Agilent Technologies DP111 - Manual (Page 29)

User Manual: Agilent Acqiris 8-bit Digitizers Page 29 of 59
The U1071A-FAMILY has a fixed threshold and uses AC coupling. Otherwise, the transitions of the clock are
defined with the aid of a threshold that is user selectable in the range [-2.0 V, 2.0 V] ([-3.0 V, 3.0 V] for the DC240).
The signals should not exceed ±5 V amplitude. For a detailed discussion on the programmed use of the external
clock, refer to the Programmer’s Guide.
For applications that require greater timing precision and long-term stability than is obtainable from the internal
clock, a 10 MHz Reference signal can be used. The External Reference is nominally at 10 MHz. However,
frequencies in the range [9.0 MHz, 10.2 MHz] will be accepted for all models other than the U1071A-FAMILY. If
you do this you may need to correct for the difference in your application since the digitizer and the driver have no
way to know about such deviations. The amplitude and threshold conditions, for an External Reference, are the same
as for the External Clock. If synchronization between several digitizers is required, the reference signal should be
applied to all of them.
3.6. Internal Calibration
The software drivers supplied include calibration functions for the timing, gain and offset settings, which can be
executed upon user request. The digitizers are never calibrated in an “automatic” way, i.e. as a side effect of another
operation. This ensures programmers have full control of all calibrations performed through software in order to
maintain proper event synchronization within automated test applications.
The model DC and DP series digitizers include a high precision voltage source and a 16-bit DAC, used to determine
the input voltage and offset calibration.
For accurate time and voltage measurements it is recommended to perform a calibration once the module has attained
a stable operating temperature (usually reached with a few minutes of digitizer operation after power on). Further
calibration should not be necessary unless temperature variations occur.
A full internal calibration of a digitizer can be very time consuming (> 100 s/digitizer), in particular for the DC2x1A
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. Calibration can usually be performed with signals present at the channel, external, and clock inputs.
However, if the calibration is found to be unreliable, as shown by a calibration failure status, it may be necessary to
remove such signals.
3.7. AS bus
The DC Series 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 digitizer, except the
DC135 and DC140 models, includes AS bus, a proprietary high bandwidth auto-synchronous bus system that allows
multiple digitizers to work together synchronously.
The AS bus 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.
The AS bus 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 AS bus connector is located on the front panel of each cPCI module. Bridges are used to connect adjacent
modules for synchronization, which is then activated through software by defining a master module and the trigger
source. Up to 7 modules may be synchronized with the AS bus. More detailed information and the commands
required to setup the AS bus clock and trigger distribution are included in the Programmer's Guide and
Programmer’s Reference Manual.
The U1071A-FAMILY uses the AS bus 2 system with a connector on the top of each card to achieve the same
functionality. It works for up to 3 modules.