Pre Production
WM8941
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 75Ω, 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:
PP, Rev 3.3, December 2007
53
w
WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
