×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

multiconductor cable problem (stray capacitance)

multiconductor cable problem (stray capacitance)

multiconductor cable problem (stray capacitance)

(OP)
Hi Everyone... I'm a bioengineering graduate (still looking for a job). Anyways I'm trying to connect a slowly varying small amplitude signal from a sensor to an amplifier using a two conductor cable. I have to use the other wire in the cable too.. I have 2 options.
1. Connect a power line (60 Hz, 120 V RMS)
2. 0 to + 5 V sinusoid with f=100 MHz

Which one would be a better option? (considering stray capacitances)...Would I have to consider any other effects?

Thanks for the help

RE: multiconductor cable problem (stray capacitance)

Another option would be to install the amplifier directly adjacent to the sensor. For low level signals it is a very common approach.

To start towards answering your question:

Why 100 MHz? Where does the 100 MHz come from? Is that a power supply (asking because the 60Hz 120V seems to be...).

Although 100 MHz would probably couple much more from wire to wire than 60 Hz, it might be cleaner and (being higher frequency) easier to filter out. It depends.

RE: multiconductor cable problem (stray capacitance)

(OP)
the 100Mhz signal is a digital signal originating from a computer. I figured it would be a good approximation for coupling to say that its a sinusoid.

I took just 1 class on instrumentation 2 years ago, and can't remember much... I only  remember reading a couple of paragraphs on stray capacitance... (i'm a biochemical engineer) The design is too complicated to explain here... But the only option I got is to use the cable to carry two signals...One is fixed, and the other, I can either use the power line or the digital signal.... It would be great if you could explain why...

RE: multiconductor cable problem (stray capacitance)

So you have a cable with 120Vac 60Hz on one set of wires, and some sort of 100MHz digital signal from a PC on another set of wires, and you need to move a small amplitude analog signal using some of those same wires?

If this isn't it, then explain about the cable again.

RE: multiconductor cable problem (stray capacitance)

(OP)
I have a varying (slowly) small amplitude signal (still didnt figure out its properties) on one set of wires. For the second set, I have 2 options...as mentioned above (a power signal or a digital signal).... Which one would be a better option...? the power one or the digital one?

RE: multiconductor cable problem (stray capacitance)

I see. Sorry I'm slow on the uptake. My excuse is that I still can't really envision how one would have a system architecture choice between AC power or a 100 MHz digital signal (but I do believe you).

Your answer depends on many things. But I'd instinctively try the higher frequency digital signal because it would seem to be much easier to filter out 100 MHz (plus and minus sidebands and who-knows-how-much noise), than to try to filter out all the low frequency rubbish found on the typical AC power line.

You'd probably want to use a differential input at the amplifier end so that much of the noise (being common mode) would be ignored.

On the face of it, it would seem to be a fairly simple exercise to remove the high frequency from the slowly varying. It's a spectrum separation approach.

Stand by for other opinions - they're all free.

RE: multiconductor cable problem (stray capacitance)

Never try to carry a low amplitude signal over a cable.

(Never)^2 try to carry a low amplitude signal over a shared cable.

Connect an amplifier directly to your sensor.  Use the cable to power the amplifier, and an ADC, and a small RF transmitter to send the amplified and sampled signal wherever you need it to go.

Mike Halloran
Pembroke Pines, FL, USA

RE: multiconductor cable problem (stray capacitance)

(OP)
Thanks for the help guys..
VE1BLL: This is an idea that I came up with while I was doing my senior design... So I've just been putting together some stuff for that...

And Mike:  "((Never)^2 try to carry a low amplitude signal over a shared cable)" Care to explain why? I'm assuming its due to the stray capacitance, but are there other factors?

RE: multiconductor cable problem (stray capacitance)

Not just 'stray capacitance', but also inductive coupling. Unless the wire pairs are individually shielded, you have to assume that there will be plenty of coupling (depending on the overall length).

But, it is possible for various signal to share even just ONE PAIR of conductors by taking advantage of the frequency spectrum.

For example, satellite TV cables from the dish to the receiver have chaning 13 or 18 Vdc power, switched 22kHz audio, sometimes a digital code riding on the 22kHz audio, and L-band RF all running on the same cable.

Many microphones provide DC bias on the same wires.

RE: multiconductor cable problem (stray capacitance)

Stray everything, and connector resistance.


Mike Halloran
Pembroke Pines, FL, USA

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close