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\subsection{Smart-photodetector electronics}
\label{sec:photo}
%
The coverage of large areas (around 17,500 m$^2$ for MEMPHYS) with
photodetectors at lowest cost implies a readout integrated electronics
circuit (called ASIC). This makes it possible to integrate: high-speed
discriminator on the single photoelectron (pe), the
digitisation of the charge on 12 bits ADC to provide numerical signals
on a large dynamical range (200~pe), the digitisation of time on 12
bits TDC to provide time information with a precision of 1~ns, and
channel-to-channel gain adjustment to homogenize the response of the
photomultipliers and to thus use a common high voltage. Such an ASIC
for readout electronics allows moreover a strong reduction of the
costs, as well as external components (high-voltage units, cables of
great quality...) since the electronics and the High Voltage may be
put as close as possible to the PMTs and the generated numerical
signals are directly usable by trigger logical units and 
the data acquisition computers (Fig.~\ref{fig:MEMPHYSPMTS}). 
%

The main difficulty in associating very fast analog electronics and
digitization on a broad dynamic range does not make it possible yet to
integrate all these functions in only one integrated circuit, but
certain parts were already developed separately as for example in the
OPERA Read Out Channel \cite{Lucotte:2004mi}
(Fig.~\ref{fig:OPERAROC}). The evolution of integrated technologies,
in particular BiCMOS SiGe 0.35$\mu$m, now make it possible to consider
such circuits and has triggered a new campaign of research and
development. 

\begin{figure}[htb]
 \begin{minipage}[c]{0.44\textwidth}
\centering
\includegraphics[width=0.85\textwidth]{./figures/ModuleArriere.eps}
\caption{\label{fig:MEMPHYSPMTS}
\it Sketch of a possible photosensor basic module composed of a matrix
of $4\times4$ 12" PMTs with the electronic box containing the High
Voltage unit and the Readout chip.}	 
\end{minipage}
%
 \begin{minipage}[c]{0.04\textwidth}
~
\end{minipage}
%
 \begin{minipage}[c]{0.44\textwidth}
\centering
\includegraphics[width=\textwidth]{./figures/electronic.eps}
\caption{\label{fig:OPERAROC}
\it Sketch of the existing Read Out electronics developed for the
OPERA Target Tracker and that is intented to be extended for MEMPHYS
by integrating the ADC and TDC.}	 
\end{minipage}
\end{figure}
%

\newpage


