Chapter 4 Analog InputNI USB-621x User Manual 4-10 ni.comAnalog Input Digital TriggeringAnalog input supports three different triggering actions:• Start trigger• Reference trigger• Pause triggerRefer to the AI Start Trigger Signal, AI Reference Trigger Signal, andAI Pause Trigger Signal sections for information about these triggers.A digital trigger can initiate these actions. All USB-621x devices supportdigital triggering. USB-621x devices do not support analog triggering.Field Wiring ConsiderationsEnvironmental 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 applymainly to AI signal routing to the device, although they also apply to signalrouting in general.Minimize noise pickup and maximize measurement accuracy by taking thefollowing precautions:• Use DIFF 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 input channels are twisted together and then covered witha shield. You then connect this shield only at one point to the signalsource ground. This kind of connection is required for signals travelingthrough areas with large magnetic fields or high electromagneticinterference.Refer to the NI Developer Zone document, Field Wiring and NoiseConsiderations for Analog Signals, for more information. To access thisdocument, go to ni.com/info and enter the info code rdfwn3.
