Keysight DAQ970A Guia Del Usuario página 218

4 Measurement Tutorials
Thermal EMF errors
Thermoelectric voltages are the most common source of error in low-level DC voltage measurements.
Thermoelectric voltages are generated when you make circuit connections using dissimilar metals at different
temperatures. Each metal-to-metal junction forms a thermocouple, which generates a voltage proportional to the
junction temperature difference. You should take the necessary precautions to minimize thermocouple voltages and
temperature variations in low-level voltage measurements. The best connections are formed using copper-to-
copper crimped connections. The table below shows common thermoelectric voltages for connections between
dissimilar metals.
Copper-to-
Copper
Gold
Silver
Brass
Beryllium Copper
Aluminium
Kovar or Alloy 42
Silicon
Copper-Oxide
Cadnium-Tin Solder
Tim-Lead Solder
Noise caused by magnetic fields
If you are making measurements near magnetic fields, you should take precautions to avoid inducing voltages in the
measurement connections. Voltage can be induced by either movement of the input connection wiring in a fixed
magnetic field or by a varying magnetic field. An unshielded, poorly dressed input wire moving in the earth's
magnetic field can generate several millivolts. The varying magnetic field around the AC power line can also induce
voltages up to several hundred millivolts. You should be especially careful when working near conductors carrying
large currents.
Where possible, you should route cabling away from magnetic fields. Magnetic fields are commonly present around
electric motors, generators, televisions, and computer monitors. Also make sure that your input wiring has proper
strain relief and is tied down securely when operating near magnetic fields. Use twisted-pair connections to the
instrument to reduce the noise pickup loop area, or dress the wires as close together as possible.
218
Approx. μV/°C
< 0.3
0.5
0.5
3
5
5
40
500
1000
0.2
5
Keysight DAQ970A/DAQ973A User's Guide