When good shielding techniques cannot keep noise from affecting measurements, the following tips can help:
Use a spectrum analyzer to help identify noise frequencies and amplitudes. Look for harmonics of noise signals and line-power frequencies. This information can help you identify noise sources.
Separate digital and analog grounds. Use a single-point ground or cluster ground connections as close together as possible to eliminate ground loops. Do not cut off the earth ground (green wire) on a power plug or use a three-wire to two-wire power-outlet adapter to break a ground connection (see photo, below). If you cannot eliminate a ground loop, an isolation amplifier on your signal lines can help.
Suspect all connectors even those from high-quality suppliers. Broken, high-resistance or corroded ground connections cause problems and increase the noise on measurement signals. Do not mix high-power conductors and low-level signal conductors in the same cable or wire bundle. Separate measurement and control signals as much as possible. Check screw-terminal connections to ensure tight contact. Use high-quality cable and connectors.
Keep connectors clean. 3M's Novec contact cleaner removes grease, silicones and dust, and the 407C audio/video head cleaner from MG Chemicals remove oil and dirt. Neither leaves a residue.
Switch-mode power supplies, motor controllers, electric welders and power-switching equipment can radiate noise. Discharge-lighting equipment and fluorescent-light ballasts can create radiated and conducted noise. You might need to replace old ballasts or install an in-line filter such as the Leviton XPF.
Use power-line EMI filters to attenuate noise conducted in and out of circuits. Some filters include a choke on the ground line to prevent it from conducting noise, too. I know of a portable-phone battery charger that caused considerable RFI over a wide area due to a faulty component and a poor design.
Look for coupling between measurement signals and electromagnetic devices such as motors, relay coils and SMPS transformers that have time-varying magnetic fields. Reorient signal leads perpendicular to magnetic fields or move wires away from these fields.
The electrolyte in aluminum capacitors used in power supplies can dry out and allow noise to pass through to sensitive measurement circuits. If you see line-frequency or SMPS switching noise, look for bad capacitors. It might cost less to replace a bad supply than its bad capacitors.
Look for slots in chassis and enclosures that can radiate RFI signals. If possible, decrease the slot width with conductive tape that connects to the conductive chassis.
Use wide-band ferrite cores, or ferrite noise suppressors, on signal cables to attenuate conducted RFI signals.
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