VTI Instruments Corp.
APPENDIX B
54
M
EASUREMENT
P
ROCESS
M
EASUREMENT
S
ETUP
After the instruments are initialized by driver
Initialize
method, or the previous measurement is
finished, the instrument is in
Idle
state. While in the
Idle
state, the user prepares for the next data
acquisition by configuring the setup parameters.
Start, Arm, and Trigger
Properties in the
Start
,
Arm
, and
Trigger
interfaces configure the data acquisition gating condition
and timing.
Start
determines when the data acquisition starts and when the instrument is ready for
the next acquisition. By the time of
Start
, all hardware configurations and filters settling need to be
complete in order to ensure valid data is acquired.
Sampling Parameters
The sampling parameters are specified by properties in
Sampling
interface.
Sampling
in the
Measuremen
t interface configures parameters common to entire system. Optionally, the user can
configure the parameters of individual input channels using the
Channels
interface (with some
restrictions).
Analog Front End
While some parameters can be changed during the data acquisition, most configuration options
should be performed prior to data acquisition to avoid data glitches. Most of front end parameters
can be configured independently for each input channel. The front end configuration includes input
range, AC/DC coupling, IEPE current, voltage/charge input function, and single-ended or
differential input mode. To ensure the most accurate measurements, it is highly recommended that
SelfCalibration
be performed prior acquiring data.
M
EASUREMENT
I
NITIATION
After the user has completed configuring the instrument, the user can initiate the data acquisition
process by calling the
Initiate
method in the
Measurement
interface. During measurement initiation,
the instrument starts to prepare for the actual data acquisition. If the ADC has not been
synchronized, the SYNC signal is sent and ADC then begins digitization. When the ADC’s
reference oscillator has been changed, the ADC sampling clock must be re-locked with the PLL.
Filter settling also occurs at this time. Once the filters have settled and the instrument is ready to
acquire valid data, the measurement moves to the
Wait for Start
state. The source of the
Start
event
is
Immediate
by default. In this case, the measurement starts automatically. Otherwise, it must be
started by an event specified by the
Source
property of the
Start
interface.
During this period, the digitized signal from the A/D converter is continuously filtered and discarded
until the filters have settled. Once settled, the filtered data samples are stored in the internal data
buffer.
M
EASUREMENT
L
OOP
Once the measurement begins, the state machine cycles through arming and triggering for the
number of times specified by
ArmCount
and
TriggerCount.
The measurement stops when the
specified number of arm and trigger loops are completed or when it is aborted by an
Abort
command
or when the FIFO buffer became full and the FIFO mode is set to
Stop
.
