National Instruments NI 784xR User Manual

Also see for NI 781xR: User manual User manual

Contents

National Instruments NI 784xR User Manual Manual pdf 52 page image

Chapter 2 Hardware Overview of the NI 78xxR© National Instruments Corporation 2-31 R Series Intelligent DAQ User ManualField Wiring Considerations(NI 783xR/784xR/785xR Only)Environmental noise can seriously affect the measurement accuracy of thedevice if you do not take proper care when running signal wires betweensignal sources and the device. The following recommendations mainlyapply to AI signal routing to the device, as well as signal routing in general.Take the following precautions to minimize noise pickup and maximizemeasurement accuracy:• Use differential AI connections to reject common-mode noise.• Use individually shielded, twisted-pair wires to connect AI signals tothe device. With this type of wire, the signals attached to the positiveand negative inputs are twisted together and then covered with a shield.You then connect this shield only at one point to the signal sourceground. This kind of connection is required for signals travelingthrough areas with large magnetic fields or high electromagneticinterference.• Route signals to the device carefully. Keep cabling away from noisesources. The most common noise source in a PXI DAQ system is thevideo monitor. Keep the monitor and the analog signals as far apart aspossible.Use the following recommendations for all signal connections to theNI 783xR/784xR/785xR:• Separate NI 783xR/784xR/785xR signal lines from high-current orhigh-voltage lines. These lines can induce currents in or voltages onthe NI 783xR/784xR/785xR signal lines if they run in parallel paths ata close distance. To reduce the magnetic coupling between lines,separate them by a reasonable distance if they run in parallel or run thelines 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 throughspecial metal conduits.Refer to the NI Developer Zone tutorial, Field Wiring and NoiseConsiderations for Analog Signals, at ni.com/zone for more information.