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Techniques to enhance op amp signal integrity in low-level sensor applications (Part 2 of 4)

December 05, 2008 | | 212700847
Shielding, grounding, ground loops, common mode, single-ended, differential mode, induced noise, isolation, and more: meet and overcome the challenges of low-level analog signal conditioning
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Editor's Note: This lengthy and insightful article is presented in four parts:

Part 1: click here
Part 2: below
Part 3: to be posted December 16, 2008
Part 4: to be posted December 18, 2008

Shielding, grounding and ground loops
Successful shield grounding cannot be divorced from the issue of good signal grounding; they go hand-in-hand. Thus both subjects will be treated as a combined topic in Part 2 of this series.

Shielding
Twisted-pair cable is quite effective in reducing interference at low frequencies, even without a shield. However, the most demanding applications require that the cable be fully shielded. Wires shielded with copper, aluminum, and tin will reduce most interference, particularly electric-field interference, and for very long cable lengths, in industrial sensor/data acquisition instrumentation, they are an absolute must. However, this type of shielding material does not work well with low frequency magnetic field-induced pick up, owing to the long cable runs, where copper and aluminum are not effective. To shield against magnetic interference, the least expensive solution is still that of twisting the signal wires to eliminate the current loop, and thus the pickup.

Grounding and ground loops
Probably nowhere does the designer confront challenges of such difficulty as those presented by protecting sensitive analog input circuits from ground-coupled interference. The circuits can be fairly well protected using an isolated ground plane (isolated from the signal source), but the problem often includes interconnection of wires and cabling to sensors and common (un-isolated) grounds that present many opportunities for ground-loops. The simplest solution is to employ instrumentation with electrically isolated inputs. However, these are typically twice the price of a non-isolated measuring device, and are only justified in process-critical situations in industrial plants. The non-isolated approach is a more appealing and cost-effective solution in less critical applications, although it does require a good understanding of grounds and ground-loops.

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