Specifications

Intel
®
810E2 Chipset Platform
R
106 Design Guide
In addition to the termination resistance and LC pi-filter, there are protection diodes connected to the
RAMDAC outputs to help prevent latch-up. The protection diodes must be connected to the same power
supply rails as the RAMDAC. An LC filter is recommended to connect the segmented analog 1.8V
power plane of the RAMDAC to the 1.8V board power plane. The LC filter is recommended to be
designed for a cut-off frequency of 100 kHz.
3.25.1. 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
255I, where I is a unit of current. Therefore, the unit current or LSB current of the DAC signals equals
73.2 µA. The reference circuitry generates a voltage across this R
set
resistor equal to a bandgap voltage
divided-by-three (409 mV). The RAMDAC reference current generation circuitry is designed to generate
a 32I reference current using the reference voltage and the R
set
value. To generate a 32I reference current
for the RAMDAC, the reference current setting resistor, R
set
, is calculated from the following equation:
R
set
= V
REF
/32I = 0.409V/32*73.2µA = 174 Ω
3.25.2. RAMDAC Board Design Guidelines
The RAMDAC layout is shown in the following three figures. An RLC network should be used between
the board power plane and the board ground plane. The recommended RAMDAC routing for a four layer
board is such that the red, green and blue video outputs be routed on the top (bottom) layer over (under)
a solid ground plane to maximize noise rejection characteristics of the video outputs. It is essential to
avoid 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.
Matching of the video routes (red, green, and blue) from the RAMDAC to the VGA connector is also
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).
The following figure shows recommended RAMDAC component placement and routing. The
termination resistance can be placed anywhere along the video route from the RAMDAC output to the
VGA connector as long as the impedance of the traces are designed as indicated in the following figure.
The pi- filters are recommended to be placed in close proximity to the VGA connector to maximize EMI
filtering effectiveness. The LC filter components for the RAMDAC/PLL power plane, de-coupling
capacitors, latch-up protection diodes, and the reference resistor are recommended to be placed in close
proximity to the respective pins.