Description of functionality

TDA7404

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4.2

AutoZero

In order to reduce the number of pins there is no AC coupling between the In-Gain and the

following stage, so that any offset generated by or before the In-Gain-stage would be

transferred or even amplified to the output. To avoid that effect a special Offset-cancellation-

stage called AutoZero is implemented. This stage is located before the Mixing-block to

eliminate all offsets generated by the Input-Stages and the In-Gain (Please notice that

externally generated offsets, e.g. generated through the leakage current of the coupling

capacitors, are not canceled).

The auto-zeroing is started every time the DATA-BYTE 0 (Input Selector/Gain) is selected

and takes a time of max. 0.3ms. To avoid audible clicks the Audioprocessor is muted before

the loudness stage during this time.

4.2.1

AutoZero remain

In some cases, for example if the µP is executing a refresh cycle of the IIC-Bus-

programming, it is not useful to start a new AutoZero-action because no new source is

selected and an undesired mute would appear at the outputs. For such applications the

TDA7404 could be switched in the AutoZero-Remain-Mode (I2 bit of the subaddress-byte).

If this bit is set to high, the DATABYTE 0 could be loaded without invoking the AutoZero and

the old adjustment-value remains.

4.3

Mixing stage

The 4 step Mixing stage offers the possibility to mix the rear selector signal or the phone

signal to any other source. Due to the fact that the mixing-stage is located behind the In-

Gain-stage fine adjustments of the main source level could be done in this way.

Figure 4.

Signal-flow of mixing-stage

Diffin

Loudness

SE1

SE2

SE3

Mono

Beep

Input Selector

In-Gain

Mixing

Mixing Level

Input / Beep or Mono

0dB

/

-100dB

-3.5dB

/

- 9.6dB

- 6dB /

- 6dB

- 12dB

/

- 2.5dB