Fluke 8808A User Manual - Page 97
External Trigger, Thermal Voltages
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A Appendices Applications Table A-5. Typical Measurement Intervals (in Seconds) for Dual Display Measurements Meas. Function D A E B O G F Range All All All All N/A N/A N/A Slow 1.2 1.0 1.2 1.0 N/A N/A N/A Med 1.0 0.85 1.0 0.85 N/A N/A N/A Fast 0.9 0.8 0.9 0.8 N/A N/A N/A External Trigger The external trigger can be used with or without settling delays, as shown in Table A-4. (Refer to Table 4-3 for trigger types.) The amount of trigger delay varies depending on differences between the primary and secondary displays, as described in the previous section. When external trigger is enabled, the meter determines the ranges for the primary and secondary (if enabled) displays based on the input at that time. The meter is then ready to begin measuring the input on the optimum range as soon as the trigger is received. If the input changes so that either display autoranges after the trigger is received, the autoranging response times (as shown in Table A-3) may be required before each measurement result is displayed. The rear panel trigger input is edge sensitive. A low to high pulse (above +3 V) will be recognized as a trigger less than 3 ms. Thermal Voltages Thermal voltages are the thermovoltaic potentials generated at the junction between dissimilar metals. Thermal voltages typically occur at binding posts and can be greater than 1 μV. When making low-level dc measurements, thermal voltages can present an additional source of error. Thermal voltages can also cause problems in the low ohms ranges. Some low-value resistors are constructed with dissimilar metals. Just handling such resistors can cause thermal voltages large enough to introduce measurement errors. Use the following techniques to reduce the effect of thermal voltages: 1. Use similar metals for connections wherever possible (e.g., copper-to-copper, goldto-gold, etc.). 2. Use tight connections. 3. Use clean connection (especially free of grease and dirt). 4. Use caution when handling the circuit under test. 5. Wait for the circuit to reach thermal equilibrium. (Thermal voltages are generated only where there is a temperature gradient.) A-7
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