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Chapter 7 Tutorial
System Cabling and Connections
4
7
Grounding Techniques
One purpose of grounding is to avoid ground loops and minimize noise.
Most systems should have at least three separate ground returns.
1. One ground for signals. You may also want to provide separate signal
grounds between high-level signals, low-level signals, and digital
signals.
2. A second ground is used for noisy hardware such as relays, motors,
and high-power equipment.
3. A third ground is used for chassis, racks, and cabinets. The AC power
ground should generally be connected to this third ground.
In general, for frequencies below 1 MHz or for low-level signals, use
single-point grounding (
see below ). Parallel grounding is superior but is
also more expensive and more difficult to wire. If single-point grounding
is adequate, the most critical points (those with the lowest levels and/or
the most precise measurement requirements) should be positioned near
the primary ground point. For frequencies above 10 MHz, use the
separate grounding system. For signals between 1 MHz and 10 MHz, you
can use a single-point system if the longest ground return path is kept to
less than 1/20 of a wavelength. In all cases, return-path resistance and
inductance should be minimized.
Circuit A Circuit B Circuit C Circuit A Circuit B Circuit C
Circuit A Circuit B Circuit C
Single-Point Ground
Separate Grounds
Parallel Grounds
34970A Refresh UG.book Page 259 Wednesday, February 17, 2010 12:34 PM