Coax cable used for DC instrumentation

A

Andy

Guest
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
 
On Mar 12, 11:18 am, Andy <andrewkgent...@gmail.com> 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.

thanks
Click to expand...
Are you looking for the 5 mV signal to be a DC or an AC signal, if AC
up to what frequency?
 
On Mar 12, 9:18 am, Andy <andrewkgent...@gmail.com> wrote:

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.
Click to expand...
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.
 
"Andy"
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.
Click to expand...
** Co-axial cable is a shielded 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?
Click to expand...
** 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
 
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.
 
I meant mic cable. Though mice are known to nibble cables.
 
"William Sommerwanker Fool "

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

** Co-ax works perfectly with microphones.

Every bit as good as shielded twisted pair cable.



..... Phil
 
On Mar 12, 7:56 pm, "Phil Allison" <phi...@tpg.com.au> wrote:
"William Sommerwanker Fool "

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

** Co-ax works perfectly with microphones.

   Every bit as good as shielded twisted pair cable.

....  Phil
Click to expand...
The OP has yet to tell us what kind of a dignal source he has, AC vs
DC, etc.
 
"Robert Macy"
"Phil Allison"

** Co-ax works perfectly with microphones.

Every bit as good as shielded twisted pair cable.
Click to expand...
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
 
On Mar 12, 5:56 pm, "Phil Allison" <phi...@tpg.com.au> wrote:
"William Sommerwanker Fool "

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

** Co-ax works perfectly with microphones.

   Every bit as good as shielded twisted pair cable.

....  Phil
Click to expand...
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
 
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.
Click to expand...

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!'
 
In article
<bd144a11-9004-40e3-99af-e73a4878c029@b36g2000pri.googlegroups.com>,
Andy <andrewkgentile@gmail.com> 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.
Click to expand...
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
 
In article <8009mfFm35U1@mid.individual.net>,
Phil Allison <phil_a@tpg.com.au> wrote:

"William Sommerwanker Fool "

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

** Co-ax works perfectly with microphones.

Every bit as good as shielded twisted pair cable.
Click to expand...
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
To e-mail, change noise into sound.
 
** Co-ax works perfectly with microphones.
Every bit as good as shielded twisted pair cable.

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.
Click to expand...
It's nice to see that Phil is disagreeing without being abusive.
 
"William Sommerwanker Fool "

5mV is down in the range of microphones and phono pickups.
I would not run such a signal through 35' of coax.
Click to expand...
Looks like I spoke too soon.
 
On Sat, 13 Mar 2010 05:51:01 -0800 (PST), Andy <andrewkgentile@gmail.com> wrote:

On Mar 13, 4:21 am, "William Sommerwerck" <grizzledgee...@comcast.net
wrote:
"William Sommerwanker Fool "
5mV is down in the range of microphones and phono pickups.
I would not run such a signal through 35' of coax.

Looks like I spoke too soon.

The signal is DC from a instrument, a pyranometer.
Click to expand...
That anything like an optical pyrometer? IOW a thermopile?

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.
Click to expand...
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.
 
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
Click to expand...
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.
 
On Mar 13, 4:21 am, "William Sommerwerck" <grizzledgee...@comcast.net>
wrote:
"William Sommerwanker Fool "
5mV is down in the range of microphones and phono pickups.
I would not run such a signal through 35' of coax.

Looks like I spoke too soon.
Click to expand...
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.
 
On Fri, 12 Mar 2010 16:17:05 -0800, "William Sommerwerck"
<grizzledgeezer@comcast.net>wrote:

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.
Click to expand...
Going to have to find some mice cables for my mice.
 
"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.
Click to expand...
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.


Your original post told us next to nothing, so it's not surprising that the
suggestion of using coax came under attack. You don't send weak
instrumentation signals down 35' of unbalanced line and expect them to
arrive unscathed. We haven't even discussed the possibility of ground loops.

Coax simply isn't a good idea. That you can't afford to do it the right way
doesn't make it any better an idea. People constantly ask for advice, then
complain that the correct advice costs too much to implement.

Go ahead and use the coax. But when it doesn't work the way you hope it
will, don't come back complaining.
 
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