User manual

User Manual: Agilent Acqiris 10-bit Digitizers Page 16 of 27
the individual trigger time stamps makes it possible to determine the time from one trigger to any other trigger. Time
differences up to 213 days can be measured. The TTI resolution sets the resolution of the trigger time stamps (see
section 3.3.5 TIMING). To maximize sampling rates and utilize memory as efficiently as possible, the digitizers
include both Single and Sequential storage modes. For both of these modes the data of all of the active channels is
acquired synchronously; all of the ADC’s are acquiring data at the same time, to within a small fraction of the
maximum sampling rate.
The Single Acquisition mode is the normal operation of most digitizer products. In this mode an acquisition consists
of a waveform recorded with a single trigger. The user selects the sampling rate and acquisition memory size and sets
the number of segments to 1 (default value).
The Sequence Acquisition mode allows the capture and storage of consecutive “single” waveforms. Sequence
Acquisition mode is useful as it can optimize the digitizer’s sampling rate and memory requirements for applications
where only portions of the signal being analyzed are important. The mode is extremely useful in almost all impulse-
response type applications (RADAR, SONAR, LIDAR, Time-of-Flight, Ultrasonics, Medical and Biomedical
Research, etc.).
In Sequence Acquisition mode the acquisition memory is divided into a pre-selected number of segments.
Waveforms are stored in successive memory segments as they arrive. Each waveform requires its own individual
trigger. The memory can be divided into any number of segments between 2 and 1000 (up to 16000 segments with
the M32M option in a DC282 and 125000 segments with options M256M or greater). In Sequence Acquisition mode
the user needs to specify the sampling rate, the total acquisition memory, and the number of segments. Note that the
Single Acquisition mode is just a special case of the Sequence Acquisition mode with the number of segments set to
1.
Sequence acquisition enables successive events, which can occur within a very short time, to be captured and stored
without loss. A crucial feature of Sequence Acquisition mode is that it has a very fast trigger rearm time. A fast
trigger rearm helps produce very low “dead time” (< 350 ns for the highest available sampling rates using internal
memory and < 1.8 μs for the highest available sampling rates with optional memory) between the segments of a
sequence acquisition. The “dead time” is the period after the end of an event when the card cannot digitize data for a
new trigger event.
3.3.4. Simultaneous multibuffer Acquisition and Readout (SAR)
The internal memory has a dual-port structure which can be exploited to permit simultaneous data acquisition and
read out. The memory can be turned into a circular buffer of a chosen number of banks, between 2 and 1000. Data
can be read out of one bank while data is acquired into any available free banks. This mechanism, together with
sequence acquisition, helps achieve a high maximum continuous event rate in spite of interrupts due to the computer
operating system. The maximum continuous event rate is the maximum value of the trigger frequency that can be
accepted without the loss of any event. Note that this mode is available for internal memory only on single modules
(no AS bus) and does not work for all cases of sampling rate and channel combination. It does not work if the ‘Start
on Trigger’ mode of acquisition is selected. It will work for the maximum sampling rate. This mode is available for
the DC222, DC252, and DC282 only. Please contact us if you have any doubts.
3.3.5. Timing
A crystal-controlled timebase is used to clock the ADC system of the digitizers. The timebase accuracy is guaranteed
to be better than 2 ppm . The digitizers also include a built-in Trigger Time Interpolator (TTI) that measures the time
from the trigger point to the first sample point. This information is essential for determining the precise relation
between the trigger or other event of interest and the digitized samples of the signal. The TTI resolution is ~13 ps.