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December 2016 TechTip

Turn All Metal Surfaces from Noise Conductors into Shields

Electrostatic Conduction of Noise Signals through Metal

In our recent series, Noise Debug 101 – Part 2, we explained “capacitive or electrostatic conduction” of noise signals and the effectiveness of shielding. To briefly review, electrostatic noise occurs when AC noise is coupled into the circuit via parasitic capacitance through the air/space. The noise source is any varying voltage and is independent of current flow. The most common sources are fluorescent lights, switching power supplies, or other common electronics in the lab.

Equipment Racks & Carts = Potential for Noise Conduction

However, even an ungrounded equipment rack, or a mobile cart, can become a large conductor of the AC Power Line noise to all cables and equipment in the rack – and negatively impact the quality of signals during a recording session.

As shown in this schematic:

Connecting the rack to ground will turn all metal surfaces from noise conductors into shields. Most of the time, instrument cases installed in the rack are internally grounded. However, this ground connection is made “by chance” and therefore may be unreliable and susceptible to conducted ground loops.

Rack Grounding Specifics

There should be only one power cord from all equipment to the AC source (wall).

Connect the metal frame of the rack directly to the AC Power Earth Ground to keep all equipment at the same “ground potential” - the equipment’s Safety Ground (on the AC power cords). A good place for this connection is either at the main power strip or isolation transformer case.

Important: Do not connect the rack to a different “Earth Ground.” This will create a ground loop conduction as previously explained in the Noise Debug 101 series.
After making this connection, check continuity with an ohmmeter by:

  • Powering off all equipment;
  • unplugging the main AC power cord from the source; and
  • checking continuity from the main AC power cord Ground to the rack chassis and each instrument ground (if available).

You should read less than 1.5 ohms for each connection.