User's Manual

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7.2.5 Line Input
If the input analogue gain is set to less than 21dB, WT32 automatically selects line input mode. In line
input mode, the first stage of the amplifier is automatically disabled, providing additional power saving.
In line input mode, the input impedance varies from 6kΩ-30kΩ, depending on the volume setting. Figure
19 and Figure 20 show two circuits for line input operation and show connections for either differential or
single-ended inputs.
Figure 18: Differential input (left channel shown)
Figure 19: Single ended input (left channel shown)
7.2.6 Output Stage
The output digital circuitry converts the signal from 16-bit per sample, linear PCM of variable sampling
frequency to a 2Mbits/s 5-bit multi-bit bit stream, which is fed into the analogue output circuitry.
The output circuit is comprised of a digital to analogue converter with gain setting and an output
amplifier. Its class AB output stage is capable of driving a signal on both channels of up to 2V pk-pk
differential into a load of 16Ω. The output is available as a differential signal between
AUDIO_OUT_N_LEFT and AUDIO_OUT_P_LEFT for the left channel, as Figure 20 shows; and
between AUDIO_OUT_N_RIGHT and AUDIO_OUT_P_RIGHT for the right channel. The output is
capable of driving a speaker directly if its impedance is at least 8Ω at reduced output swing and if only
one channel is connected or an external regulator is used.
Figure 20: Speaker output (left channel shown)
The analogue gain of the output stage is controlled by a 3-bit programmable resistive divider, which sets
the gain in steps of approximately 3dB.
The multi-bit bit stream from the digital circuitry is low pass filtered by a third order filter with a pole at
20kHz. The signal is then amplified in the fully differential output stage, which has a gain bandwidth of
typically 1MHz.