SCB-68 Connector Block

# Caution the scb 68 is not designed for any input

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Unformatted text preview: 2. Select the value for R2 (10 k is recommended). Use Equation 5-12 to calculate the value for R1. Base the R1 calculation on the following values: Maximum Vin you expect from the transducer Maximum voltage (<10 VDC) that you want to input to the DAQ device Accuracy Considerations For best results when attenuating voltage, you should choose a resistor that has the following characteristics: Low wattage of approximately 1/8 W Precision of at least 5% Temperature stable Tolerance of 5% AXL package (suggested) Carbon or metal film (suggested) Verify that R1 and R2 drift together with respect to temperature; otherwise, the system may consistently read incorrect values. Adding Components You an build a two- or three-resistor circuit for attenuating voltages at the single-ended inputs, differential inputs, analog outputs, and digital inputs of the SCB-68. Single-Ended Input Attenuators To build a two-resistor circuit for attenuating voltages at the single-ended inputs of the SCB-68, refer to Figure 5-20. SCB-68 Shielded Connector Block User Manual 5-20 ni.com Chapter 5 Adding Components for Special Functions ACH<i> + Vin AIGND RF,G + RB,D Vm Figure 5-20. SCB-68 Circuit Diagram for SE Input Attenuation Install resistors in positions B and F, or positions D and G, depending on the channel you are using on the SCB-68. Use Equations 5-14 or 5-15 to calculate the gain of the circuit: RB G = ----------------------( RB + RF ) RD G = -----------------------( RD + RG ) (5-14) (5-15) Differential Input Attenuators To build a three-resistor circuit for attenuating voltages at the differential inputs of the SCB-68, refer to Figure 5-21. ACH<i> + Vin ACH<i+8> RF RE + Vm RG Figure 5-21. SCB-68 Circuit Diagram for DIFF Input Attenuation Install resistors in positions E, F, and G of the chosen differential channel pair. Use Equation 5-16 to determine the gain of the circuit: RE G = -----------------------------------( RE + RF + RG ) (5-16) National Instruments Corporation 5-21 SCB-68 Shielded Connector Block User Manual Chapter 5 Adding Components for Special Functions Analog Output and Digital Input Attenuators To build a two-resistor circuit for attenuating voltages at the DAC0OUT, DAC1OUT, and TRIG1 pins on the SCB-68, refer to the pad positions in Figure 5-22. ACH<i> + Vin ACH<i+8> CF + RE Vm Figure 5-22. SCB-68 Circuit Diagram for Digital Input Attenuation Use positions R1 and RC1 for TRIG1, and determine the gain according to Equation 5-17: RC1 G = ---------------------------( RC1 + R1 ) Use positions R2 and RC2 for DAC1OUT, and determine the gain according to Equation 5-18: RC2 G = ---------------------------( RC2 + R2 ) Use positions R3 and RC3 for DAC0OUT, and determine the gain according to Equation 5-19: RC3 G = ---------------------------( RC3 + R3 ) (5-17) (5-18) (5-19) Special Considerations for Analog Input When calculating the values for R1 and R2, consider the input impedance value from the point of view of Vin, as Figure 5-23 shows. SCB-68 Shielded Connector Block User Manual 5-22 ni.com Chapter 5 Adding Components for Special Functions R1 + + Input Impedance Vin R2 Figure 5-23. Input Impedance Electrical Circuit Zin is the new input impedance. Refer to Appendix A, Specifications, in the device user manuals at ni.com/manuals for the input impedance. Equation 5-20 shows the relationship among all of the resistor values: ( R 2 Input Impedance ) Z in = R 1 + -------------------------------------------------------( R 2 + Input Impedance ) (5-20) Special Considerations for Analog Output When you use the circuit shown in Figure 5-19 for AO, the output impedance changes. Thus, you must choose the values for R1 and R2 so that the final output impedance value is as low as possible. Refer to Appendix A, Specifications, in the device user manuals at ni.com/manuals for device specifications. Figure 5-24 shows the electrical circuit you use to calculate the output impedance. R1 Zout Output Impedance R2 Figure 5-24. Electrical Circuit for Determining Output Impedance Equation 5-21 shows the relationship between R1, R2, and Zout, where Zout is the old output impedance and Zout2 is the new output impedance: ( Z out + R 1 ) R 2 Z out2 = -------------------------------------Z out + R 1 + R 2 (5-21) National Instruments Corporation 5-23 SCB-68 Shielded Connector Block User Manual Chapter 5 Adding Components for Special Functions Special Considerations for Digital Inputs If you use the Vin voltage of Figure 5-20 to feed TTL signals, you must calculate Vin so that the voltage drop on R2 does not exceed 5 V. Caution A voltage drop exceeding 5 V on R2 can damage the internal circuitry of the DAQ device. NI is not liable for any device damage or personal injury resulting from improper use of the SCB-68 and the DAQ device. SCB-68 Shielded Connector Block User Manual 5-24 ni.com Specifications A This appendix lists the SCB-68 specifications. These ratings are typical at 25 C unless otherwise stated. Analog Input Number of channels 68-pin DAQ devices ....................... Eight differential, 16 single-ended 100-pin DAQ devices ..................... 32 differential, 64 single-ended Temperature sensor Accuracy ......................................... 1.0 C over a 0 to 110 C range Output ......
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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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