VTI Instruments Corp.
48 EX1629 Basic Operation
COMPLETION RESISTOR
In a standard quarter-bridge strain application, the completion resistor value must match the
nominal resistance of the active strain gage. The required completion resistor is normally enabled
through the appropriate setting of the EU conversion. For example, setting a quarter-bridge 350
conversion automatically enables the 350 Ω completion resistor. However, there are potentially
desirable configurations that are not covered by the standard conversions. An example of this is a
quarter-bridge configuration that provides voltage output instead of strain (this is sometimes
useful as a “sanity” check). In that case, the completion resistor must be specifically configured.
Creating a nonstandard EU conversion begins by selecting the standard conversion that provides
the desired conversion equation. Following that, the nominal settings of the completion resistor
and input multiplexer can be changed to fit the requirements of the application.
The available settings for the completion resistor are full, 120, 350, user, and off. 120 and 350
refer to the 120 Ω and 350 Ω resistor paths. Full refers to a low-impedance connection between the
–excitation source and the –Excitation connection (Pin 2 of the input connector). The typical
resistance in full mode is 125 mΩ. User refers to the user-specified resistor path, which may be
populated as a factory option. Off refers to the disabling of all of the resistor paths.
The default value of the completion resistor is full.
INPUT MULTIPLEXER
The connection of the EX1629’s signal conditioning circuitry is governed by the setting of its
input multiplexer, which connects the measurement path to the input measurement lines (±Sense),
the back-half resistors, the calibration source, or ground. The required input multiplexer
configuration is normally controlled through the appropriate setting of the EU conversion. For
example, setting a quarter-bridge 350 conversion automatically configures the input multiplexer
for quarter mode, as shown in Figure 2-3. However, there are potentially desirable configurations
that are not covered by the standard conversions. An example of this is a quarter-bridge
configuration that provides voltage output instead of strain. In that case, the input multiplexer
must be specifically configured.
Creating a nonstandard EU conversion begins by selecting the standard conversion that provides
the desired conversion equation. Following that, the nominal settings of the completion resistor
and input multiplexer can be changed to fit the requirements of the application.
The available settings for the input multiplexer are shown in Table 3-1, mapped to the specific
connections of the instrumentation amplifier inputs.
TABLE 3-1: INPUT MULTIPLEXER SETTINGS
The default value of the input multiplexer is full.
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