindent {\footnotesize Fig.13. Laser beam refraction angle distribution from
a scan of a 10 cm diameter glass wafer (for simplicity only the x-component,
\( \vartheta _{x} \), is shown).}{\footnotesize \par}
\vspace{0.5cm}

The deposition of the active sensor surface on the new high quality polished
glass wafers is still in a (late) development stage, the thicknesses of the
various layers still to be optimized. Fig.14. shows the typical current-voltage
characteristics of these sensor cells. 
\medskip{}

\vspace{-0.1cm}
{\par\raggedright \resizebox*{1.1\textwidth}{0.3\textheight}{\includegraphics{almyiii-iv.eps}} \par}

\vspace{0.5cm}
\noindent {\footnotesize Fig.14. Current-voltage characteristics measured on
the diodes of the new a-Si:H sensor cells (for three glass wafers - gc08, gc09 and gc10). (a) Measurement on the illuminated
cell (light) and non-illuminated cell (dark) are shown in a logarithmic scale.
(b) Measurement on a non-illuminated cell is also presented in a linear scale.}{\footnotesize \par}
\vspace{0.5cm}



The dark current measured at the bias voltage of 1V is uniformly distributed  over all sensor strips (see Fig.15.)

 

\vspace{0.5cm}
{\par\centering \resizebox*{0.9\textwidth}{0.28\textheight}{\includegraphics{x-iv-new.eps}} \par}

\vspace{0.5cm}
\noindent {\footnotesize Fig.15. Dark current distribution over the sensor strips, at bias voltage of 1V, for the newly produced sensor cells}{\footnotesize \par}
\vspace{0.5cm}


The performance of the ALMY III sensor modules with the newly produced sensor
cells is shown on Fig.16. and Fig.17. The position resolution of \( \sim 3\, \mu m \),
the refraction angle of less than 3~\( \mu rad \) (without calibration procedure),
the transmittance of 84\% and a uniform sensor sensitivity to illumination are
observed.

{\par\raggedright \resizebox*{1.1\textwidth}{0.45\textheight}{\includegraphics{newelc-news.eps}} \par}

\noindent {\footnotesize Fig.16. (a) Position resolution \( \Delta x \) and
(b) refraction angle \( \vartheta _{x} \) (no calibration corrections applied)
of an ALMY III module with newly produced sensor cell on a polished parallel
glass substrate.}{\footnotesize \par}

{\par\raggedright \resizebox*{1.1\textwidth}{0.50\textheight}{\includegraphics{newelc-news-tran.eps}} \par}

\noindent {\footnotesize Fig.17. (a)Transmission (\( \tau  \)) and (b) sensitivity
(ADC) of an ALMY III module with newly produced sensor cell on a polished parallel
glass substrate.}{\footnotesize \par}


\vspace{1.5cm}
V. SENSITIVITY DEGRADATION (AGING STUDIES)
\vspace{0.5cm}

It is observed in the studies of the solar cells that the sensitivity of a-Si:H
deteriorates under illumination. Thus, the effect of the long-term illumination
on the performance of ALMY sensors needed to be investigated. After several
hours of illumination of an ALMY~II module with a 690 nm laser beam of 3~mm
diameter and 10~mW/cm\( ^{2} \) intensity the local sensitivity degradation
causes significant errors in the position measurement (see Fig.18.). The effect
is also referred to as the aging effect.

However, in the normal operation mode, using a 780 nm laser beam of 10~mW/cm\( ^{2} \)
intensity (giving the maximum unsaturated signal) the degradation is reduced
to a tolerable rate. After 500 hours of continuous illumination the sensitivity
loss measured on 4 different sensors is only 2\% (see Fig.19.). This corresponds
to the local position measurement error of <5 \( \mu m \) after 20 years of
operation in the ATLAS muon spectrometer.

{\par\raggedright \resizebox*{1.0\textwidth}{0.9\textheight}{\includegraphics{age_141.eps}} \par}

\vspace{0.5cm}
\noindent {\footnotesize Fig.18. The scan of the sensor surface shows the local
degradation of the sensitivity (relative light intensity) and of the position
resolution \( \Delta x \), \( \Delta y \) (\( \mu m \)) after several hours
of illumination with a 690 nm laser beam (10 mW/cm\( ^{2} \) intensity). The
transmittance and the beam refraction angle (\( \mu rad \)) are not affected
by illumination.}{\footnotesize \par}
\vspace{0.5cm}

{\par\centering \resizebox*{0.8\textwidth}{0.4\textheight}{\includegraphics{all_aged_in_one.eps}} \par}

\vspace{0.5cm}
\noindent {\footnotesize Fig.19. Long-term monitoring of the sensitivity loss
on 4 ALMY sensors under illumination with 780 nm laser beam of 3 mm diameter
and 10~mW/cm\( ^{2} \) intensity.}{\footnotesize \par}
\vspace{0.5cm}

At the wavelength of 690 nm the sensitivity of a-Si:H is 10 times higher than
at 780 nm, implying that the aging effect observed with 780 nm laser shouldn't
increase when the ALMY system is operated using 690~nm laser beams of ten times
lower intensity. This is demonstrated by illuminating one sensor continuously
for 1500 hours with a 690 nm laser beam of 1 mW/cm\( ^{2} \) intensity. The
sensitivity degradation of only 2\% was observed (see Fig.20.) indicating the
possibility to operate the system using the visible light.

{\par\centering \resizebox*{0.7\textwidth}{0.35\textheight}{\includegraphics{aged_with_690.eps}} \par}

\vspace{0.5cm}
\noindent {\footnotesize Fig.20. Long-term monitoring of the sensitivity degradation
of a-Si:H sensor under illumination with a 690 nm laser beam of 3 mm diameter
and 1 mW/cm\( ^{2} \) intensity.}{\footnotesize \par}
\vspace{0.5cm}

VI. RADIATION HARDNESS
\vspace{0.5cm}

The ALMY sensors have been irradiated with neutrons and \( \gamma  \)-rays.
The sensitivity and transmission of the active sensor surface don't deteriorate
under irradiation doses of up to 10 MRad for \( \gamma  \) and up to 10\( ^{14} \)
n/cm\( ^{2} \) for neutrons. The ALMY II readout electronics has been irradiated
with up to 10\( ^{13} \)n/cm\( ^{2} \) and no degradation in performance is
observed.

\vspace{0.5cm}
ACKNOWLEDGMENTS
\vspace{0.5cm}

\medskip{}
REFERENCES
\medskip{}

\newpage

\vspace{0.5cm}
APPENDIX
\vspace{0.5cm}

{\par\centering \textbf{\large REPORT ON ALMY III SENSOR ELECTRONICS}\large \par}
\vspace{0.5cm}

Work was mainly done in MPI Munich during periods: 

February 2000 - June 2000 

December 2000 April-May 2001 

\bigskip{}
During this time was done:
\smallskip{}

\medskip{}
\textit{In Hardware}
\medskip{}

Designed completely new sensor module, ALMY-V, based on the same semitransparent
sensor, which was used in the old MPI design. New sensor is much smaller, has
many times less power consumption, has easy to use optional set of interfaces:
RS232, RS485, CANbus. Besides there are no connectors, instead sensor pins are
directly bonded to analog PCB, significantly improving reliability. New sensor's
PCBs are of 4 layers, instead of 8-12 of the old one and used only industry
standard components, instead of ASIC as in old one. So they cost much less.
Sensor module consists of two boards: 

1. Analog board ALMY-VN, including analog multiplexors, transimpedance amplifiers
and ADC. 

2. Interface board, including ATMEL microcontroller, regulated power supply
and three optional interfaces. 

We manufactured 6 experimental sensor modules, which are still under testing.

\medskip{}
\textit{In Software}
\medskip{}

Designed embedded program for ATMEL microcontroller, using RS485 interface Modernized
the Stefan Shaels ALICA3 program for PC, to make it work with the new sensors
(this was made by student Akobyan Davit).

In May 2001 was completely finished design of Semiconductor Laser controller
board. This board contains also power regulator for all sensor modules, used
with the laser and converter RS232 (standard for PC) - RS485 interface. Using
one such board, arbitrary quantity of ALMY-V sensors (limited by optics) and
small unregulated power supply it is possible to build completely finished one-axis
geometry monitoring system. All design files are in electronic workshop of MPI
Munich and it is expected to have experimental units in the autumn 2001.

\bigskip{}
Varuzhan Danielyan

\newpage

{\par\centering \textbf{\large ALMY III SENSOR ELECTRONICS}\large \par}
\vspace{0.5cm}

New ALMY sensor (ALMY-VN) electronics consists of two printed circuit boards
(PCB) ALMY-VN and ALMY-V-IFC . ALMY-VN (see almy-vn scheme) board contains all
analog circuits. Below they are listed with brief description. 

\medskip{}
\noindent {*} Resistor networks R1...R16. Through this resistors are closed
photocurrent paths. 
\medskip{}

\noindent {*} Analog multiplexers U1...U8. U1, U2 are used for multiplexing
signals from X strips with odd numbers (X01, X03, X63) and U3, U4 for even numbered
X strips (X02, X04, X64). Similarly U5, U6 are used for odd numbered Y strips
and U7, U8 - for even numbered Y strips. One can see, that corresponding of
the strip numbers and multiplexer inputs is not straight. For example, X01 is
connected to U1:S8 input, X03 to U1:S7 and so on descending until X15 to U1:S1,
but then X17 is connected to U1:S9, X19 to U1:S10 and so on, ascending until
X31 to U1:S16. Such inhomogeneous connection is chosen to optimize the PCB layout.
Multiplexer addresses are switched by microcontroller and it can be done in
arbitrary sequence. 

\noindent {*} Voltage divider for X strips biasing: R17, R18, C1, and C2. Currently
the bias voltage is set equal to approximately 1 Volt, but the setting can be
easily changed, by using another value for R18 resistor. The buffer capacitor
C2 should be large enough to prevent the bias voltage deviation, caused by current
through R21 resistor, which is changing according to the selected strip photocurrent
(the voltage on U9:6 pin is always equal to the one on the U9:5, which is proportional
to X strip photocurrent). In previous versions of ALMY sensor this deviation
brought to crosstalk between X and Y coordinates. 
\medskip{}

\noindent {*} X transimpedance amplifier on U9 and Y transimpedance amplifier
on U10. The second stage of the X amplifier shifts signal level from the bias
voltage to ground. Therefore R21 should be equal to R20, and R22 to R23. As
the amplifiers are of transimpedance type, their gain is measured in impedance
(resistance) units. The gains of X and Y amplifiers are equal and can be calculated
by formulas:

\noindent Xgain = R19{*}(R22/R21) = 3900000 Ohm = 3.9V/\( \mu  \)A 

\noindent Ygain = R26{*}(R28/R27) = 3.9V/\( \mu  \)A

\medskip{}
\noindent {*} Analog to Digital Converter (ADC) U11. This is 12-bit 4-channel
ADC with internal reference voltage of 2V, which corresponds to maximal code
value of 4095. Except of the main measured voltages of X and Y coordinates,
there is possibility for diagnostic purposes to measure values of Vbias and
Vss (the negative supply voltage). Having 4 times larger dynamic range, we can
choose large enough zero shifting of amplifiers outputs, to prevent negative
values for the worst conditions: an operational amplifiers with maximal input
voltage offset values, working at maximal or minimal rated temperatures. These
zero shifts are made by R24, R25 divider for X and R29, R30 for Y and are equal
about 190-200 channels (of 4095) for amplifiers with zero input voltage offset.
D1 and D2 Schottky diodes protect the ADC inputs from negative polarity transients. 

\medskip{}
\noindent {*} Inductorless low-noise DC/DC converter U14 to generates negative
voltage -4.1V for powering analog multiplexers and operational amplifiers. This
part is optional, when not used on the board the negative voltage can be supplied
from J2:14 pin of J2 connector.

\bigskip{}
The described schematic is based on assumption, that photocurrents can be measured
from the any side of photo-strip. To check this hypothesis, the simulation was
made, using Microsim 8.0 PSPICE software. The photocurrent generating junctions
in the crossings of strips were modeled as ideal current source in parallel
with PN diode. To simplify calculations the sensor model size was chosen 8x8
instead of 64x64. As the real sensor strip has resistance about 50k, to simulate
this distributed resistance, between each pare of junctions is connected a 6k
resistor. The manufacturer supplied multiplexer and OA PSPICE models were used
(almy-sim scheme). To compare signals from the odd and even numbered strips,
the illuminated (current generating) area was chosen of square shape 3x3 junctions
with the same current value. The simulation results are presented on sim-plot.pdf.
Readout time for each strip is equal 25us, so on the time (horizontal) axis
we have 8 zones. The square marked curve is for Y, and diamond marked - for
X. On the Y plot to illuminated strips correspond to zones 2...4 (from 25us
to 100us), on X plot to zones 6...8 (from 125us to 200us). At the beginning
of each zone we have large transients, caused by stray capacitance of multiplexers
and amplifier settling processes. Apparently the transients for odd and even
strips have different shapes. But settled values at the end of each zone are
equal with more than 4 decimal digit precision. According to this plots, sampling
for AD conversion is made at the end of each zone. For the real sensor duration
of zone (including safety margins) is set by the microcontroller software and
is chosen about 70us and can be made even smaller without affecting precision. 

\vspace{1cm}
ALMY-V-IFC (see almy-v-ifc scheme) board contains microcontroller with corresponding
auxiliary parts, power supply section and 3 optional interface circuits. 

\medskip{}
\noindent {*} As the main microcontroller is used Atmel 8-bit AT90S8515 controller
chip, having on-chip 8Kbyte Flash program memory, 512 Data EEPROM and 512 SRAM.
Flash and EEPROM can be programmed in-site, using serial SPI interface connection
through J2:15...J2:20 pins. The EEPROM can be reprogrammed by the microcontroller
software, and can be used to store sensor's address (instead of hardware switches
or jumpers), correction coefficients (if needed) and any other sensor-dependent
information. In future, this chip can be replaced with Atmel Atmega161 with
the same pin-out. This new part has capability to be reprogrammed in-site through
the same interface, which is used to communicate with the main computer (RS232,
RS485 or CANbus, see below). The U5 IC is used to generate RESET signal when
supply voltage falls below 4V, preventing possible corruption of EEPROM contents.
The clock frequency for the sensor can be optionally generated by on-chip generator,
using Y1 quartz, or by external quartz generator Y2. The frequency depends on
chosen interface. Typically it is equal 7.3728 MHz for RS interfaces and 16MHz
or 8MHz for CANbus. 

\medskip{}
\noindent {*} Power supply section consists of low-dropout regulator U7, which
is adjusted by R6, R17 divider for 4.5V output voltage, voltage dropping circuit
D2, R20, to generate about 4.2V voltage for the analog board and optional inductorless
DC/DC converter U9 to generate -4.1V for powering the analog board. The chosen
voltage values are the optimal to minimize the analog board noise. The main
noise sources seem to be analog multiplexers, not sensor and not amplifiers.
And the multiplexer noise is rapidly reduced with decreasing of voltage. On
the other hand, the minimal voltage rating for the most digital ICs used is
4.5V and maximal difference between analog and digital supply voltages, for
good working of microcontroller-ADC interface is 0.3V. The dropout voltage of
the U7 chip is very small, so that sensor in whole can be powered by only on
5V...12V external power supply. The voltage of the external supply can be unregulated
(well works simple wall transformer), but to minimize the power dissipation
inside the sensor, better to use minimal value, but guaranteed not less than
5V on the input terminals of the sensor (J2:1, J2:5). The current consumption
is minimal for the RS485 interface version (less than 40mA when waiting and
less than 60mA when transmitting data). So, in case of input voltage 5V, inside
the sensor will be dissipated only about 0.2 - 0.3 W. 

\medskip{}
\noindent {*} As RS232 interface is used Maxim MAX3222 chip, which can work
properly at the supply voltages less than 4V. 

\medskip{}
\noindent {*} As RS485 interface is used Linear Technology LTC1480 IC. 

\medskip{}
\noindent {*} CANbus interface consists of CAN controller U2 and CAN transceiver
U3. 

\medskip{}
\noindent {*} All connections of the boards are made by two connectors: J1 for
stacking connection with the analog board and J2 to connect to the external
circuits: power, interface and in-system programmer. J2 is miniature, high density
(1.25mm pitch) connector, which has a matching pare with crimp housing, allowing,
thus, to connect different terminal wires to different external connectors,
depending on the interface type and hardware construction of the sensor in whole.

\vspace{1cm}
\textit{\large The software} consists of two parts: embedded microcontroller
program and PC program. The measurement will be made by the next procedure: 

\medskip{}
\noindent {*} The main PC switches the laser on and sends broadcasting message
to all connected to interface line sensors. 

\medskip{}
\noindent {*} All sensors simultaneously receiving this message perform the
photocurrent measurement for all 128 strips and save result in internal memory. 

\medskip{}
\noindent {*} The main PC after a necessary for measurement time = number of
averaging 

\medskip{}
\noindent {*} 10ms switches the laser off. 

\medskip{}
\noindent {*} The main PC afterwards, sequentially address by address reads
the data from sensors. If RS485 interface with 115Kbit/sec is used, readout
time for one sensor will be about 30ms.

\vspace{1cm}
\textit{\large Main advantages} of the new design are: 

\noindent 1. Small outline dimensions of the boards: 60x60mm. With the box,
assembled sensor will be slightly larger. 

\noindent 2. Much more reliable design: - there are not any contact connections
in analog circuits, instead wires are directly bonded to the analog board; -
there are not trimming potentiometers used. 

\noindent 3. Instead of very critical regulated power supplies (+8V, -8V, +5V)
with separate analog and digital grounds, new sensor needed only one, non-regulated
(in range +5...+12V) power supply. It works well even from usual wall transformer. 

\noindent 4. Much less power dissipation: 0.25W (for RS485 interface, Vsuppl
= 5V) compared with more than 1.5W of the old one. This parameter is very critical
because of laser beam thermal distortions. 

\noindent 5. Only standard components used, not ASIC. So the sensor can be manufactured
any time in any quantities. 

\noindent 6. Only 4 layer PCBs, compared with 8 (or 10?) of the old (much cheaper). 

\noindent 7. RS232 standard serial interface for testing and simple applications
is available directly on sensor, without external interface board. 

\noindent 8. RS485 standard interface, which will give possibility to connect,
using simple cheap adapter (30..50USD), up to 30 sensors to one standard serial
port of PC. 10. CANbus interface for more complicated applications.

\vspace{1cm}
\textit{\large Future improvements: }{\large \par}

The new one with the same size and pinout will replace the AT90S8515 microcontroller:
Atmega161, without any changes on the digital board. Using this new controllers
we will have new, important, properties. 

\medskip{}
\noindent {*} Controller will be able to be reprogrammed in-site without SPI
in system programming interface, but by using the same main interface with PC,
which is used for data acquisition: RS232, RS485 or CANbus. This property maybe
especially useful when sensors are used in large setups with hard radiation
conditions and limited (if any) possibility for maintenance, to refresh the
Flash program memory contents. 

\medskip{}
\noindent {*} The new controller has hardware multiplication instruction and
twice larger program memory. It seems to be possible to have the Gaussian fitting
algorithm immediately inside the microcontroller. So instead of 128 2-byte words
(total 256 bytes) could be transmitted only 2 4-byte numbers (8 bytes), one
for X and one for Y.

\bigskip{}
\noindent The Atmel Corporation claims, that the samples of these new controllers
will be available in July 2000.

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{\par\raggedright \resizebox*{1.1\textwidth}{1\textheight}{\rotatebox{-90}{\includegraphics{almy-v-ifc.ps}}} \par}

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{\par\raggedright \resizebox*{1.3\textwidth}{1\textheight}{\includegraphics{almy-sim.ps}} \par}

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