Measure voltage on telephone line without adding noise?

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.

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

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Hi mtpoweraoki1302,
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BTCare Community Mod
If we have asked you to email us with your details, please make sure you are logged in to the forum, otherwise you will not be able to see our ‘Contact Us’ link within our profiles.
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Measure voltage on telephone line without adding noise?

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