SA541 Sames, SA541 Datasheet - Page 10

• Manufacturer Part Number: SA541
• Description: PULSE/DTMF SWITCHABLE DIALLERS
• Manufacturer: Sames

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7.
8.
SA541
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.
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.
sames
Figure 2: Earth Loop Recall Application