multifunction-rio-user-manual.pdf - 第48页

Chapter 2 Hardware Overv iew of the NI 78x x Multifunction Reconfigurable I/O U ser Manual 2-30 ni.com Field Wiring Considerations (NI 783 x /784 x /785 x Only) Environmental noise can se riously affect the meas urement …

Chapter 2 Hardware Overview of the NI 78xx

Disk Drive Power Connector Installation

Before installing the disk drive power connector, you must install and set

up the R Series PCI Express device as described in the Getting Started with

R Series Multifunction RIO document. Complete the following steps to

install the disk drive power connector.

1. Power off and unplug the computer.

2. Remove the computer cover.

3. Attach the PC disk drive power connector to the disk drive power

connector on the device, as shown in Figure 2-19.

Note The power available on the disk drive power connectors in a computer can vary. For

example, consider using a disk drive power connector that is not in the same power chain

as the hard drive.

Figure 2-19. Connecting to the Disk Drive Power Connector

4. Replace the computer cover, and plug in and power on the computer.

5. Use the NI 78xx Calibration Utility to run a self-calibration on the

PCIe-784x/785x device. Refer to the Self-Calibration section of

Chapter 3, Calibration (NI 783x/784x/785x Only), for more

information.

Note Connecting or disconnecting the disk drive power connector can affect the analog

performance of your device. To compensate for this, NI recommends that you

self-calibrate after connecting or disconnecting the disk drive power connector.

1 Device Disk Drive Power Connector 2 PC Disk Drive Power Connector

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.

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