Agilent Technologies DC152 - Manual (Page 34)

A full internal calibration of a digitizer can be very time consuming (> 100 s/digitizer), in particular for the HZ
models. Therefore, several other options are available. They are documented in the Programmer's Reference
Manual. A program can always be started with the digitizer in an uncalibrated state and data taken can be used for
many kinds of testing. However, as soon as good data respecting the specifications of the instrument is required a
calibration of at least the current acquisition state is needed. The full internal calibration has the advantage that it
generates the calibration constants needed for any possible configuration of the instrument; its disadvantage is the
time taken. If a more selective calibration is done it will allow the generation of good data in the current acquisition
state. This calibration will remain useable whenever that acquisition state is used again for as long as the temperature
of the instrument does not change significantly. A fast calibration of a channel in a configuration can be done in
around a second. Many applications can save time by only performing calibration for the configurations that will
actually be used.
An application program will close devices if it exits normally. When a 10-bit digitizer is closed in this way its power
consumption will be reduced. This has the consequence that if another application program is started later an
appropriate warm-up period may have to be respected before a calibration is called for and full performance
expected.
3.7. ASBus
2
The digitizers may be used in applications that require many data acquisition channels. In such cases it is possible to
use more than one digitizer in a standard CompactPCI/PXI crate. Each DC Series 10-bit digitizer includes ASBus
2
, a
proprietary high bandwidth auto-synchronous bus system that allows multiple digitizers to work together
synchronously.
ASBus
2
distributes both the clock and trigger signals along a plug-in front panel bus, between all the digitizers that
participate in the system. It allows any digitizer to act as the trigger source and any other digitizer to act as the clock
source (acquisition master), enabling all the digitizers to be clocked at the same time. Synchronizing the clock signals
between the devices improves the accuracy of cross-channel measurements and is essential for accurate time
correlation between signals on different channels.
ASBus
2
is intended to connect modules of the same type, i.e. of the same model number, although some exceptions
to this rule might be possible. If modules with the same model number, but different memory length options, are
connected only the shortest memory length can be used.
The ASBus
2
connector is located on the front panel of each module. Bridges are used to connect adjacent modules
for synchronization. An ASBus
2
multi-instrument is activated through software by a function call. Up to 5 6U
modules, or 3 3U modules, may be synchronized with the ASBus
2
. More detailed information and the commands
required to set up the ASBus
2
clock and trigger distribution are included in the Programmer's Guide and Reference
manuals.
3.8. Special Front Panel Inputs and Controls
Connector Function
MMCX CLK IN 50 Ω External Reference or External Clock Input
MMCX I/O A User configurable (see below)
MMCX TR OUT Signal occurs after an accepted TRIGGER.
MMCX I/O B User configurable (see below)
The I/O A and I/O B signals are 3.3 V compatible CMOS. This means that, on input, low is < 0.7 V and high must be
in the range [1.7 V, 5.0 V]. An unconnected signal will be high. This definition ensures TTL compatibility. On
output, the low level will be in the range [0 V, 0.7 V] and the high level in the range [1.7 V, 3.3 V] for HiZ. The high
level output will typically give 0.8 V into 50
Ω. The I/O A or B connectors can be used for an Enable trigger input or
the following output signals: 10 MHz reference clock, Acquisition skipping to next segment, Acquisition is active,
Trigger is armed.
3.9. External Trigger Output
When the digitizer is ready to be triggered and a valid trigger signal occurs, a trigger output is generated for external
use. It is always available on the Front Panel Trigger Out MMCX connector. The pulse ends when the data
acquisition for the trigger in question is complete.
User Manual: Family of 10-bit Digitizers Page 34 of 55