VTI Instruments EX1629, Руководство пользователя

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EX1629 Basic Operation 55
Each input channel provides a unique, precision 55 kΩ resistor that can be connected locally
(switch C) for quarter-bridge shunting or remotely (switch B) for full- or half-bridge shunting.
Since there is a resistor per channel, all 48 input channels may be shunted simultaneously.
If an alternate shunt resistor is required, an external resistor may be connected into each of three
front panel connectors, one for each group of 16 input channels. Each resistor may be connected
locally (switches D, F) for quarter-bridge shunting or remotely (switches A, F) for full- or half-
bridge shunting. Up to three front panel resistors may be used simultaneously, but not for more
than one channel in each 16-channel block.
For highest accuracy, the value of this external resistor should be precisely known. The connection
resistance characteristic in the specifications refers to the total resistance of the connection
switches. This resistance must be considered in conjunction with the raw resistor value when
determining the theoretical simulated strain of the shunt calibration process. The connection
resistance is the same for local or remote connection.
Finally, the EX1629 offers the ability to read and control a TEDS-equipped remote resistor
(switches A, E) for full- or half-bridge shunting. Up to three TEDS remote resistors may be used
simultaneously, but not for more than one channel in each 16-channel block.
Table 3-4 summarizes the shunt calibration capabilities and the specific connection switches they
utilize.
Setting Usage Switches
front_panel_remote 1 per 16 channels A, F
front_panel_local 1 per 16 channels D, F
internal_remote every channel B
internal_local every channel C
teds_remote 1 per 16 channels A, E
T
ABLE
3-4: S
HUNT
C
ALIBRATION
S
ETTINGS
Shunt calibration configuration involves two discrete and disjoint operations. Each channel is
assigned a shunt calibration mode, as described above. Separately, each shunt is enabled
(connected) or disabled (disconnected).
The default shunt calibration mode is internal_remote. The default shunt enable setting is disabled.
S
ELF
-C
ALIBRATION
In order to deliver high measurement accuracy over a wide ambient operating temperature range,
the EX1629 provides the ability to perform an instrument self-calibration. During self-calibration,
the input signal conditioning paths are internally disconnected from the input jacks and connected
instead to a calibration bus that is driven by an internal, precision calibration source. Through
measurement of the conditioning paths at multiple calibration source points, software
compensation for circuitry drift since the last full calibration is conducted. Once self-calibration is
performed, the accuracy specifications listed in Section 1 are valid for 30 days and over a ±5 ºC
ambient temperature change. Note, however, that the unit must still undergo a full calibration at
least once a year, regardless of the use of self-calibration.
Self-calibration should be conducted as often as practical, especially if the ambient environment
has changed significantly since the previous calibration. However, fast ambient environmental
changes should ideally be followed by a period of thermal stabilization before conducting self-
calibration to allow the internal circuitry to stabilize to a new thermal operating condition. The
self-calibration process completes quickly and does not require removal of the actual input
connections, making it convenient to run often.