Agilent Technologies DP111 - Manual (Page 25)

User Manual: Agilent Acqiris 8-bit Digitizers Page 25 of 59
3.3.2. Acquisition Memory
Data from the ADC is stored in on-board acquisition memory. The amount of memory in use for acquisition can be
programmed and is selectable from 1 point to the full amount of acquisition memory available.
For technical reasons, a certain memory “overhead” is required for each waveform, reducing the available memory
by a small amount. In order to simplify programming, an interface function recommends the best sampling rate and
the maximum possible number of data points, taking into account the available memory, the requested time window,
the number of segments (in Sequence mode), as well as the required memory overhead.
To ensure maximum sampling rate and high timing resolution, we strongly recommend the use of long acquisition
memories whenever possible. For example, the model DC110 or DP110 with 2 Mpoints of memory can record a
signal over a 2 ms period with a sampling rate of 1 GS/s (1 ns per point). The fast sampling rate ensures that all high
frequency signal components, up to the full 250 MHz bandwidth of the digitizer, are accurately recorded. If the
memory were reduced to just 20 Kpoints then the sampling rate would need to be reduced to just 10 MS/s
(20,000/2 ms) to record the same 2 ms period. All frequencies above 5 MHz would then be incorrectly digitized and
important signal characteristics may be distorted or even completely missed.
3.3.3. Single and Sequence Acquisition Modes
Digitizers acquire waveforms in association with triggers. Each waveform is made of a series of measured voltage
values (sample points) that are made by the ADC at a uniform clock rate. 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. For most modules the memory can be divided into any number of segments between 2 and 200 (up to 8000
segments with options M32, M16, M8M and M4M, 4000 segments with option M2M, 2000 with option M1M). The
U1071A-FAMILY can have up to 1000 segments for the standard memory and up to 64000 with option -256. 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” (less than 800 ns for the highest available sampling rates in most
units, however 350 ns for the U1071A-FAMILY internal memory and 1.5 μs for its external 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. To complement this mode of operation the digitizer can also measure and store
the arrival time of each trigger using the information from the on board Trigger Time Interpolator (TTI). Readout of
the individual trigger time stamps makes it possible to determine the time from one trigger to any other trigger in the
sequential acquisition. The TTI resolution sets the resolution of the trigger time stamps (see section 3.3.5 Timing and
the table above).
3.3.4. U1071A-FAMILY Simultaneous multibuffer Acquisition and Readout
(SAR)
For this module the internal memory has a dual-port structure which can be exploited to permit simultaneous data
acquisition and read out. When the special SAR option is present the internal 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. It will work for
both 1 and 2 converter/channel modes. It does not work if the ‘Start on Trigger’ mode of acquisition is selected nor