Measure voltage on telephone line without adding noise?

[Lance B]

I am using a PCI 6013, and made a voltage divider (two resistors in series - 10MOhm and 1Mohm - so the telephone network doesn't see the additional load) to bring the 40 volt onhook v down to 4 volts for the card to measure (the off hook v will read .5 v down from 5v). I assume I must use the differential configuration to read this value? And how to keep from adding noise to the line?

Any help is greatly appreciated.

[Russell]

Hmmm, are you making a phone tap? 🙂

I really don't know anything about telephone line specifications and what are the thresholds between noise and signal, but I do know about the DAQ board! Yes, I would say differential is the best configuration for you to use. This is almost a must to help reject the common-mode noise already on the phone line. It will also produce a lot less noise onto the phone line as opposed to referenced single ended mode. I doubt you'll put much noise on the phone line. You could probe the line with a scope with and without your "project" connected to see how it affects it.

Sounds like fun!

Russell
Applications Engineer
National Instruments
http://www.ni.com/support

[Ben]

Phone lines can float relative to local ground.

You may want to look into transformer coupling or opto-isolation.

Ben

[Lance B]

This is what I was concerned about. Can you spell out a simple solution, or is this a complicated issue?

Thanks again,

Lance


Message Edited by Support on 02-26-2008 09:47 AM

[Russell]

Here are a couple of NI solutions:

1) The SCXI-1125 isolation module. It provides 8 channels of up to +-300Vrms of isolation.

2) There is also the SCC-AI series signal condition modules for lower channel counts that also provides +-300Vrms of isolation.

--Russell

[Ben]

THe big issue is common-mode rejection not just issolation.

All I can say at this point is, if you want to inject noise into a phone line, then tie it to ground!

Radio Shack may sell a phone tap. this could help.

Ben