Coax cable used for DC instrumentation

[Andy]

On Mar 13, 7:51 am, "William Sommerwerck" <grizzledgee...@comcast.net>
wrote:

The signal is DC from a instrument, apyranometer. 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.

"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.

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.

 

[Robert Macy]

On Mar 12, 9:36 pm, "Phil Allison" <phi...@tpg.com.au> wrote:

"Robert Macy"
"Phil Allison"

** 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.

....  Phil

Engineering's goal is to design a system until it is 'good enough' and
often, and in many applications and locations, coax can be
interchangeable with STP. Upon that premise I can agree with your
claim.

However, in general, STP will be better:
Floating mike, somewhat reasonable, except...
1 if a person holds the mike, there is a connection to 'gnd', which
creates a gnd loop.
2 if the shield is not terminated as a total and complete envelope
wrapped around ALL internals of the mike, there is unbalanced
capacitance, which allow noise sources to inject signals into
different circuit locations inside the mike. In other words, direct
pickup from those noise sources.

 

[whit3rd]

On Mar 13, 5:51 am, Andy <andrewkgent...@gmail.com> wrote:

[about a 5 mV signal at a distance from the measuring
node]

The signal is DC from a instrument, a pyranometer.

That isn't the RIGHT information; what is the output impedance?
If it's like a thermocouple (one kind of pyrometer), coaxial
cable is CONTRAINDICATED because the outer shield
and inner conductor will have different temperature lags
in case of a chill wind on the apparatus.

If it's a thermocouple, with low impedance (under 10 ohms),
then unshielded twisted pair will probably work fine; don't
ground ANYTHING (the high temperature insulation is
failure-prone anyhow) except next to the meter.
Use Cat5 wire (it's cheap) and consider dedicating
an extra pair, for preamp power, at each node.
Either a splice, or a preamplifier, requires a small
box for protection. Small meaning the size of
a connector (print server computers inside a
DB-25 connector shroud are not uncommon).

If it's a temperature measurement, with slow response,
the measurement end can take several readings and
average (or even integrate over an exact interval) to get
rid of AC errors; or you can apply an overall grounded
shield to reduce AC pickup.

Signal sources have four important characteristics, GIVE ALL
FOUR YOUR ATTENTION. Signal amplitude (5 mV);
source impedance (totally unknown); bandwidth (DC);
resolution (totally unknown).

Of the four, we answer-ers only got one (the 5 mV), and
a half (if it were REALLY DC, you could measure once in
January and just refer to that number until June).

 

[Phil Allison]

"Robert Macy"

** 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.

However, in general, STP will be better:
Floating mike, somewhat reasonable, except...
1 if a person holds the mike, there is a connection to 'gnd', which
creates a gnd loop.

** ROTFL !!

You are a real scream - pal.

One red hot fucking nut case.


2 if the shield is not terminated as a total and complete envelope
wrapped around ALL internals of the mike,


** Mics do tend to be wrapped in metal cases with metal mesh over the
capsule.

You must have a look at one sometime.

If you ever stop wanking.



...... Phil

 

[Phil Allison]

"William Sommerwenker"

"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.

** Shame Wanker Bill has no idea what the " very good reason " is.



...... Phil

 

[Phil Allison]

"William Sommerwanker"

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

** He won't be complaining.

His signal is a DC voltage.


..... Phil

 

[William Sommerwerck]

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

** He won't be complaining.
His signal is a DC voltage.

And, as we all know, DC signals are immune to noise and inteference.

 

[Phil Allison]

"William Sommerwanker Lying Turd "

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

** He won't be complaining.
His signal is a DC voltage.

And, as we all know, DC signals are immune to noise and inteference.

** There will not be any with a 35 foot run of RG59 co-ax.

The receiving device very likely has a LPF incorporated and dead easy to add
one of it does not.

So piss off to HELL

- you PITA know nothing wanker !!!



...... Phil

 

[Robert Macy]

On Mar 13, 3:50 pm, "Phil Allison" <phi...@tpg.com.au> wrote:

"Robert Macy"



** 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.

However, in general, STP will be better:
Floating mike, somewhat reasonable, except...
1 if a person holds the mike, there is a connection to 'gnd', which
creates a gnd loop.

**  ROTFL  !!

  You are a real scream - pal.

  One red hot fucking nut case.

2 if the shield is not terminated as a total and complete envelope
wrapped around ALL internals of the mike,

** Mics do tend to be wrapped in metal cases with metal mesh over the
capsule.

  You must  have a look at one sometime.

  If you ever stop wanking.

.....   Phil

1 prefer to stick to technical dicussions, where I can learn
something.

2 taken apart more than one microphone, designed several, even
designed one unintentionally, but it was six feet in diameter and I
was targetting a completely dfferent function.

I routinely work in the nanovolt ranges, the femtoamp ranges in some
very robust environments, also often 140 dB dynamic ranges, so noise
pickup is a BIG issue.

 

[Phil Allison]

"Robert Macy"
"Phil Allison"

** 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.

However, in general, STP will be better:
Floating mike, somewhat reasonable, except...
1 if a person holds the mike, there is a connection to 'gnd', which
creates a gnd loop.

** ROTFL !!

You are a real scream - pal.

One red hot fucking nut case.

2 if the shield is not terminated as a total and complete envelope
wrapped around ALL internals of the mike,

** Mics do tend to be wrapped in metal cases with metal mesh over the
capsule.

You must have a look at one sometime.

If you ever stop wanking.

1 prefer to stick to technical dicussions,

** ROTFL - you are nothing but a rabid nut case.

Piss off.



..... Phil

 

[William Sommerwerck]

I routinely work in the nanovolt ranges, the femtoamp
ranges in some very robust environments, also often
140 dB dynamic ranges, so noise pickup is a BIG issue.

There's another issue we haven't discussed, and that's electrochemical
reactions. BNC connectors aren't well-sealed against the environment. You
don't "stuff" getting into the connector and corroding the connection.

An XLR connector offers no only a balanced line, but the contacts have
greater surface area, and you can seal off the connector will silicone.

 

[Gus]

On Mar 12, 12:18 pm, 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

Does your "sensing device" support differential inputs.. I would think
that would determine if twisted pair is the solution or not.

 

[whit3rd]

On Mar 14, 3:23 pm, Gus <gusr...@comcast.net> wrote:

Does your "sensing device" support differential inputs.. I would think
that would determine if twisted pair is the solution or not.

That's not the whole story; if the source is floating
(not locally ground-referenced) the twisted pair is a good solution
for non-differential input, as well.

 

[pedr]

"Andy" <andrewkgentile@gmail.com> wrote in message
news:bd144a11-9004-40e3-99af-e73a4878c029@b36g2000pri.googlegroups.com...

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
The RG terminations refer to the RADIO FREQUENCY and pulse properties of

the line. If you are only measuring voltages which change slowly (not more
than 50Hz components) ANY coax will do. Think of it as a screened test lead
( of course you should earth the outer screen)
Hope this helps
P.W.