VXI VT1422A - Manual (Page 155)

Programming the VT1422A for Data Acquisition and Control 153 Chapter 4
Compensating for System Offsets
System Wiring Offsets
The VT1422A can compensate for offsets in the system’s field wiring.
Apply shorts to channels at the Unit-Under-Test (UUT) end of the field
wiring and then execute the CAL:TARE (@<ch_list>) command.
The instrument will measure the voltage at each channel in <ch_list> and
save those values in RAM as channel tare constants.
Important Note for
Thermocouples
•
CAL:TARE must not used on field wiring that is made up of
thermocouple wire. The voltage that a thermocouple wire pair
generates cannot be removed by introducing a short anywhere
between its junction and its connection to an isothermal panel (either
the VT1422A’s Terminal Module or a remote isothermal reference
block). Thermal voltage is generated along the entire length of a
thermocouple pair where there is any temperature gradient along that
length. To CAL:TARE thermocouple wire this way would introduce
an unwanted offset in the voltage/temperature relationship for that
thermocouple. If a thermocouple wire pair is CAL:TARE’d
inadvertently, see “Resetting CAL:TARE” on page 154.
•
CAL:TARE should be used to compensate wiring offsets (copper wire,
not thermocouple wire) between the VT1422A and a remote
thermocouple reference block. Disconnect the thermocouples and
introduce copper shorting wires between each channel’s HI and LO,
then execute CAL:TARE for these channels.
Residual Sensor Offsets
To remove offsets like those in an unstrained strain gage bridge, execute the
CAL:TARE command on those channels. The module will then measure the
offsets and as in the wiring case above, remove these offsets from future
measurements. In the strain gage case, this "balances the bridge" so all
measurements have the initial unstrained offset removed to allow the most
accurate high-speed measurements possible.
Operation
After CAL:TARE <ch_list> measures and stores the offset voltages, it then
performs the equivalent of a *CAL? operation. This operation uses the tare
constants to set a DAC which will remove each channel offset as "seen" by
the module’s A/D converter.
The absolute voltage level that CAL:TARE can remove is dependent on the
A/D range. CAL:TARE will choose the lowest range that can handle the
existing offset voltage. The range that CAL:TARE chooses will become the
lowest usable range (range floor) for that channel. For any channel that has
been "CAL:TARE’d" Autorange will not go below that range floor and
selecting a manual range below the range floor will return an Overload value
(see the table “Maximum CAL:TARE Offsets” on page 154).