Intel 815 - Manual (Page 103)

Integrated Graphics Display Output
R
Intel
®
815 Chipset Platform Design Guide 103
8.1.2 Reference Resistor (Rset) Calculation
The full-swing video output is designed to be 0.7V according to the VESA video standard. With
an equivalent DC resistance of 37.5
Ω
(two 75
Ω
in parallel; one 75
Ω
termination on the board
and one 75
Ω
termination within the display), the full-scale output current of a RAMDAC channel
is 0.7/37.5
Ω
= 18.67 mA. Since the RAMDAC is an 8-bit current-steering DAC, this full-scale
current is equivalent to 255 I, where I is a unit current. Therefore, the unit current or LSB current
of the DAC signals equals 73.2
µ
A. The reference circuitry generates a voltage across this R
set
resistor equal to the bandgap voltage divided by three (407.6 mV). The RAMDAC reference
current generation circuitry is designed to generate a 32-I reference current using the reference
voltage and the R
set
value. To generate a 32-I reference current for the RAMDAC, the reference
current setting resistor, R
set
, is calculated using the following equation:
R
set
= VREF / 32*I = 0.4076 V / 32*73.2 µ A = 174 Ω
8.1.3 RAMDAC Board Design Guidelines
Figure 50 shows a general cross-section of a typical four-layer board. The recommended
RAMDAC routing for a four-layer board is such that the red, green, and blue video outputs are
routed on the top (bottom) layer over (under) a solid ground plane to maximize the noise rejection
characteristics of the video outputs. It is essential to prevent toggling signals from being routed
next to the video output signals to the VGA connector. A 20-mil spacing between any video route
and any other routes is recommended.
Figure 50. Cross-Sectional View of a Four-Layer Board
Board Cross Section
Board
components
RAMDAC / PLL circuitry
Graphics chip
One solid, continuous
ground plane
Digital power plane
RAMDAC_board_xsec
Bottom of board
Segmented analog power
plane for RAMDAC / PLL
Low-frequency
signal traces
Ground plane
Analog traces
Video
connector
Top of board
Avoid clock routes or
high-frequency routes in
the area of the RAMDAC
output signals and
reference resistor.
Matching the video routes (i.e., red, green, blue) from the RAMDAC to the VGA connector also is
essential. The routing for these signals should be as similar as possible (i.e., same routing layer(s),
same number of vias, same routing length, same bends, and jogs).