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
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)
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)
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)
If this isn't it, then explain about the cable again.
RE: multiconductor cable problem (stray capacitance)
RE: multiconductor cable problem (stray capacitance)
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)^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)
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)
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)
Mike Halloran
Pembroke Pines, FL, USA