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SA545

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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