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Problem using SM8223AP caller ID chip

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taskmn57

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Hi all,

I am using a caller ID chip which can detect both FSK and DTMF caller IDs on its input there is differential amplifier which we use a protection circuit to connect [TIP, RING] to it and there are very precise resistors in the input network.
I could run one of these chips some years ago but now I don't have the documents of that design.
the problem I have is that the chip can detect rings but when I monitor pos and neg inputs of amplifier I see that it has allways a 50Hz noise on it so can't detect caller IDs ?!:-( I just used the application circuit but I don't know whay it is not working, unfortunately I don't have documents of the project that I did before so I need someones help.

Can anybody help?
I would appreciate any help.

Hossein
 

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