Anecdote - hope it's not boring...
Shielding is basically trying to wrap a Faraday shield around your measurement system and its signal input wires.
That way capacitive coupled current flows in your shield wire instead of your signal wire, and that's the objective.
It is important that signal wire and shield wire be at approximately same voltage, that way there's not much capacitive coupling inside the cable between shield and signal conductors.
That leads to an interesting corollary - if your signal isn't grounded but your shield is, you will have voltage between shield and signal wires hence capacitive coupling between them.
So some books say that proper way is to connect shield to low side of signal at the point of measurement.
Hardly anybody does that. But most signals aren't very much elevated from ground so most folks get away with simply grounding the shield someplace.
For extreme cases, one can drive the shield to same voltage as the signal wire by using another amplifier. We had one instrument that went to that trouble and for good reason.
It was a "Magnetic Flowmeter", a huge electromagnet on an insulated section of pipe with two electrodes contacting the fluid. The electromagnet was excited by several amps of 60hz AC and induced a tiny 60hz voltage in the fluid, proportional to fluid's velocity. The signal produced at the electrodes was something like zero to 1.65 millivolts ac for zero to 10 gpm through the pipe.
That feeble signal traveled 250 feet to control room through a power plant stuffed with huge motors and electrical equipment, all operating at 60 hz.. "Can't possibly work", i said.
Each of the two signal wires contacting the fluid was wrapped with its own foil shield, and each shield was driven from receiving end to same voltage as the signal wire it enclosed. The pair of shielded signal wires was itself wrapped with third shield, that one tied to case of the electromagnet/pipe. So that third outer shield was tied to low side of signal at point of measurement.
So - no capacitive current was coupled into the signal wires . Capacitive current was coupled into the driven shields, but that current flowed only in the shields , meeting the objective.
Were those shields not driven but grounded, there'd have been zero to 1.65 millivolts between the signal wire and its shield, ergo capacitive coupling.
The system worked better than one could believe. It is a testament to the perseverance of 1960's circuit designers.
old jim
