MC34014P Looks interesting from the few application notes I found, but for whatever reason I cannot seem to find data sheets on this product... Could you give me a direct link? This looks like it may be worth investigating. Additionally, is there additional certifications needed for telco connectivity? A complete third party product that could save us additional certifications may be an attractive alternative (depending on unit price). John Clark Indianapolis, IN -----Original Message----- From: Brian Hopkins [mailto:brian.hopkins@TELUSPLANET.NET] Sent: Wednesday, May 24, 2000 4:35 PM To: PICLIST@MITVMA.MIT.EDU Subject: Re: [EE]: Impedance Matching a Phone Line John: Telco subscriber loops (twisted pair cable) have a nominal characteristic impedance (Z) of 900 ohms. That is the classic comprise number used when designing subscriber side equipment, and why all the CO line simulators (artificial lines) use that value. This of course is not the DC loop resistance (R) which will depend on your distance from the CO and the gauge of cable in your loop makeup. So..if you design your line drive electronics to have an output impedance of 900 ohms you should have maximum power transfer, and minimize reflections (echo) for... most.... average...typical ... loops your equipment may connect to. I can't find any of my tech college references, but these sites confirm what I recall was drilled into me. http://www.mnm.com.sg/measurement/teltone/tls5.htm http://teltone.com/telecom_solutions/test_tools/test_tls-3a_specs.html http://www.wilcominc.com/t240.htm http://www.egyed.com/phonework.html Any references you see to 600 ohms (Z) relate to 'open wire' , used for toll circuits (open wire carrier) in the pre-microwave days which were the pre-fibre optic days. The equipment side of the telco was designed to match that 600 ohms. Old standards...old history... Brian Hopkins ----- Original Message ----- From: "Clark, John" To: Sent: Wednesday, May 24, 2000 12:51 PM Subject: [EE][OT] Impedance Matching a Phone Line > I am trying to connect the output of a D/A signal to an open phone line to > play audio (sort of similar to the "DTMF decoding with a PIC" thread that is > going on, only I need to transmit DTMF and music). I have been doing quite > a bit of searching for specifications on telco line impedance and so forth, > but I have yet to find a definitive guide. I am also strongly leaning to > finding a 3rd party product that would accept a standard 47K ohm impedance > signal from an RCA phono plug shielded type of connector. If anyone has > information, especially on the latter, I would really appreciate it. > > Thanks. > > > John Clark > Indianapolis, IN