WM8776 datasheet(53/55 Pages) WOLFSON

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WM8776

PP Rev 1.91 June 2004

It is recommended that a low pass filter be applied to the output from the DAC for hi-fi

applications. Typically a second order filter is suitable and provides sufficient attenuation of

high frequency components (the unique low order, high bit count multi-bit sigma delta DAC

structure used in WM8776 produces much less high frequency output noise). This filter is

typically also used to provide the 2x gain needed to provide the standard 2Vrms output level

from most consumer equipment. Figure 43 shows a suitable post DAC filter circuit, with 2x

gain. Alternative inverting filter architectures might also be used with as good results.

+VS

-VS

10uF

Ω

7.5K

Ω

680pF

1.8k

Ω

10k

Ω

4.7k

Ω

4.7k

Ω

1.0nF

Figure 43 Recommended Post DAC Filter Circuit

To ensure that system ‘pop’ noise is kept to a minimum when power is applied or removed, a

transistor clamp circuit arrangement may be added to the output connectors of the system. A

recommended clamp circuit configuration is shown below.

When the +VS power supply is applied, PNP transistor Q10 of the trigger circuit is held on

until capacitor C49 is fully charged. With transistor Q10 held ‘on’, NPN transistors Q4 to Q5

of the clamp circuits are also switched on holding the system outputs near to GND. When

capacitor C49 is fully charged transistors Q10 and Q4 to Q5 are switched off setting the

outputs active.

When the +VS power supply is removed, PNP transistor Q11 of the trigger circuit is switched

on. In turn, transistors Q4 to Q5 of the clamp circuits are switched on holding the outputs of

the evaluation board near to GND until the rest of the circuitry on the board has settled.

Note: It is recommended that low Vcesat switching transistors should be used in this circuit

to ensure that the clamp is applied before the rest of the circuitry has time to power down.

Important: If a trigger circuit such as the one shown is to be used, it is important that the +VS

supply drops quicker than any other supply to ensure that the outputs are clamped during the

period when ‘pop’ noise may occur.

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