multifunction-rio-user-manual.pdf - 第50页

Chapter 3 Calibration (NI 783x /784x /785x Only) R Series Multifunction RIO User Manua l 3-2 ni.com drifts, particularly those due to changes in temp erature. During the self-calibration process, the AI an d AO channels …

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.

Connecting I/O Signals

This appendix describes how to make input and output signal connections

to the NI 78xx I/O connectors.

Figure A-1 shows the I/O connector pin assignments and locations for

PCI-7811/7813 and PXI-7811/7813.

Figure A-2 shows the I/O connector pin assignments and locations

for PCI-7830/7831/7833, PCIe-7841/7842/7851/7852, and the

PXI-7830/7831/7833/7841/7842/7851/7852/

7853/7854.

Note The PXI-7830 and PCI-7830 do not have Connector 2 (RDIO).