SA545 Datasheet, PDF (12/18 Pages) Sames – PULSE/DTMF SWITCHABLE DIALLERS WITH 10 REPERTORY MEMORIES

SA545
8. Oscillator Circuit
This requires an external 560KHz ceramic resonator connected between OSC IN
and OSC OUT, providing a timing reference for all chip functions.
Consult your resonator supplier for recommendation of a suitable resonator type.
9. Tone Filtering
The spectral purity of the DTMF output is sufficient for most applications. However,
where lower distortion DTMF tones are required, an on-chip Darlington pair is
provided (accessible via pins 6 & 7) for use in a low pass active filter.
Figure 3 shows how a 2-pole Sallen and Key filter may be implemented. The typical
component values have been chosen to give a second order Butterworth response
with a cut-off frequency of about 3,5KHz and a nominal pass-band insertion loss of
0,5dB. The relative values of R3 and R4 may be adjusted to achieve the desired
DTMF amplitude which varies with supply voltage.
DTMF Tone Filtering
Figure 3: 2-Pole Sallen and Key Filter Circuit
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