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Problem in Connecting MT8870 DTMF Decoder to detect the tone from the phone Exchange

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Joe87

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I have implemented the MT8870 Decoder like the application note design. The circuit works will if using a local telephone, but if I tried to decoded the tones from the telephone exchange the circuit most off the time will be not able to detect the tone.... is this related to the 600ohm impedance or related to R and C values ???!!!!
I'll be thankful if anyone have some ideas .... :)
 
A telephone exchange does not produce DTMF dialing tones. It produces a dial tone and a busy signal.
A telephone produces DTMF tones.
 
sorry for this... I was trying to say that: when someone calls from another telephone(not located in the same phoneline) and press some numbers , the circuit get some of them... and I tried to change the value of the resisters but the response was almost the same..
 
Even the exchanges send CLIP info in DTMF tones in the European and Asian exchanges and in the form of FSK in US, Canada and some other nations, just before the ringing voltage is applied.
the trouble is perhaps due to received level from the distant user,and the amplification needed.
 
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I do banking with my telephone and the DTMF receiver at the bank detects my tones perfectly.
 
I didnt use any resistors or caps as the datasheet schematic stated (on CM8870)

but thats how i got it to work started ripping out parts, it now works on my pc audio line and any cell phone, in, out and through my calls

I think i ended up putting a resistor back in somewhere there.
 
The problem was in the gain. with a higher gain I can now receive all the digits...
Thanx for help :)...
 
You do not design a circuit by ripping out parts. You plan the circuit before you build it. An input coupling capacitor is probably needed to keep the DC in the telephone or on the telephone line from saturating the decoder IC. The resistors set the gain of the decoder and must have the correct values.
 
you are right AG, but the equipment the bank uses has been designed well
the 8870 can work and detect. at very low levels.
if we both are on phone , i am sure i can detect the DTMF you send from Canada, LoL
let the O P optimize his design.
 
nono, i didn't design it, i used the circuit off the datasheet, after checking my work 3x, still getting no response, thats when i started ripping out parts!! im not going to question it too much cause i dont get it, an noone else does either, but thats wat i did to get mine WORKING!!
 
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