WM8946 datasheet(86/175 Pages) WOLFSON

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WM8946

Production Data

PD, July 2012, Rev 4.3

VIDEO BUFFER

The WM8946 provides a current mode output video buffer with an input 3

rd order Butterworth low pass

filter (LPF) and clamp. The video buffer is powered from LDOVDD – typically 3.3V. The video buffer is

compatible with PAL and NTSC video formats.

The low pass filter (LPF) is intended to remove images in the video DAC output waveform at multiples

of the DAC clock frequency. The input clamp supports AC coupling at the input to the video buffer.

Figure 33 Video Buffer Lowpass Filter Frequency Response Gain=0dB

The current mode output employed by the WM8946 video buffer allows operation at lower supply

voltages than voltage mode video buffers. The current mode output also provides inherent protection

against short circuits during jack insertion and removal. A current reference resistor (positioned close

to the WM8946) ensures that the signal swing at the output of the buffer is the same as that at the

receiving equipment (e.g. a television set), thus providing excellent signal reproduction.

For best performance, the input to the video buffer should be AC coupled and terminated to 75

.

Note that the input clamp and pull-down features described below are only applicable to the AC-

coupled input configuration.

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

performance. PCB vias and sharp corners should be avoided and parasitic capacitance minimised

on signal paths; these should be kept as short and straight as possible. The LDOVDD supply should

be decoupled as close to the WM8946 as possible. See the “External Components” section for more

information.

The video buffer is enabled using the VB_ENA register bit. The gain of the video buffer is selected

using VB_GAIN; this can be set to 0dB or 6dB (corresponding to 6dB or 12dB unloaded). The LPF

response can be adjusted by setting the VB_QBOOST register; this provides a small amount of

additional gain in the region of the cut-off frequency.

The input signal clamp is enabled using VB_CLAMP; this controls the DC component of the video

signal for compatibility with the WM8941. The video buffer pull-down can be enabled using VB_PD;

this may be used during power-up of the video buffer in order to align the signal levels between the

source and the WM8946. Note that the pull-down should not be enabled during normal operation of

the video buffer; it should be enabled when the video buffer is first powered up, and subsequently

disabled (e.g. After 20ms) once the circuit has settled.

A programmable DC offset can be applied to the output signal using the VB_DISOFF register field;

this can be set to 0mV, 20mV or 40mV offset.

Note that the VMID reference (see “Voltage References and Master Bias”) must be enabled when

using the WM8946 video buffer. VMID is enabled by setting VMID_ENA, as defined in Table 41.

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