An Update on Shielding
Jon Titus, Contributing Editor -- Design News, November 1, 2009
When you measure a sensor's signal, you don't want ambient electromagnetic noise added to it. So you rely on shielded twisted-pair wires to protect the sensor's signal. The shield provides a path to ground for ambient electrical noise. (A shield also reflects some of the ambient noise and in some cases lets noise through to the signal wires.)
The quality of the shield greatly affects how much noise gets to your signal wires. The photo below shows examples of shielded wires. The twisted-pair cable, A, has a 100 percent aluminum-foil shield and separate "drain" wires that connect to the shield and simplify a ground connection. These separate wires have a lower resistance than the foil alone. The inexpensive RG-58/U coaxial patch cable, B, has a foil shield and about 20 small-gauge shield wires. The RG-58C/U coaxial cable, C, provides a stranded center conductor and a woven copper shield. The shield still has small openings that very-high-frequency signals could penetrate. High-quality coax cable can provide up to about 95 percent woven-shield coverage. (Go with a 100 percent shield if you can.)
Now that you have a shielded cable, where do you ground it? I have recommended grounding it at the measurement end. Grounding at both ends can cause ground loops that compound problems and can add powerline noise to sensor signals. In some cases, you can ground one shield end and run the other shield end to ground through a small-value capacitor.
But recently I read this rule of thumb: When a length of cable shield exceeds 1/20th of the wavelength of the highest-frequency noise signal, you should ground both ends of the shield. But what's the source of the 1/20th rule? David Ballo, an application development engineer in the Component Test Div. at Agilent Technologies uncovered a reference I paraphrase below. (See References below.)
The key assumption in single-point grounding is that the wavelength of the highest noise frequency is long relative to the physical dimensions involved, so you assume everything electrical occurs simultaneously and uses lumped-circuit analysis. But when wavelengths are short enough that the physical dimensions become significant, you need to use distributed circuit analysis. Then, a single-point grounded shield starts to look like an antenna. You have an optimum "antenna" when the shield length equals one-quarter wavelength of the noise signal. Then you must ground both ends. The author's rule of thumb is that you need multipoint grounding when the shield length exceeds 1/20th wavelength of the highest frequency of interest.
I'll have more to report on the 1/20th rule of thumb and shielding in my next Tips column. In the meantime, I welcome comments that practically or theoretically explain the 1/20th wavelength figure at jontitus@comcast.net.
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For a good video on the subject, see www.iet.tv/search/index.html?spres=5243 where Bill provides much of the underlying theory of how the various stray effects cause havoc in systems. It is my contention that much system noise is self inflicted, and often driven by the very line filters needed to reduce radiated transmission.
Philip Oakley - 2009-25-11 18:44:56 EST -
This topic is way more complex than indicated here and needs to be examined from several perspectives. A look at the low-frequency equivalent circuit of a shielded cable, for example, shows that all conductors are a series R and L. At frequencies above the transition frequency of the shield's R and L (typically 5 to 10 kHz), virtually all the voltage appearing over the length of the shield are magnetically coupled to the inner conductors. This effectively makes the cable a common-mode choke. Therefore, little coupling from shield (ground loop) currents occurs in the received signal above about 50 kHz. Another often forgotten but related phenomenon is called SCIN (shield current induced noise), wherein current flow in the shield magnetically induces differential voltage in the twisted pair. Shields consisting of braid only fare well in this regard, but those with a "drain wire" are very bad. The shield current tends to flow in the lower resistance drain wire which is NOT symmetrically aligned with the inner conductors, resulting in unequal induced voltages (i.e., differential "noise"). The topic of how to connect shields on balanced audio cables was debated for years by the Audio Engineering Society. In audio, it's standard practice to ground the shields at both ends, but this aggravates the SCIN problem. The best compromise is the capacitive grounding at the receive end, so at low (audio) frequencies shield current is minimal, but at higher frequencies the ground is effective. Neutrik makes the "EMC" type XLR-3 connector with the capacitor array built-in and is effective to over 1 GHz. There has always been a conflict in grounding practices between the professional audio community and the data/telecom community, mainly because the latter can be cavalier about low-frequency noise (which is disaster in an analog signal chain with 120 dB+ dynamic range. And there are other issues, too, but space is limited here. -- Bill Whitlock, Fellow of the AES, Senior Member of IEEE
Bill Whitlock - 2009-24-11 20:53:45 EST -
Grounding shields at one end or capacitive ground termination can be way worse than double ended grounding. Many designs have been limited by an uninformed aversion to "ground loops". If your design is self contained (not reliant on mains power) then double ended grounding should be the standard.
Alexander Rivera - 2009-24-11 16:19:21 EST
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