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I would like to use 50mV FSD signal to sense DC current at a distance of ~70 feet with a twisted pair cable and then perform signal condition there.

Does anyone have any experience with this?

" Originally asked" Is it advicible to take lower side shunt 50mV FSD signal at a distance of about 70feet with STP cable , and then make signal conditioning there at remote location. I need to enquire does experienced members do this way?. BTW I am trying to sense DC current.

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  • \$\begingroup\$ Without any context the 70 foot distance is meaningless. What is this question about? \$\endgroup\$ – Andy aka Jun 20 '14 at 15:37
  • \$\begingroup\$ Sorry but I cannot follow you? . Its a mV signal running over a long distance cable. \$\endgroup\$ – Adi Jun 20 '14 at 15:41
  • \$\begingroup\$ @Adi, information about the exepected frequency would is very important as capacitance increases with wire length. If you are transmitted a high frequency signal then attenuation can be a big problem here. \$\endgroup\$ – sherrellbc Jun 20 '14 at 15:49
  • \$\begingroup\$ @sherrellbc , its DC signal. \$\endgroup\$ – Adi Jun 20 '14 at 15:54
  • \$\begingroup\$ @Adi, Ha! Please excuse my ignorance. I read that entire post, yet apparently dismissed that information. Of course there is the matter of how quickly it is changing. That is, is it an indictor representing water flow (slow changes) or perhaps something that may change a bit more quickly. \$\endgroup\$ – sherrellbc Jun 20 '14 at 15:55
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STP I assume means screened/shielded twisted pair and if you want to measure current flowing down one of the conductors then do so at any point. "Condition" the signal where the measurement is made or do the conditioning half locally and half remotely. The minimum amount of conditioning to be done locally is to amplify the shunt signal so that the wires from the amplified shunt conditioning circuit do not pick-up excessive extraneous EMI on route to the final circuit that monitors the shunt current.

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  • \$\begingroup\$ I am aware of the EMI issues, however in this application the customer is asking to bring all the signals at one place and do conditioning there .Thats why the question , is it wise to do this . I am little uncomfortable with this. \$\endgroup\$ – Adi Jun 20 '14 at 15:49
  • \$\begingroup\$ Experience tells me it is unwise because 70ft is a long way to bring a small signal even if it is basically just a slow moving DC signal. Can't you "steal" a bit of power from the DC you are trying to measure and locally power an op-amp? \$\endgroup\$ – Andy aka Jun 20 '14 at 16:49
  • \$\begingroup\$ Actually even sending powersupply towards sensor node won't be a problem.But some times customers try to impose plenty of their wisdome which creates problem :). \$\endgroup\$ – Adi Jun 20 '14 at 17:25
  • \$\begingroup\$ Sounds like not an engineering problem but a customer relations exercise therefore I vote to close this question LOL! I reckon you could phantom power it - what is the DC signal voltage present on the cable at the measurement point and can you steal any current (before the measurement point of course? \$\endgroup\$ – Andy aka Jun 20 '14 at 17:48
  • \$\begingroup\$ Its 0-50mV available on the panel, its already wired. \$\endgroup\$ – Adi Jun 20 '14 at 18:00
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If the signal can be low-pass filtered (you say it is DC current), and your source impedance is low (presumably a shunt) and your input impedance high and suitably filtered, there's really nothing especially tricky about running a 50mV signal for 20 or 30 meters. We do it all the time with thermocouple signals which are in the same voltage range (sometimes less).

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  • \$\begingroup\$ Thanks for sharing your experience. I feel Thermocouples are electrically Isolated from the device , by I expect to get a good amount of conducted noise. \$\endgroup\$ – Adi Jun 20 '14 at 16:14
  • \$\begingroup\$ Actually, they're typically either grounded or ungrounded. It's possible to make the signal conditioner sensitive to noise by bad design, but that's a different question. If there is electrical noise on the shunt wrt ground you certainly might have severe problems, but that's nothing to do with the cable length. Had no problems in an installation with 50mV shunts and 5-15kA currents flowing around. \$\endgroup\$ – Spehro Pefhany Jun 20 '14 at 16:19
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Given the low level of the signal and the length of the cable I would say this was a bad idea. Yes, it can be done, but the signal integrity would be bad.

You will need to do at least one of two things to the signal to ensure good communication:

  1. Amplify the signal. Make the signal a higher voltage. A simple op-amp may do this for you nicely.

  2. Optionally turn it into a differential signal. Send the signal twice - once normally and once inverted. You can then subtract one signal from the other to get a nice clean signal out the other end. Again, op-amps can do this for you.

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