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Unformatted text preview: 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. Single-Ended Inputs
When you measure ground-referenced single-ended signals, the external signal supplies its own reference ground point, and the DAQ device should not supply one. Therefore, you should configure the DAQ device for NRSE input mode. In this input mode, connect all the signal grounds to AISENSE pin, which connects to the negative input of the instrumentation amplifier on the DAQ device. RSE input mode is not recommended for grounded signal sources. To leave the SCB-68 inputs in the factory configuration with jumpers in the series position (F or G, depending on the channel), do not use the open positions that connect the input to AIGND, A, and C (refer to Figure 5-1, SCB-68 Shielded Connector Block User Manual 3-4 ni.com Chapter 3 Connecting Signals Analog Input Channel Configuration Diagram for ACH<i> and ACH<i+8>). Any signal conditioning circuitry requiring a ground reference should be built in the custom breadboard area using AISENSE as the ground reference instead of building the circuitry in the open component positions. Referencing the signal to AIGND can cause inaccurate measurements resulting from an incorrect ground reference.
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. Differential Connection Considerations (DIFF Input Mode)
A differential connection is one in which the DAQ device AI signal has its own reference signal, or signal return path. These connections are available when the selected channel is configured in DIFF input mode. The input signal is tied to the positive input of the instrumentation amplifier, and its reference signal, or return, is tied to the negative input of the instrumentation amplifier. On DAQ devices that support both single-ended and DIFF input modes, using DIFF input mode commits two channels, ACH<i> and ACH<i+8>, to each signal. You should use differential input connections for any channel that meets any of the following conditions: The input signal is low-level (less than 1 V). The leads connecting the signal to the DAQ device are longer than 10 ft (3 m). The input signal requires a separate ground-reference point or return signal. The signal leads travel through noisy environments. Differential signal connections reduce noise pickup and increase common-mode noise rejection. Differential signal connections also allow input signals to float within the common-mode limits of the instrumentation amplifier. National Instruments Corporation 3-5 SCB-68 Shielded Connector Block User Manual Chapter 3 Connecting Signals Differential Connections for Ground-Referenced Signal Sources
Figure 3-2 shows how to connect a ground-referenced signal source to a channel on the DAQ device configured in DIFF input mode. ACH+ or ACH<i> Instrumentation Amplifier PGIA ACH or ACH<i+8> CommonMode Noise and Ground Potential Vm + Measured Voltage GroundReferenced Signal Source + + Vs + Vcm AISENSE* AIGND I/O Connector Measurement Device Configured in DIFF Input Mode *AISENSE is not present on all devices. Figure 3-2. Differential Input Connections for Ground-Referenced Signals With this connection type, the instrumentation amplifier rejects both the common-mode noise in the signal and the ground potential difference between the signal source and the DAQ device ground, shown as Vcm in Figure 3-2. SCB-68 Shielded Connector Block User Manual 3-6 ni.com Chapter 3 Connecting Signals Differential Connections for Nonreferenced or Floating Signal Sources
Figure 3-3 shows how to connect a floating signal source to a channel on the DAQ device configured in DIFF input mode. ACH+ or ACH<i> Instrumentation Amplifier PGIA ACH or ACH<i+8> Vm + Measured Voltage + Floating Signal Source + Vs Bias Resistor (see text) AISENSE* AIGND I/O Connector Measurement Device Configured in DIFF Input Mode *AISENSE is not present on all devices. Figure 3-3. Differential Input Connections for Nonreferenced Signals Using Bias Resistors
Figure 3-3 shows a bias resistor connected between ACH or ACH<i+8>, and AIGND. This resistor provides a return path for the 200 pA bias current. A value of 10 k to 100 k is usually sufficient. If you do not use the resistor and the source is truly floating, the source is not likely to remain within the common-mode signal range of the PGIA, and the PGIA saturates, causing erroneous readings. You must reference the source to the respective channel ground. National Instruments Corporation 3-7 SCB-68 Shielded Connector Block User Manual Chapter 3 Connecting Signals Common-mode rejection might be improved by using another bias resistor between ACH+ or ACH<i>, and AIGND. This...
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