hi mike,
i recognise the top bit of that article but not seen the lower stuff
before. is very good actually, is prompting lots of questions. im
currently in the process of reducing the noise in my measurement
system.. hours and hours ive spent fiddling gradually reducing the
noise but was wondering what to do about the magnetic signals its
picking up.
some questions arising from that doc
- refering to the bit on capacitive coupling.. if you notice the
vnoise source is grounded, i wonder if the source of the noise wasnt
referenced to the same ground whether you would still get
interference.. if not then would that be one of the benefits of having
the measurements system floating?
- for the bit on measuring floating sources, i read somewhere about
using the shield as a return path for bias currents being better than
sending them back down the signal cables themselves. ie take figure 17
and disconnect the ground at the source. also i wonder if this would
work for grounded signals although would need to go through a resistor
to limit any potential ground loop current.
- not come accross the concept of magnetic shielding before.. is
this essentially building a faraday cage around either the source thats
made from iron. if so does this i guess trap the flux within the cage..?
- ive often considered using balanced signals for my measurement
system but have no idea just how much difference this would actually
make. i imagine you would have to have a lot of noise to justify the
added circutary. also if you're going to add circuitary to invert your
signal you may aswell boost it at the same time and add another amp to
boost the +ve version too. also for fig 23 i think you could also use
the shield for bias current return path again.
seriously though thanks so much for that link.. is really informative.
measurement noise is such an under-studied subject.. most people know
about twisted pairs and that you need to ground sheilds. but grounding
them to the common at the source is the essential thing.. before i
connected them to earth which added a horrific amount of noise to my
signals.
many thanks
mike
