July 24, 2015 David Lawrence

This directory contains 4 files used to configure the fADC-125
module:

threshold
offset
lookback
window

Each contains a single number in plain text format. The numbers 
are read during the prestart transition of CODA in the cpp_test_list
readout list. These numbers are used to configure all 72 channels.
Note that the fADC-125 module is capable of having different threshold
and offset values for each channel. This is not utilized for this
test setup since it is assumed that the rates will be low enough
that the full sample spectrum may be recorded for every channel,
every event. Thresholds and offsets can therefore be effectively 
adjusted in the postprocessing/analysis stage. Thus, the use of
common values for the threshold and offset in the acquisition 
stage.

Details on each file follow.

threshold
==========================
The value for "threshold" is set to "0" to effectively disable
the threshold. This forces every channel to be read out for
every event. Keep in mind though that the readout list is 
hardcoded to only read out the first connector (i.e. first
24 channels).


offset
==========================
The value of offset is used to program a DAC (Digital to Analog
Converter) that applies a DC offset to the channels. This is used
to compensate for any DC offset on the signal lines and to push the
signals up slightly. Because the samples are recorded as unsigned
integers, any reading that goes below zero results in an underflow
condition. The convention used in Hall-D has been to adjust the
offset value until the samples read 100 counts when no signal is
present. This must be done empirically by taking data and adjusting 
the value of offest and then taking data again. By way of example,
in the EEL 126 lab at Jefferson Lab with no inputs attached, the
following values were observed:

offset   pedestal
------   --------
36864      2364
16384       937
 4371        93


lookback
==========================
This value specifies the number of samples to look back from when the
trigger is received. This must be adjusted based on how long it
takes the trigger signal to get to the module after the detector
signals have made it there.

window
==========================
This indicates how many samples should be read out for the event. This
value must be less than the "lookback" value. The samples recorded to
disk will be from (trig_time-lookback) to (trig_time-lookback+window).
Intially, one may need to set a large lookback and a large window value
just to identify where the signal lies in time relative to the trigger.
These should be tightened up a bit though once that is determined since
the amount of data one has to deal with will be directly proportional
to "window".