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Unformatted text preview: tions Table 5-1. Component Location for Analog Input Channels in DIFF Input Mode (Continued) Channel ACH4 ACH5 ACH6 ACH7 A R30 R32 R34 R36 B RC20 RC22 RC24 RC26 C RC21 RC23 RC25 RC27 D R31 R33 R35 R37 E RC8 RC9 RC10 RC11 F R12 R14 R16 R18 G R13 R15 R17 R19 Conditioning Analog Output Channels
Figure 5-2 illustrates the generic AO channel pad configuration, and Table 5-2 describes the AO component locations and labels. Figure 5-3 shows the AO channel configuration for DAC0OUT.
DACOUT (A) (B) AOGND Figure 5-2. Analog Output Channel Configuration Diagram Table 5-2. Component Location for Analog Output Channels in DIFF Input Mode Channel DAC0OUT DAC1OUT A R3 R2 B RC3 RC2 National Instruments Corporation 5-3 SCB-68 Shielded Connector Block User Manual Chapter 5 Adding Components for Special Functions R3 DAC0OUT + RC3 AOGND C Figure 5-3. Analog Output Channel Configuration Diagram for DAC0OUT Conditioning PFI0/TRIG1
Figure 5-4 illustrates the digital input channel configuration, and Figure 5-5 shows the digital input channel configuration for PFI0/TRIG1.
PFI0/TRIG1 (R1) 11 (RC1) 44 DGND Figure 5-4. Digital Input Channel Configuration Diagram R0 PFI0/TRIG1 + RC1 DGND C Figure 5-5. Digital Input Channel Configuration Diagram for PFI0/TRIG1 SCB-68 Shielded Connector Block User Manual 5-4 ni.com Chapter 5 Adding Components for Special Functions Accuracy and Resolution Considerations
When you measure voltage to subsequently measure current, take the following steps to maximize measurement accuracy: 1. 2. 3. Refer to the accuracy tables in Appendix A, Specifications, of the DAQ device user manual at ni.com/manuals. Use Equation 5-1 to determine the code width, which is the smallest signal change that a system can detect. Divide code width by the resistor value to determine the minimum current value you can measure. Range Code Width = -----------------------------------------Resolution Gain 2 (5-1) In Equation 5-1, range defines the values between and including the minimum and maximum voltages that the ADC can digitize. For example, the range is 20 when you measure a signal between 10 to 10 V. Gain, which is determined by the input limits of the application, is a value you apply to amplify or attenuate the signal. Gain is expressed in decibels and is defined as: Gain = 20 Log ( f ) (5-2) Resolution, or the smallest signal increment that can be detected by a measurement system, is either 12 or 16 bits, depending on the DAQ device. Open Thermocouple Detection
As an option, you can build open thermocouple detection circuitry by connecting a high-value resistor between the positive input and +5V. A resistor of a few M or more is sufficient, but a high-value resistor allows you to detect an open or defective thermocouple. If the thermocouple opens, the voltage measured across the input terminals rises to +5 V, a value much larger than any legitimate thermocouple voltage. You can create a bias current return path by using a 100 k resistor between the negative input and AIGND. National Instruments Corporation 5-5 SCB-68 Shielded Connector Block User Manual Chapter 5 Adding Components for Special Functions Differential Open Thermocouple Detection
Use position A to connect a high-value resistor between the positive input and +5V. Leave the jumpers in place (positions F and G) for each channel used. Single-Ended Open Thermocouple Detection
Use position A for one channel and C for the next channel when you connect a high-value resistor between the positive input and +5V. Leave the jumpers at positions F and G in place for each channel used. Sources of Error
When making thermocouple measurements with the SCB-68, the possible sources of error are compensation, linearization, measurement, and thermocouple wire errors. Compensation error can arise from two sources--inaccuracy of the temperature sensor and temperature differences between the temperature sensor and the screw terminals. The temperature sensor on the SCB-68 is specified to be accurate to 1 C. You can minimize temperature differences between the temperature sensor and the screw terminals by keeping the SCB-68 away from drafts, heaters, and warm equipment. Thermocouple output voltages are nonlinear with respect to temperature. Conversion of the voltage output to temperature using either look-up tables or polynomial approximations introduces linearization error. The linearization error is dependent upon how closely the table or the polynomial approximates the true thermocouple output. For example, you can reduce the linearization error by using a higher degree polynomial. Measurement error is the result of inaccuracies in the DAQ device. These inaccuracies include gain and offset. If the device is properly calibrated, the offset error should be zeroed out. The only remaining error is a gain error of 0.08% of full range. If the input range is 10 V and the gain is 500, gain error contributes 0.0008 20 mV, or 16 V of error. If the Seebeck coefficient of a thermocouple is 32 V/C, this measurement error adds 0.5 C of uncertainty t...
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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