SCB-68 Connector Block

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Unformatted text preview: connection creates a slight measurement error caused by the voltage divider formed with the output impedance of the floating source, but it also gives a more balanced input for better common-mode rejection. Single-Ended Connection Considerations A single-ended connection is one in which the DAQ device AI signal is referenced to a ground that can be shared with other input signals. The input signal is tied to the positive input of the instrumentation amplifier, and the ground is tied to the negative input of the instrumentation amplifier. You can use single-ended input connections for input signals that meet the following conditions: The input signal is high-level (greater than 1 V). The leads connecting the signal to the DAQ device are less than 10 ft (3 m). The input signal can share a common reference point with other signals. DIFF input connections are recommended for greater signal integrity for any input signal that does not meet the preceding conditions. In single-ended modes, more electrostatic and magnetic noise couples into the signal connections than in differential modes. The coupling is the result of differences in the signal path. Magnetic coupling is proportional to the area between the two signal conductors. Electrical coupling is a function of how much the electric field differs between the two conductors. SCB-68 Shielded Connector Block User Manual 3-8 ni.com Chapter 3 Connecting Signals Single-Ended Connections for Floating Signal Sources (RSE Input Mode) Figure 3-4 shows how to connect a floating signal source to a channel on the DAQ device configured for RSE input mode. ACH + Instrumentation Amplifier PGIA Floating Signal Source + Vs AISENSE* AIGND + Vm Measured Voltage I/O Connector Measurement Device Configured in RSE Input Mode *Not all devices support RSE input mode. Figure 3-4. Single-Ended Input Connections for Nonreferenced or Floating Signals Single-Ended Connections for Grounded Signal Sources (NRSE Input Mode) To measure a grounded signal source with a single-ended configuration, configure the DAQ device in NRSE input mode. The signal is then connected to the positive input of the DAQ device instrumentation amplifier, and the signal local ground reference is connected to the negative input of the instrumentation amplifier. The ground point of the signal should, therefore, be connected to AISENSE. Any potential difference between the DAQ device ground and the signal ground appears as a common-mode signal at both the positive and negative inputs of the instrumentation amplifier, and this difference is rejected by the amplifier. If the input circuitry of a DAQ device were referenced to ground, in this situation (as in the RSE input mode), this difference in ground potentials would appear as an error in the measured voltage. National Instruments Corporation 3-9 SCB-68 Shielded Connector Block User Manual Chapter 3 Connecting Signals Figure 3-5 shows how to connect a grounded signal source to a channel on the DAQ device configured for NRSE input mode. GroundReferenced Signal Source CommonMode Noise and Ground Potential ACH + Vs + Vcm AIGND AISENSE* + Instrumentation Amplifier PGIA + Vm Measured Voltage I/O Connector Measurement Device Configured in NRSE Input Mode *Not all devices support NRSE input mode. Figure 3-5. Single-Ended Input Connections for Ground-Referenced Signals Connecting Analog Output Signals When using the SCB-68 with a 68-pin or 100-pin DAQ device, the AO signals are DAC0OUT, DAC1OUT, EXTREF, and AOGND. DAC0OUT is the voltage output channel for AO channel 0. DAC1OUT is the voltage output channel for AO channel 1. EXTREF is the external reference input for both AO channels. AOGND is the ground reference signal for both AO channels and the external reference signal. Note For more information, refer to the device user manual at ni.com/manuals for detailed signal connection information for AO signals. SCB-68 Shielded Connector Block User Manual 3-10 ni.com Chapter 3 Connecting Signals Figure 3-6 shows how to make AO connections and the external reference connection to the SCB-68 and the DAQ device. EXTREF External Reference Signal (optional) + + Vref Load VOUT 0 DAC0OUT AOGND Load VOUT 1 + DAC1OUT SCB-68 Figure 3-6. Connecting AO Signals Connecting Digital Signals When using the SCB-68 with a 68-pin or 100-pin DAQ device, the DIO signals are DIO<0..7> and DGND. DIO<0..7> are the eight single-ended DIO lines, and DGND is the ground reference. You can program all lines individually to be inputs or outputs. Note For more information, refer to the device user manual at ni.com/manuals for detailed signal description and connection information. Figure 3-7 illustrates several common DIO applications and signal connections. Digital input applications include receiving TTL signals and sensing external device states such as the state of the switch shown in Figure 3-7. Digital output applications include sending TTL signals and driving external devices such as the LED shown in Figure...
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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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