multifunction-rio-user-manual.pdf - 第49页
© National Instruments Corporation 3-1 Mult ifunction Reconfigurable I/O User Manual 3 Calibration (NI 783 x /784 x /785 x Only) Calibration is the process of determin ing and/or adjusting the accuracy of an instrument t…
Chapter 2 Hardware Overview of the NI 78xx
Multifunction Reconfigurable I/O User Manual 2-30 ni.com
Field Wiring Considerations (NI 783x /784x /785x Only)
Environmental noise can seriously affect the measurement accuracy of the
device if you do not take proper care when running signal wires between
signal sources and the device. The following recommendations mainly
apply to AI signal routing to the device, as well as signal routing in general.
Take the following precautions to minimize noise pickup and maximize
measurement accuracy:
• Use differential AI connections to reject common-mode noise.
• Use individually shielded, twisted-pair wires to connect AI signals to
the device. With this type of wire, the signals attached to the positive
and negative inputs are twisted together and then covered with a shield.
You then connect this shield only at one point to the signal source
ground. This kind of connection is required for signals traveling
through areas with large magnetic fields or high electromagnetic
interference.
• Route signals to the device carefully. Keep cabling away from noise
sources. The most common noise source in a PXI DAQ system is the
video monitor. Keep the monitor and the analog signals as far apart as
possible.
Use the following recommendations for all signal connections to the
NI 783x/784x/785x:
• Separate NI 783x/784x/785x signal lines from high-current or
high-voltage lines. These lines can induce currents in or voltages on
the device signal lines if they run in parallel paths at a close distance.
To reduce the magnetic coupling between lines, separate them by a
reasonable distance if they run in parallel or run the lines at right angles
to each other.
•Do not run signal lines through conduits that also contain power lines.
• Protect signal lines from magnetic fields caused by electric motors,
welding equipment, breakers, or transformers by running them
through special metal conduits.
Refer to the NI Developer Zone tutorial, Field Wiring and Noise
Considerations for Analog Signals, at
ni.com/zone for more information.
© National Instruments Corporation 3-1 Multifunction Reconfigurable I/O User Manual
3
Calibration
(NI 783x /784x /785x Only)
Calibration is the process of determining and/or adjusting the accuracy of
an instrument to minimize measurement and output voltage errors. On the
NI 783x/784x/785x, onboard calibration DACs (CalDACs) correct these
errors. Because the analog circuitry handles calibration, the data read from
the AI channels or written to the AO channels in the FPGA VI is already
calibrated.
Three levels of calibration are available for the NI 783x/784x/785x
to ensure the accuracy of its analog circuitry. The first level, loading
calibration constants, is the fastest, easiest, and least accurate. The
intermediate level, internal self-calibration, is the preferred method of
assuring accuracy in your application. The last level, external calibration, is
the slowest, most difficult, and most accurate.
Loading Calibration Constants
The NI 783x/784x/785x is factory calibrated before shipment at
approximately 25 °C to the levels indicated in the device specifications.
Refer to the Multifunction Reconfigurable I/O Specifications, available at
ni.com/docs, for more information calibration levels. The onboard
nonvolatile Flash memory stores the calibration constants for the device.
Calibration constants are the values that were written to the CalDACs to
achieve calibration in the factory. The NI 783x/784x/785x hardware reads
these constants from the Flash memory and loads them into the CalDACs
at power-on. This occurs before you load a VI into the FPGA.
Self-Calibration
With self-calibration, the NI 783x/784x/785x can internally measure and
correct almost all of its calibration-related errors without any external
signal connections. NI provides software to perform an self-calibration.
This internal self-calibration process, which generally takes less than
two minutes, is the preferred method of assuring accuracy in your
application. Self-calibration minimizes the effects of any offset and gain
Chapter 3 Calibration (NI 783x/784x/785x Only)
R Series Multifunction RIO User Manual 3-2 ni.com
drifts, particularly those due to changes in temperature. During the
self-calibration process, the AI and AO channels are compared to the
device onboard voltage reference. The offset and gain errors in the analog
circuitry are calibrated out by adjusting the CalDACs to minimize these
errors.
Note The NI 78xx Calibration Utility does not support NI 781x devices.
If you have NI-RIO installed, you can find the self-calibration utility at
Start»All Programs»National Instruments»NI-RIO»Calibrate 78xx
Device. Device is the PXI-783x/784x/785x or PCI-783x device.
Immediately after self-calibration, the only significant residual calibration
error is gain error due to time and temperature drift of the onboard voltage
reference. You can minimize gain errors by performing an external
calibration. If you are primarily taking relative measurements, then you can
ignore a small amount of gain error and self-calibration is sufficient.
The Flash memory on the NI 783x/784x/785x stores the results
of a self-calibration so the CalDACs automatically load with the newly
calculated calibration constants the next time the device is powered on.
External Calibration
An external calibration refers to calibrating your device with a known
external reference rather than relying on the onboard reference. The
NI 783x/784x/785x has an onboard calibration reference to ensure
the accuracy of self-calibration. The reference voltage is measured at the
factory and stored in the Flash memory for subsequent self-calibrations.
Externally calibrate the device annually or more often if you use it at
extreme temperatures.
During the external calibration process, the onboard reference value is
re-calculated. This compensates for any time or temperature drift-related
errors in the onboard reference that might have occurred since the last
calibration. You can save the results of the external calibration process to
Flash memory so that the NI 783x/784x/785x loads the new calibration
constants the next time it is powered on. The device uses the newly
measured onboard reference level for subsequent self-calibrations.
To externally calibrate your device, use an external reference several times
more accurate than the device itself. For more information on externally
calibrating your device, refer to the NI 783xR/784xR/785xR Calibration
Procedure for NI-RIO, found on
ni.com/docs.