Advanced Information
WM8181
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.
LEVEL SHIFT
WM8181
DEPENDENT ON
VOUT DC LEVEL
BLACK
REFERENCE
LEVEL
MCLK
VSMP
DOUT
VINP
SYSTEM
ASIC
VOUT
LEVEL
SHIFT
VINM
CLAMP
VRT VRB
CCD
SENSOR
VREFIN
SET VOLTAGE TO MATCH
VPP FROM SENSOR.
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.
WM8181
4.7k
MCLK
VSMP
DOUT
VINP
4.7k
220n
SYSTEM
ASIC
VOUT
200p
VINM
CLAMP
VRT VRB
VREFIN
CCD
SENSOR
SET VOLTAGE TO MATCH
VPP FROM SENSOR.
Figure 9 a.c. Coupled Negative Video
AI Rev 3.0 January 2000
WOLFSON MICROELECTRONICS LTD
11
WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
