EC-8.4 Grounding, Shielding and Where Noise Gets In
You can tell noise that was in the signal from noise your own wiring added, find a ground loop, decide where a shield should be connected, and choose between averaging and filtering knowing what each one costs.
Before:S4-5. Probability Theory & Stochastic ProcessesPE2-4. Sensing Techniques and Sensor SystemsEC-1. Circuit AnalysisUnlocks:EC-13. Laboratory Practice, in Sequence
The most frustrating hours in a laboratory are spent chasing something that is not in the circuit at all, and this topic is the map of where it usually comes from instead. It is placed here rather than in a later module because everything after it, from a sensor reading to an embedded converter, inherits the same set of problems.
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Noise that is in the signal, and noise the measurement added
Thermal and shot noise come with the physics and set a floor you cannot go below. Interference from a nearby supply or a mobile handset came from your wiring, and unlike the floor it can be removed entirely.
NPTEL: Measurement and Instrumentation (IIT Guwahati) · CourseGround loops, and the single-point ground
Two connections to ground by different routes make a loop that picks up whatever passes through it. Bringing every ground to one physical point removes the loop, and it is the first thing to try when a reading hums.
NPTEL: Electrical Measurement and Electronic Instruments (IIT Kharagpur) · CourseShielding, and what a shield is connected to
A shield works by giving interference somewhere to go, so a shield connected at neither end does nothing and one connected at both ends can make a loop. Connecting it at the source end only is the usual answer.
NPTEL: Electrical Measurement and Electronic Instruments (IIT Kharagpur) · CourseCapacitive and inductive coupling between wires
A changing voltage on one wire couples to a neighbour through the capacitance between them; a changing current couples through the magnetic field around it. Separation helps the first, and keeping the loop area small helps the second.
NPTEL: Measurement and Instrumentation (IIT Guwahati) · CourseDifferential measurement, and the twisted pair
If both wires pick up the same interference, a measurement of the difference between them rejects it. Twisting the pair makes the pickup on the two wires as nearly equal as possible, which is why data cable is twisted.
NPTEL: Transducers for Instrumentation (IIT Delhi) · CourseAveraging, filtering and what each costs you
Averaging a hundred readings improves a random error by a factor of ten and costs a hundred times the time. Filtering removes noise outside the band you care about and costs you speed of response, so neither is free.
NPTEL: Measurement and Instrumentation (IIT Guwahati) · Course
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