MT8870 decodes only few numbers

[niket_304]

mt8870 application note

I hav built the circuit of MT8870 as shown in application note (with single ended input, 100K gain resistor, 390K betn 16-17 pin, 1-4 shorted).....
the problem is
it detects only few dtmf tones like 4,5,6,7,8,9
It does not detect 1,2,3,*,0,#.
does anyone have ne idea???

 

[hamed_sotoudi]

betn16

mye be there isnt any isolator in your circuit. connect one of your phone line to GND because phone line is -48 volt and your circuit is 5 volt.

 

[niket_304]

hey i ve used audio isolation transformer.....

 

[hamed_sotoudi]

I maked a control embedded system and it worked very good. change your IC may be it is damaged. and check for your capacitors in input and output pins of tramsformer.