WM8141

Production Data

WOLFSON MICROELECTRONICS LTD

PD Rev 3.0 October 2000

26

APPLICATIONS RECOMMENDATIONS

INTRODUCTION

The WM8141 is a mixed signal device, therefore careful PCB layout is required. The following

section contains PCB layout guidelines, which are recommended for optimal performance from the

WM8141, and some typical applications circuits.

PCB LAYOUT

1)

Use separate analogue and digital power and ground planes. The analogue and digital ground

planes should be connected as close as possible to, or underneath, the WM8141.

2)

Place all supply decoupling capacitors as close as possible to their respective supply pins and

provide a low impedance path from the capacitors to the appropriate ground.

3)

Avoid noise on AGND, pin 13.

4)

Avoid noise on reference pins VRT, VRB and VRX. Place the decoupling capacitors as close

as possible to these pins and provide a low impedance path from the capacitors to analogue

ground.

5)

Input signals should be screened from each other and from other sources of noise to avoid

cross- talk and interference.

6)

Minimise load capacitance on digital outputs. Capacitive loads of greater than 20pF will

degrade performance. Use buffers if necessary and keep tracks short.

TYPICAL APPLICATIONS DIAGRAMS

The WM8141 is intended to be used in three types of architecture.

•

Monochrome

•

Colour Pixel-By-Pixel

•

Colour Line-By-Line

Each of these architectures is outlined in this section.

The output from a CCD sensor usually has a high impedance and must therefore be buffered

as close to the sensor as possible. The sensor manufacturers’ datasheets specify the buffer circuit

to use.

Initially, the designer must decide if CDS and RLC are to be used. The WM8141 supports both of

these functions and Wolfson recommend using both CDS and pixel-by-pixel RLC for optimal

performance. In this case a low value a.c. coupling capacitor is required between the sensor and the

WM8141. Experiments have shown that a 100pF capacitor is the optimum value to use, however this

may vary for particular applications depending on speed of operation and PCB layout.