WM8941 datasheet(53/96 Pages) WOLFSON

53 / 96 page

Pre Production

WM8941

PP, Rev 3.3, December 2007

VIDEO BUFFER

DESCRIPTION

The WM8941 incorporates a current mode output video buffer capable of operating from a 2.5V

supply, with an input 3rd order Low Pass Filter (LPF) and clamp. The gain through this buffer can be

programmed as 0dB or 6dB (=6dB or 12dB unloaded) via the control interface. The current mode

output means that the signal swing seen at the output of the buffer will be the same as that at the

connection to the receiving equipment (e.g. a TV). Note that the input to the receiver should be AC

coupled and terminated to 75

Ω, as is standard, for best performance.

Figure 25 Video Buffer

The input clamp should be enabled when using AC coupling at the input to the video buffer, using

the VBCLAMPEN register bit.

Care should be taken with PCB layout, designing for at least 1GHz frequencies to avoid degrading

performance. Vias and sharp corners should be avoided and parasitic capacitance minimised on

signal paths, which should be kept as short and straight as possible. The VBVDD supply should be

decoupled as close to the pin as possible. See the “External Components” section for more

information.

LOW PASS FILTER

A low pass filter is integrated at the video buffer input, which is intended to remove images in the

video DAC output waveform at multiples of the DAC clock frequency. A 3rd order Butterworth filter is

used, with the following characteristics:

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