\section{Global sensor performance}
\label{sec:sensor}

Global aspects of the sensor performance were also studied and
are discussed below.

\subsection{Configuration load}
The per-pixel five-bit configuration data were loaded and
read back to check for bit errors. Each load transfered
141\,kbits into the sensor configuration registers. The
load and readback cycle was repeated 25,000 times and
the data written and readback were compared. No bits
were seen to have been corrupted in this process. This
sets an upper limit on the bit error rate of $1\times 10^{-9}$
at 90\% confidence level.
%TO DO: DATA SPEED

\subsection{Pickup between pixels}
\label{sec:sensor:pickup}

As mentioned in section~\ref{sec:pixel:peds}, pickup was
observed between pixels when large numbers fired at the
same time. For this reason,
all the results described in section~\ref{sec:pixel} were
done with only a small number of unmasked pixels.
Typical pickup behaviour is illustrated in
figure~\ref{fig:sensor:pickup} where the threshold
scan for a single pixel is shown with just that pixel
unmasked and also with all pixels unmasked.
It is clear the distribution when all pixels are enabled
is uncorrelated with the
pedestal of the actual pixel being studied;
it depends on the pedestals of the
other pixels firing. The scan with pickup has a much
larger RMS and this has been used to characterise the
onset of pickup.
%
\begin{figure}[ht!]
\begin{center}
%\includegraphics[width=6cm]{sensor/Run470422A.png}
%\includegraphics[width=6cm]{sensor/Run470436A.png}
\includegraphics[width=6cm]{sensor/pickups1.png}
\includegraphics[width=6cm]{sensor/pickups2.png}
\caption{\sl 
Effect of pickup on a typical pixel with only the 
pixel enabled (left) and with all other pixels enabled (right).
}
\label{fig:sensor:pickup}
\end{center}
\end{figure}

Many studies to analyse this effect were performed. It was found
that the level of pickup depended on the number of pixels unmasked
but not their geometric positions relative to each other. One
example of these studies which shows this result clearly is shown
in figure~\ref{fig:sensor:masking}. In this study, a single pixel
on one edge of the sensor was unmasked. The RMS of this pixel
from a threshold scan was then determined as an increasing number
of pixels
were unmasked along the other edge of the sensor, i.e. at the
furthest point from the single pixel. It is seen that pickup on the
isolated pixel has the same behaviour as that of all the pixels
along the far edge. Hence, the pickup is seen to be unrelated to
position.
%
\begin{figure}[ht!]
\begin{center}
%\includegraphics[width=7cm]{sensor/pickupvsn.png}
\includegraphics[width=7cm]{sensor/droop.png}
\caption{\sl 
Onset of pickup, as measured by the RMS determined from a
threshold scan, as a function of the number of unmasked channels.
Each of the 16 lines corresponds to a single pixel, one
of which (in red) is on the opposite side of the sensor to the others.
%The x axis corresponds to the number of rows of 42 pixels
%which were unmasked, in addition to a single edge row which
%was always active.
%The line labelled ``63,0'' corresponds to an isolated pixel
%on one edge of the sensor,
%while all the other lines correspond to a subset of pixels in
%the row along
%the opposite edge. It is seen that pickup starts around an
%x axis value of 6, which corresponds to 294 pixels unmasked.
It is seen that pickup starts on most pixels when around 300
pixels are unmasked and it is independent of the pixel position.
}
\label{fig:sensor:masking}
\end{center}
\end{figure}

The pickup is not due to coupling directly between
monostables as this would not be affected by masking pixels.
It is thought that the pickup is caused by a power mesh
being shared between the pre-shape comparator and monostable
components. 
If a large number of pixels fire and
give hits in the monostables, the common power level
droops
and causes the comparators in other pixels to
fire. 
The power meshes to these two components have been
separated in the next version of the sensor.

It is important to note that this power coupling effect
does not affect normal operation of the sensor. 
It is only when doing sensor tests involving threshold
scans with low thresholds,
such as a pedestal determination, 
that large numbers of pixels fire simultaneously.
When operating
as a particle detector, then the threshold setting
is high enough that only small numbers of pixels fire and
hence the power droop is never large enough to cause spurious
hits. In the mode, all pixels can be operated unmasked and
the sensor has high efficiency.


\subsection{Hit corruption}
The ``hit override'' mode described in
section~\ref{sec:design:storage} allows hit data to be
stored in the sensor memory without requiring any
pixels to be above threshold. In this mode, all seven
banks of pixels in each row of 42 pixels will store
data for every timestamp and hence the 19 data registers
will be filled within the first three timestamps.

This test mode results in predictable values for the
MUX address, timestamp and row address for every data word
stored. These can be compared with the values read out
to check for corruption. (Note, the six-bit hit data 
themselves are not predictable as noise can cause them
to be randomly set.) A low level of corruption, at
around 0.1\%, was seen for the MUX address and timestamp
bits, although the actual rate varied from sensor to sensor.
This was traced to a design fault; the signal to write to
the SRAM did not have a wide enough voltage range to guarantee
the value would be written correctly. The rate of corruption
was eliminated by applying an external voltage level of 2.7\,V
to increase the range and this was used for all the
results shown in this paper.
This design fault has been corrected in the next version
of the sensor.
