VTI Instruments EX1629, Руководство пользователя

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VTI Instruments Corp.
22 EX1629 Introduction
E
XPLANATION OF
S
PECIFICATIONS
This section provides explanatory notes to certain elements of the EX1629 specifications that may
be confusing or easily misunderstood.
Sampling Rate
The EX1629 ADCs run at 50 kSa/s and data is decimated down by an integer factor to the user-
selected sample rate. The EX1629 is capable of supporting a sampling rate of 10 kSa/s on all
48 channels simultaneously. Over a limited number of channels, however, the EX1629 is capable
of supporting a sample rate of 25 kSa/s. In order to realize this higher sampling rate, the number of
operating channels must be limited to 16 maximum and these channels must all exist on the same
analog board (i.e. channels 0 through 15, 16 through 31, or 32 through 47 can be selected). If these
conditions are not met, an error will occur.
Bridge Excitation
Performance of the excitation source is quantified in two ways. Set point accuracy refers to the
absolute accuracy of the excitation output compared to its nominal programmed value.
Conversely,
stability refers to the drift characteristics of the source once it has been programmed
to a specific value. Since the EX1629 provides the ability to measure the excitation source and use
the measured value in the EU conversion, it is the source’s stability that ultimately effects strain
measurement accuracy, not its set point accuracy.
While the source’s performance characteristics are provided, they should not be added to the listed
quarter-bridge and full-bridge accuracy tables, as these accuracy tables already contain the effects
of the excitation source stability. The source characteristics are listed for reference and for the
possibility that a user might use the EX1629 to provide bridge excitation, but another piece of
equipment to measure the bridge output. In that case, the excitation performance is required in
order to calculate the total system uncertainty. For this analysis, it must be noted that the listed
characteristics are for each source independently.
Bridge Completion
The characteristic input connector lead resistance refers to the residual resistance that the input
connector represents in a quarter-bridge configuration. Specifically, referring to Figure 2-3, this is
the resistance between Pin 1 (+Excitation) of the connector and the local connection of the
+Excitation Sense line. Similarly, it is the resistance between Pin 2 (-Excitation) and the point at
which the completion resistor is shunted. These resistances serve to slightly desensitize a quarter-
bridge configuration, even in the absence of external lead wire resistances. This resistance is
specified from 30 mΩ to 60 mΩ, depending on the selected channel. This results in an
uncompensated gain error on a 350 Ω bridge of 86 ppm to 172 ppm.
This error is not reflected in the quarter-bridge accuracy table, however, because it is assumed that
lead wire compensation will be done to remove the effects of external lead wire resistance. That
process will simultaneously and completely compensate for this resistance, as it is matched within
each channel to within 5 mΩ. If the lead wire compensation is done via shunt calibration, the
30 mΩ possible difference between channels is unimportant, as each channel will undergo a
unique shunt calibration. If, however, the lead wire compensation is done theoretically (as one
might do if the connecting cable is well characterized), an average compensation of 129 ppm
should be used for the internal resistance. This would leave a possible uncompensated error of
only 43 ppm.
Automatic measurement is possible, conversely, using the traditional shunt calibration process.
Shunt calibration is the process of placing a known resistance in parallel with one of the bridge
elements to create a known simulated strain. For quarter-bridge configuration, this element is
usually the internal completion resistor. In this method, the deviation of the actual measured strain