In article
,
Andy wrote:

Hello,
I am working with an instrument which outputs 0-5mV, and that signal
has to reach a sensing device 35' away. I am concerned with noise,
and I don't think shielded cable will work well enough. So I am
considering using coax cable. But I don't understand coax. There are
numerous "RG" ratings and ohm ratings. Suppose I used RG62 with 50
ohm terminations. What do I need to know about the termination and
the cables in order to properly use this? What's the difference
between RG62 and say RG59? What are the ohm ratings referring to, and
what terminations would I need for an ohm rating of 50? Is there a 50
ohm load which gets put somewhere in the system? Clearly I have many
questions. Any help would be appreciated.

From your description of your application, not one single thing you
mentioned is of any significance at all.

With a 5 mV signal, it's almost a certainty that you'll need a shielded
cable, but shielded may not be enough. It might be necessary for you to
choose a cable that has low microphonics (some cables can generate a
voltage when you whack them). Generally, better-grade microphone cables
have less of problem with it.

Another thing you might look into is the percentage of coverage some
particular shield configuration offers. RF coaxial cables in particular
usually do not have great shield coverage. Foil shields can offer much
better shielding.

All cables have capacitance, and longer cables have more. As a very
general rule, larger-diameter cables will likely have lower capacitance
than skinny ones. Make sure the capacitance of the cable you choose will
not affect your signal by attenuating the higher frequency components.

Perhaps best of all would be to amplify the signal *before* you send it
down the cable.

Isaac