Integrated Graphics Display Output; Analog Rgb/Crt; Ramdac/Display Interface - Intel 815EG Design Manual

R
8 Integrated Graphics Display
Output
8.1

Analog RGB/CRT

8.1.1

RAMDAC/Display Interface

Figure 50 shows the interface of the RAMDAC analog current outputs with the display. Each DAC
output is doubly terminated with a 75 Ω resistance. One 75 Ω resistance is from the DAC output to
the board ground and the other termination resistance exists within the display. The equivalent DC
resistance at the output of each DAC output is 37.5 Ω. The output current of each DAC flows into
this equivalent resistive load to produce a video voltage without the need for external buffering.
There is also an LC pi-filter that is used to reduce high-frequency glitches and noise and to reduce
EMI. To maximize performance, the filter impedance, cable impedance, and load impedance
should be the same. The LC pi-filter consists of two 3.3 pF capacitors and a ferrite bead with a
75 Ω impedance at 100 MHz. The LC pi-filter is designed to filter glitches produced by the
RAMDAC while maintaining adequate edge rates to support high-end display resolutions.
®
Intel 815EG Chipset Platform Design Guide

Integrated Graphics Display Output

89