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
25k
Beep
Input Selector In-Gain Mixing
Mixing Level
Input / Beep or Mono
0dB / -100dB
-3.5dB / - 9.6dB
- 6dB / - 6dB
- 12dB / - 2.5dB
