Advanced Information

WM8181

WOLFSON MICROELECTRONICS LTD

AI Rev 3.0 January 2000

11

CCD SCANNER

The differential nature of the WM8181 allows it to interface as easily to CCD sensors as to CIS. The

negative going video simply requires that the VINP and VINM pins are swapped over so that the

video signal is applied to VINM and the reference voltage is applied to VINP. The d.c. level of the

CCD output must lie within the input limits of the WM8181. A level shifter may be required to ensure

this. See Figure 8.

SYSTEM

ASIC

CCD

SENSOR

V

OUT

WM8181

VSMP

DOUT

MCLK

VINP

VINM

CLAMP

VREFIN

V R T

V R B

BLACK

REFERENCE

LEVEL

LEVEL

SHIFT

LEVEL SHIFT

DEPENDENT ON

V

SET VOLTAGE TO MATCH

V

Figure 8 d.c. Coupled Negative Video

USING THE INTERNAL CLAMP

When using a CCD it is recommended that the designer use a.c. (capacitive) coupling between the

CCD output buffer and the input to the WM8181, shown in Figure 9. A CCD sensor has a negative

going video signal superimposed on a d.c. voltage of around 6V. The series capacitor between the

CCD buffer and the input to the WM8181 removes this large d.c. voltage while still allowing the video

signal through.

SYSTEM

ASIC

CCD

SENSOR

V

OUT

WM8181

VSMP

DOUT

MCLK

VINP

VINM

CLAMP

VREFIN

V R T

V R B

4.7k

4.7k

220n

200p

SET VOLTAGE TO MATCH

V

Figure 9 a.c. Coupled Negative Video