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'[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.
The phone line impedance should be 600 or 900 Ohms.
You can match to this with an opamp.
Use 47k in series with inverting input with a capacitor (where C = 1/
2*PI*47k*F, F = lowest frequency .. say 20Hz).
Now make feedback resistor = Gain Required x 47k. Say gain is 5, then
feedback resistor is 235k (use 220k or maybe a 500k pot if you want to
adjust the level).
The output of the op-amp goes to a 600 Ohm isolation transformer (Farnell
have small PCB mounting versions for Telecom) via a resistor equal to your
line impedance, the other side of the winding goes to +ve.
This now gives you a balanced, isolated drive to the line. You will need to
do a little more on the line side depending on your application (line seize
and hold circuitry etc)
Hope this is useful to you as a starting point.
In a message dated 5/24/00 2:53:21 PM Eastern Daylight Time, ININ.COMJohnC
> 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
Take a look at the Motorola MC34014P speech circuit for interface to the
Hope this helps,
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