PRELIMINARY
Document type: Title: Revision date: Revision:
User's Manual (MUT) Mod. V1729 4 Channel 12 Bit Sampling ADC 22/06/2005 3
NPO: Filename: Number of pages: Page:
00109/04:V1729.MUTx/03 V1729_REV3.DOC 38 17
BUSY is a signal destined to inform the external world that the board is not ready to
receive a signal (it is either idle, or in the process of transferring data between the
MATACQ chips and the RAM, or at the beginning of the acquisition phase). Its transition
to 0 indicates then that the board is now ready to record a signal.
3.3 Calibrations
In order to attain the optimal metrological performance, the V1729 board requires
calibrations. These remain valid for several weeks, even several months.
3.3.1 Calibration of the interpolator
The temporal interpolator (vernier) requires a calibration. This calibration, based on a
statistical measure, will require the acquisition of several thousand events and can last
from one second to a couple of minutes depending on the type of bus and on the
software used. It can be done with the inputs connected to any source, but the trigger
must be asynchronous in relation to the clock. The result of this calibration will be a
“square” histogram, of which the sides correspond to two successive rising edges of Fp.
The corresponding value on the left side (MINVER) will be the « zero » of the vernier,
and the one on the right side (MAXVER) will correspond to a vernier of 1/Fp (in other
words 10 or 20ns). The intermediate values will next be deduced at the time of the
acquisitions by a simple proportional calculation.
The diagram in Fig 3.1, which follows from that in Fig 2.5, shows the sequencing of such
a calibration. It is optimized so as to be the fastest possible. In particular :
The PRETRIG and POSTTRIG are fixed at their minimal value.
NB_OF_COL_TO_READ has to be set to 1.
Only the vernier values are read in the RAM. In other words, if NCH channels are read,
only the 2*NCH first data are read and only the NCH+1 to 2*NCH words are conserved
(see 4.7).
After a certain number of acquisitions (at least 10000), the boundaries of the vernier
MINVER and MAXVER can be calculated.
At first approximation, it is possible to simply use the minimal and maximal values of the
vernier for MAXVER and MINVER. This method is sufficient to obtain temporal
resolutions in the region of 70ps RMS.
To have greater precision, one must in order to find the two sides fix a threshold
corresponding to half the mean number of ADC counts in the « square » distribution.
To obtain even more precision, it is possible to use more complex methods.
A second, much faster method of calibration is available. For this :
Charge NB_OF_COL_TO_READ at 0.
Position the trigger in auto mode.
Authorize the internal random trigger.
Launch a Start_ACQUISITION.
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