Shield Current Induced Noise

The Rane Library has many interesting audio relaed technical documents. The most famouse of them is Note 110 Sound Systems Interconnections - it is a classic. Just few days ago I happened to look around at The Rane Library, and found some really interesting material related to audio systems and ground loops:

Grounding and Shielding Audio Devices document points out that the use of entirely balanced interconnection with both ends of the shield connected to chassis ground at the point of entry provides the best available performance. The document goes through several grounding practices. The Audio Engineering Society is developing a recommended practices document which also condones chassis-grounding balanced shields, among other things.

Pin 1 Revisited document tells that cable shields are essentially an extension of the shielding enclosure of equipment, and they should be connected directly to that shielding enclosure. To make equipment cheaper to build, manufacturers started connecting cable shields to the circuit board’s common trace, then took that trace to the chassis. The problem is that any voltage drop across the wiring that is common to both the shield current and the circuit’s path to ground will be injected into the audio circuitry.

SCIN: Shield Current Induced Noise tells about testing audio cable shielding properties on different cable types and different frequencies. The article also covers induced cable current effects. The article details this testing arrangement for testing SCIN:

 

1 Comment

  1. Tomi Engdahl says:

    Optimize mixed-signal Circuit/PCB design using noise modeling, part 1
    http://www.edn.com/design/pc-board/4458816/Optimize-mixed-signal-Circuit-PCB-design-using-noise-modeling–part-1

    Earth-ground noise sources

    Earth-ground noise is very difficult to quantify since it varies greatly depending on the application and length of cable runs. In order to start a path to modeling this type of problem, it helps to visualize RFI current paths to optimize shielding, ground connections, and ferrite placement. Earth ground connections within an instrument cabinet are not the concern here, although it can be an issue for circuits sensitive to signal levels below 10mV. Here we are considering ground connections a few feet or more apart. This includes two devices that may communicate with each other, but are plugged into different power strips. Connections less than 10m apart will likely not have much of a low frequency voltage difference (f 50V), so the standard caution to watch for ground loops with very distant connections apply. Be careful of rules of thumb, as shorter runs (<15m) may require shield grounding at both ends to address magnetic coupling concerns.

    A rough estimate for this type of noise model can start with amplitudes of 1-2 volts P-P at frequencies up to 100 MHz in series with a voltage source at 50/60Hz. The goal here is to provide as much immunity as can be afforded given cost and space, rather than a specific amount of attenuation unless you have already made field measurements of the phenomenon to gauge the voltage differential and spectral content.

    In order to quickly answer the question of "is a noise source going to affect my circuit?", we need a means to estimate what noise source amplitudes would be acceptable for most systems.

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