Intel 815 - Manual (Page 101)

Integrated Graphics Display Output
R
Intel
®
815 Chipset Platform Design Guide 101
8 Integrated Graphics Display
Output
8.1 Analog RGB/CRT
8.1.1 RAMDAC/Display Interface
Figure 49 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 current output from 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.