WM8141 Wolfson Microelectronics Ltd., WM8141 Datasheet - Page 26

• Manufacturer Part Number: WM8141
• Description: WM8141 : 12-BIT 6MSPS Cis/ccd Analogue Front End/digitiser
• Manufacturer: Wolfson Microelectronics Ltd.

WM8141
APPLICATIONS RECOMMENDATIONS
INTRODUCTION
PCB LAYOUT
TYPICAL APPLICATIONS DIAGRAMS
WOLFSON MICROELECTRONICS LTD
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.
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The WM8141 is intended to be used in three types of architecture.
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.
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.
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, pin 13.
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.
Monochrome
Colour Pixel-By-Pixel
Colour Line-By-Line
PD Rev 3.0 October 2000
Production Data
26