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Unformatted text preview: 2. Select the value for R2 (10 k is recommended). Use Equation 512 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 threeresistor circuit for attenuating voltages at the singleended inputs, differential inputs, analog outputs, and digital inputs of the SCB68. SingleEnded Input Attenuators
To build a tworesistor circuit for attenuating voltages at the singleended inputs of the SCB68, refer to Figure 520. SCB68 Shielded Connector Block User Manual 520 ni.com Chapter 5 Adding Components for Special Functions ACH<i> + Vin AIGND RF,G + RB,D Vm Figure 520. SCB68 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 SCB68. Use Equations 514 or 515 to calculate the gain of the circuit: RB G = ( RB + RF ) RD G = ( RD + RG ) (514) (515) Differential Input Attenuators
To build a threeresistor circuit for attenuating voltages at the differential inputs of the SCB68, refer to Figure 521.
ACH<i> + Vin ACH<i+8> RF RE + Vm RG Figure 521. SCB68 Circuit Diagram for DIFF Input Attenuation Install resistors in positions E, F, and G of the chosen differential channel pair. Use Equation 516 to determine the gain of the circuit: RE G = ( RE + RF + RG ) (516) National Instruments Corporation 521 SCB68 Shielded Connector Block User Manual Chapter 5 Adding Components for Special Functions Analog Output and Digital Input Attenuators
To build a tworesistor circuit for attenuating voltages at the DAC0OUT, DAC1OUT, and TRIG1 pins on the SCB68, refer to the pad positions in Figure 522.
ACH<i> + Vin ACH<i+8> CF + RE Vm Figure 522. SCB68 Circuit Diagram for Digital Input Attenuation Use positions R1 and RC1 for TRIG1, and determine the gain according to Equation 517: RC1 G = ( RC1 + R1 ) Use positions R2 and RC2 for DAC1OUT, and determine the gain according to Equation 518: RC2 G = ( RC2 + R2 ) Use positions R3 and RC3 for DAC0OUT, and determine the gain according to Equation 519: RC3 G = ( RC3 + R3 ) (517) (518) (519) 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 523 shows. SCB68 Shielded Connector Block User Manual 522 ni.com Chapter 5 Adding Components for Special Functions R1 + + Input Impedance Vin R2 Figure 523. 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 520 shows the relationship among all of the resistor values: ( R 2 Input Impedance ) Z in = R 1 + ( R 2 + Input Impedance ) (520) Special Considerations for Analog Output
When you use the circuit shown in Figure 519 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 524 shows the electrical circuit you use to calculate the output impedance.
R1 Zout Output Impedance R2 Figure 524. Electrical Circuit for Determining Output Impedance Equation 521 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 (521) National Instruments Corporation 523 SCB68 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 520 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 SCB68 and the DAQ device. SCB68 Shielded Connector Block User Manual 524 ni.com Specifications A This appendix lists the SCB68 specifications. These ratings are typical at 25 C unless otherwise stated. Analog Input
Number of channels 68pin DAQ devices ....................... Eight differential, 16 singleended 100pin DAQ devices ..................... 32 differential, 64 singleended 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.
 Spring '12
 Joshi

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