AN2583

Typical application

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The typical AIO application has audio sources in the system micro-controller and in the fax

chipset originating with the phone line. These are summed into the input amplifier which in

turn drives the filter along side the internally generated 256/512 Hz tones (see Figure 2).

Figure 2.

M41T00AUD application example

input can be added at a different level by changing its corresponding input resistor thus

allowing signals of varying levels to be normalized.

The input capacitor, C, blocks any DC into the part. Since the M41T00AUD uses a single

Ω and C =

0.1 uF, signals below approximately 80 Hz will be attenuated.

After the input amplifier is the low-pass filter with its knee at 8 kHz. With the input capacitor,

these form a band pass filter over the range of 80 Hz to 8 kHz.

Also input to the LPF is the 256/512 Hz signal from the RTC timing chain. Software controls

whether it is passed into the LPF and which of the two frequencies is selected. Software

also controls the gain. A mute bit can be set to shut off all audio and a four-bit GAIN field

controls the digital gain stage selecting between –33 dB and +12 dB of gain in 3 dB steps.

it can readily drive over 300 mW of power into an 8

Ω speaker.

uC

PWM / DAC

TONE

GENERATOR

Fax

Chipset

MODEM

Phone

Line

LPF

ADJ

GAIN

AOUT+

AOUT–

M41T00AUD Audio section

RTC Timing

Chain

256/512Hz

C

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