Ramdac Board Design Guidelines - Intel 855GME Design Manual

®
Intel 855GME Chipset and Intel
Integrated Graphics Display Port
6.1.3

RAMDAC Board Design Guidelines

For the DAC to successfully run at speeds up to 350 MHz, care shall be taken when routing the
analog RAMDAC signals. Intel recommends that each analog R, G, and B signal be routed
single-endedly. The analog RGB signals shall be routed with an impedance of 37.5 Ω . Intel
recommends that these routes be routed on an inner routing layer and that it be shielded with VSS
planes, if possible. Spacing between DAC channels and to other signals shall be maximized;
20-mil spacing is recommended. The RGB signals require pi filters that shall be placed near the
VGA connector. It consists of two 3.3 pF caps with a 75 Ω at 100 MHz FB between them. The
RGB signals shall have a 75 Ω , 1 percent terminating pull-down resistor. The complement signals
(R#, G#, and B#) shall be grounded to the ground plane.
Intel recommends that the pi filter and terminating resistors be placed as close as possible to the
VGA connector. After the 75 Ω termination resistor, the RGB signals shall continue on to their pi
filters and the VGA connector, but shall now ideally be routed with a 75 Ω impedance (~ 5 mil
traces).
The RGB signals also require protection diodes between 1.5 V and ground. These diodes shall have
low C ratings (~5 pF max) and small leakage current (~ 10 µA at 120° C) and shall be properly
decoupled with a 0.1 µF cap. These diodes and decoupling shall be placed to minimize power rail
inductance. The choice between diodes (or diode packs) shall comprehend the recommended
electrical characteristics in addition to cost.
The RGB signals shall be length matched as closely as possible, from the Intel
Graphics Memory Controller Hub (82855GME) to the VGA connector and shall not exceed
200 mils of mismatch.
154
®
6300ESB ICH Embedded Platform Design Guide
®
855GME chipset