Advanced Information

WM8181

WOLFSON MICROELECTRONICS LTD

AI Rev 3.0 January 2000

9

CCD

Outputs

VSMP

S/H

Reset

DOUT

MCLK

Reset

Video Pixel 1

Reset

Video Pixel 2

Video Pixel 3

D11

→ D0 PIXEL - 2

D11

→ D0 PIXEL - 1

D11

→ D0

PIXEL 0

LATENCY = 32.5 MCLK PERIODS

0000

0000

0000

Video Pixel 0

Figure 5 16:1 Operation

APPLICATIONS RECOMMENDATIONS

INTRODUCTION

The WM8181 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

WM8181, and some typical application 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 WM8181.

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 pins 1 and 2.

4)

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

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

ground.

5)

When VREFIN is used as an external reference control, any noise on VREFIN will degrade the

performance of the ADC. In this case, VREFIN must be carefully de-coupled to AGND.

6)

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

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

TYPICAL APPLICATIONS

The WM8181 is intended for colour scanner applications using a line-by-line architecture and

monochrome scanners, as used in fax machines.

The low pincount and simple digital interface gives the scanner designer the opportunity to place the

ADC near to the sensor. This allows the video information to be converted into the digital domain as

early as possible in the signal chain and minimises analogue noise problems. In the typical

architecture of a flatbed scanner, this means that only power and digital signals appear on the ribbon

minimising crosstalk between the digital clocks and analogue video signals. Care must be taken to

avoid any increase in EMI generated by the higher clock rates on the ribbon cable.

CIS SCANNER

The WM8181 is ideal for use in CIS based scanners where the video output is supplied on a single

output pin. This is true of the majority of colour CIS and all monochrome CIS.

In general, CIS devices provide a video output that becomes more positive for more illumination. This

situation corresponds to the d.c. Coupled Positive Video diagram, Figure 6. The value of the black

reference voltage should be set to be slightly less than the black level output from the CIS to ensure

that the black never saturates.