Hi all,
I'm designing a system that uses a MT8870 DTMF decoder to detect tones on a phone line and the typical phone line interface is done via a 1:1
600ohm "telephony" transformer, like this :-. ,--------o------ To MT8870 Line )|( | )|( z -' '-. A | | Gnd | V z | Gnd
The two zeners are here to ensure the voltage does not go above Vcc applied to MT8870. My problem is that the transformers for these applications are bulky and take a lot of space. Hence I was wondering if I could replace the transformer by a diode bridge and optocoupler pair like this:
Vcc | ___ .----------. | +-+----|___|---|-. .-|-' A A | | |/ | Line -+ | | V -> | | -(-+ | - |> | A A | | | | +-+------------|-' '-|-- To MT8870 '----------'
My main concern is the frequency that the optocoupler can support. I have a bunch of 4N25 here and considering the max rise time and fall time with a security margin, I'd say it can transfer signals up to at least 30kHz which to me is enough for this application as DTMF signals only go up to 2kHz. Can anyone confirm that my calculations are right, and that the arrangement of the transistor side of the 4N25 is fine?
Thanks a lot for your help. Cheers Olivier