VTI Instruments EX1629, Руководство пользователя

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EX1629 Triggering 65
C
ONFIDENCE
M
EASUREMENT
S
YSTEM
The Confidence Measurement System runs in parallel with the main bridge measurement system,
but largely asynchronously. The Confidence Measurement System scans the various Confidence
Measurement Sources on each main input channel enabled in the scanlist. This scanning occurs at
the same rate as the main input channel sampling rate, up to 500 Sa/s. Above 500 Sa/s and below
1000 Sa/s, the Confidence Measurement System continues to sample at 500 Sa/s, inserting data
into the FIFO when new data is available. At 1000 Sa/s, confidence data is available with every
other dataset. The confidence data is filtered by a transfer function represented by the following
differential equation:
y (n) = 0.01 x (n) + 0.99 y (n-1), where y (n) is the filtered confidence data and
x (n) is the measured confidence data. This function serves to reduce noise variance.
Confidence data is acquired for every main input channel enabled in the scanlist. Which
confidence data is acquired is controlled by the confidence scanlist. There is only one confidence
scanlist in the system, and thus the same confidence data is sampled for every main input channel.
The amount of confidence data returned is basically the product of the length of the main input
channel scanlist with the length of the confidence scanlist. For example, if the main input scanlist
is configured to measure channels 0, 1, 3, and 5, and the confidence scanlist is configured to
mExcite and –Excite, the total number of confidence data values returned will be 8.
To be clear, there is a single FIFO in the EX1629 that holds both main input channel data as well
as confidence measurement system data.
ADC C
LOCK AND
S
YNCHRONIZATION
As was previously described, each input channel is an independent measurement system with its
own synchronous analog to digital converter (ADC). Input data is acquired synchronous to a
sample clock that is distributed to each ADC. In addition to the sample clock, a synchronization
signal is used to reset each of the ADCs and align them to acquire data synchronously. The
triggering system must be re-synchronized each time a change is made to the sample clock source,
sample frequency, or filtering parameters.
LXI L
IMIT
E
VENTS
LXI Limit Events allow the EX1629 to send triggers to other instruments when an over-limit
condition is detected for bridge measurements. Each of the eight LXI Trigger Bus lines can have a
minimum and/or maximum limit defined for any and all of the 48 bridge measurement channels.
When an over-limit condition occurs, the trigger line is driven high. Because the limit comparison
is performed after filtering, the group delay of the chosen filter also delays trigger assertion. When
all filters are disabled and at a sample rate of 1 kSa/s, the typical delay is 14 ms.
Limit events also allow the user to latch their trigger signal. When latching is enabled, the trigger
line is driven high when the first over-limit condition occurs and remains high until the instrument
is reset or until the next measurement sequence is initiated. When latching is disabled, the state of
the trigger line is decided once per scan. When all chosen limit conditions are satisfied, the line is
driven low. Whenever any limits are exceeded, the line is driven high.