WM8148 datasheet(38/43 Pages) WOLFSON

38 / 43 page

WM8148

Production Data

WOLFSON MICROELECTRONICS LTD

PD Rev 4.0 April 1999

APPLICATIONS RECOMMENDATIONS

INTRODUCTION

The WM8148 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 WM8148,

and some typical applications circuits.

PCB LAYOUT

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 WM8148.

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.

Avoid noise on AGND (in particular on pins 30, 35 and 40).

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.

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

cross talk and interference.

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

CCDs are available in four basic types that are all compatible with the WM8148.

• Monochrome single output.

• Monochrome odd-even output.

• Colour 3 output.

• Colour 6 output.

Each of these applications 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 Reset Level Clamping is to be used. The WM8148

supports both of these functions and Wolfson recommend using both CDS and pixel-by-pixel

clamping for optimal performance. In this case a low value a.c. coupling capacitor is required

between the sensor and the WM8148. 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.

WM8148 Datasheet(PDF) 38 Page - Wolfson Microelectronics plc