SCB-68 Connector Block

The following sections discuss the use of single

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Unformatted text preview: s, ACH<i> and ACH<i+8>, to each signal. Figure 3-1 summarizes the recommended input modes for both types of signal sources. National Instruments Corporation 3-1 SCB-68 Shielded Connector Block User Manual Chapter 3 Connecting Signals Signal Source Type Floating Signal Source (Not Connected to Building Ground) Grounded Signal Source Input Examples: Ungrounded thermocouples Signal conditioning with Isolated outputs Battery devices Examples: Plug-in instruments with nonisolated outputs ACH(+) + V 1 Differential (DIFF) CommonMode Voltage + ACH(+) ACH() + V 1 + ACH() R + AIGND CommonMode Voltage + AIGND Refer to the Using Bias Resistors section for information on bias resistors. NOT RECOMMENDED ACH ACH Single-Ended -- Ground Referenced (RSE) + V 1 + + V 1 + Vg CommonMode Voltage + AIGND CommonMode Voltage + AIGND + Ground-loop losses, Vg, are added to measured signal. ACH + V 1 Single-Ended -- Nonreferenced (NRSE) + ACH AISENSE + V 1 CommonMode Voltage + AISENSE CommonMode Voltage + R AIGND + AIGND Refer to the Using Bias Resistors section for information on bias resistors. Figure 3-1. Summary of AI Connections SCB-68 Shielded Connector Block User Manual 3-2 ni.com Chapter 3 Connecting Signals Nonreferenced or Floating Signal Sources A floating signal source is a signal source that is not connected in any way to the building ground system, but has an isolated ground-reference point. Instruments or devices with isolated outputs are considered floating signal sources, and they have high-impedance paths to ground. Some examples of floating signal sources are outputs for thermocouples, transformers, battery-powered devices, optical isolators, and isolation amplifiers. The ground reference of a floating source must be tied to the ground of the DAQ device to establish a local or onboard reference for the signal. Otherwise, the measured input signal varies as the source floats outside the common-mode input range. Differential Inputs When measuring differential floating sources, you must configure the device for DIFF input mode. To provide a return path for the instrumentation amplifier bias currents, differential floating sources must have a 10 to 100 k resistor connected to AIGND on one input if they are DC coupled or on both inputs if sources are AC coupled. You can install bias resistors in positions B and D of the SCB-68, as shown in Figure 5-1, Analog Input Channel Configuration Diagram for ACH<i> and ACH<i+8>. Single-Ended Inputs When measuring single-ended floating signal sources, you must configure the DAQ device to supply a ground reference by configuring the DAQ device for RSE input mode. In this mode, the negative input of the instrumentation amplifier on the DAQ device is tied to the analog ground. To use the SCB-68 with single-ended inputs, where ACH<i> and ACH<i+8> are used as two single-ended channels, configure the SCB-68 in its factory-default configuration. In the factory-default configuration, jumpers on the SCB-68 are in the two series positions, F and G, as shown in Figure 5-1, Analog Input Channel Configuration Diagram for ACH<i> and ACH<i+8>. In this configuration, you should connect all signal grounds to AIGND. Note Some versions of the SCB-68 use hardwired 0 resistors as the factory-default jumpers. In such cases, to move these jumpers to and from the factory-default positions, you must solder and desolder on the SCB-68 circuit card assembly. When soldering, refer to Appendix E, Soldering and Desoldering on the SCB-68. National Instruments Corporation 3-3 SCB-68 Shielded Connector Block User Manual Chapter 3 Connecting Signals Ground-Referenced Signal Sources A grounded signal source is connected in some way to the building system ground; therefore, the signal source is already connected to a common ground point with respect to the DAQ device (assuming that the host computer is plugged into the same power system). Nonisolated outputs of instruments and devices that plug into the building power system fall into this category. The difference in ground potential between two instruments connected to the same building power system is typically between 1 and 100 V, but the difference can be much greater if the power distribution circuits are improperly connected. If a grounded signal source is incorrectly measured, this difference may appear as a measurement error. The connection instructions for grounded signal sources are designed to eliminate this ground potential difference from the measured signal. Differential Inputs If the DAQ device is configured for DIFF input mode, where ACH<i> and ACH<i+8> are used as a single differential channel pair, ground-referenced signal sources connected to the SCB-68 need no special components. You can leave the inputs of the SCB-68 in the factory configuration with the jumpers in the two series positions, F and G. Refer to Figure 5-1, Analog Input Channel Configuration Diagram for ACH<i> and ACH<i+8>, for a diagram of this configuration. Note Some versions of the SCB-68 use hardwired...
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This note was uploaded on 05/19/2012 for the course ELEN 3030 taught by Professor Joshi during the Spring '12 term at Marquette.

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