Coax cable used for DC instrumentation

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.

thanks

Reply to
Andy
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Are you looking for the 5 mV signal to be a DC or an AC signal, if AC up to what frequency?

Reply to
hrhofmann

What do you mean, 'noise'? Pickup from heavy machinery? Thermal resistor noise? Limited ADC resolution?

And what IS your signal? Source impedance and bandwidth are important.

There are lots of signals (like a thermocouple output) that would work better without coaxial cable, and lots of solutions (like a 4-20 mA current transmitter) that a 'sensing device' could conceivably support. Researching coaxial cable is not yet called for.

Reply to
whit3rd

"Andy"

** Co-axial cable is a shielded cable.

** Probably nothing - unless your mysterious "instrument" outputs signals in the MHz range.

A run of co-axial cable with no special termination will present a capacitive load to the source of about 22pF per foot - so in your example about 770pF for RG59. Long as that capacitance is OK, along with whatever load the input of the sensing device presents - you will be fine.

BTW

Co-axial cable has similar interference cancelling properties to twisted pair cable.

..... Phil

Reply to
Phil Allison
5mV is down in the range of microphones and phono pickups. I would not run such a signal through 35' of coax.

You need a balanced system -- like a mice cable -- where the shield does not carry the signal.

Reply to
William Sommerwerck

I meant mic cable. Though mice are known to nibble cables.

Reply to
William Sommerwerck

"William Sommerwanker Fool "

** Co-ax works perfectly with microphones.

Every bit as good as shielded twisted pair cable.

.... Phil

Reply to
Phil Allison

run

The OP has yet to tell us what kind of a dignal source he has, AC vs DC, etc.

Reply to
hrhofmann

run

Using physically realizable materials, the only signal carrying system one can construct in a manner that will be the most immune to external noise sources is a shielded balanced twisted pair system. Coax is prone to adding noise to the signal, since the shield has impedance, carries the signal, and therefore can add noise to the signal as the shield carries noise.

However, even poorly made twisted pair cable can actually add noise to the signal as the shield attempts to do its function. For example, Belden's high quality HDMI cable, if used to carry analog, only shieldls in the 35 to 60dB range. Of course its worse at the higher frequency range.

Robert Macy, PE

Reply to
Robert Macy

"Robert Macy" "Phil Allison"

Using physically realizable materials, the only signal carrying system one can construct in a manner that will be the most immune to external noise sources is a shielded balanced twisted pair system.

** Fraid that is dead wrong.

Coax is prone to adding noise to the signal, since the shield has impedance, carries the signal, and therefore can add noise to the signal as the shield carries noise.

** Only true where the source has a common ground path with the receiving device and an earth loop is created by the shield.

For a floating source, like a microphone, co-axial cable works just as well as STP despite not being balanced.

This is not a very well known fact and derives from the inherent symmetry of co-axial cable which rejects external magnetic field induction in a similar way twisting a pair of wires does.

.... Phil

Reply to
Phil Allison

RG-62 is a 93 ohm cable. It has a lower capacitance per foot than most other coax in that size because of its construction.

--
Lead free solder is Belgium's version of 'Hold my beer and watch this!'
Reply to
Michael A. Terrell

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

Reply to
isw

That must explain why pretty well all pro microphone cable is twisted pair, then. And always has been even before phantom etc power.

--
*Who are these kids and why are they calling me Mom?

    Dave Plowman        dave@davenoise.co.uk           London SW
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Reply to
Dave Plowman (News)

well

of

similar

It's nice to see that Phil is disagreeing without being abusive.

Reply to
William Sommerwerck

Looks like I spoke too soon.

Reply to
William Sommerwerck

The signal is DC from a instrument, a pyranometer. The output (0 to

5mV) is going to be scaled by 2000 to get 0 - 10V into a data acquisition card. My concern is that having a signal in the low millivolt range, and a cable of some 30-50' (exact length is not yet known), noise would be a problem. This is a residential project, and so there is no heavy machinery to speak of. Although I know this guy has a shop and uses a lot of standard shop tools. So, my decision to use coax is based on the fact that it's easy to make good field connections and it "appear" to have better noise immunity than shielded. There is a braided shield in coax; only a foil shield in most shielded pairs.
Reply to
Andy

That anything like an optical pyrometer? IOW a thermopile?

A balanced, shielded cable to the instrumentation amp is standard industry usage. Common-mode noise pickup is rejected. Nothing wrong with Beldin cables with foil shield.

Grant.

Reply to
Grant

5mV) is going to be scaled by 2000 to get 0 - 10V into a data acquisition card. My concern is that having a signal in the low millivolt range, and a cable of some 30-50' (exact length is not yet known), noise would be a problem. This is a residential project, and so there is no heavy machinery to speak of. Although I know this guy has a shop and uses a lot of standard shop tools. So, my decision to use coax is based on the fact that it's easy to make good field connections and it "appear" to have better noise immunity than shielded. There is a braided shield in coax; only a foil shield in most shielded pairs.

"Common sense" suggests that you amplify the signal at the sending end, not the receiving end.

"Common sense" also suggests that you use a balanced cable with a separate shield. This is the way microphones are wired, and for very good reason.

As someone pointed out, a foil shield provides 100% coverage (or close to it), while braided shields rarely do.

Reply to
William Sommerwerck

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I'm not sure what your assumptions of my project are, but they have led you to believe that I lack common sense. I have considered this approach of amplifying at the sending end, and I may end up building a voltage to current converter circuit to achieve this. This more expensive approach would require a climate controlled nema 3r control panel to be mounted on a roof, adding quite a few dollars to the overall cost. I have 12 temperature sensors also mounted on the system. I can not afford to build a sending circuit for each.

Reply to
Andy

Going to have to find some mice cables for my mice.

Reply to
Meat Plow

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