Sunday, December 9, 2012

Reducing Measurement Errors with Proper Cabling Part 2

This is part 2 of a two part post where we look at reducing measurement errors with proper cabling and grounding methods. The principles covered in this post can be applied to basic measurement setups, DAQ systems, and automated test systems. Click here to read part 1.

Radio Frequency Interference 
Most voltage-measuring instruments can generate false readings in the presence of large, high-frequency signals. Possible sources of high-frequency signals include nearby radio and television transmitters, computer monitors, and cellular telephones. High-frequency energy can be coupled to a DMM on the system cabling. To reduce the interference, try to minimize the exposure of the system cabling to high-frequency RF sources. If your application is extremely sensitive to RFI radiated from the instrument, use a common mode choke in the system cabling as shown below to attenuate instrument emissions. Note you most likely will see a choke on your computer monitor video input cable used for this purpose, look for a cylindrical hard object that has a small subsection of the cable running through the center of it.

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.

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. When I worked in calibration we would run into the problem of measurement fluctuations caused by magnetic fields when calibrating milli-ohm and milli-volt meters. To reduce these fluctuations we built a metal box that would surround the meter and block magnetic fields. The box had a small opening just big enough to read measurements and change settings.

Low-Level AC Measurement Errors 
When measuring AC voltages less than 100 mV, be aware that these measurements are especially susceptible to errors introduced by extraneous noise sources. An exposed test lead will act as an antenna and the internal DMM will measure the signals received. The entire measurement path, including the power line, act as a loop antenna. Circulating currents in the loop will create error voltages across any impedances in series with the instrument’s input. For this reason, you should apply low-level AC voltages to the instrument through shielded cables. You should also connect the shield to the input LO terminal. Be sure to minimize the area of any ground loops that cannot be avoided. A high-impedance source is more susceptible to noise pickup than a low impedance source. You can reduce the high-frequency impedance of a source by placing a capacitor in parallel with the instrument’s input terminals. You may have to experiment to determine the correct capacitance value for your application. Most extraneous noise is not correlated with the input signal. You can determine the error as shown below.

Correlated noise, while rare, is especially detrimental. Correlated noise will always add directly to the input signal. Measuring a low-level signal with the same frequency as the local power line is a common situation that is prone to this error. You should use caution when switching high-level and low-level signals on the same switch card or module. It is possible that high-level charged voltages may be discharged onto a low-level channel. It is recommended that you either use two different modules or separate the high-level signals from the low-level signals with an unused channel connected to ground.

As always if you have anything to add use the comments section below and if you have any questions feel free to email me.

Click here to read part 1


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