Specifications
Intel
®
810E2 Chipset Platform
R
Design Guide 109
Figure 68. Recommended LC Filter Connection
R
VCCDA
Display PLL and RAMDAC
lc_filter.vsd
VCCDACA1 VCCDACA2
VSSDA VSSDACA
Board Power Plane
L
C
DPLL Analog Power
Board
Ground
Plane
RAMDAC Analog Power
Board Ground Plane
The resistance from the inductor to the board 1.8V power plane represents the total resistance from the
board power plane to the filter capacitor. This resistance, which can be a physical resistor, routing/via
resistance, parasitic resistance of the inductor or combinations of these, acts as a damping resistance for
the filter and effects the response of the filter.
The LC filter topology shown in the above figure is the preferred choice since the RAMDAC minimum
voltage level requirement does not place constraints on the LC filter for the DPLL. The maximum current
flowing into the DPLL analog power is approximately 30 mA, much less than that of the RAMDAC, and
therefore, a filter inductor with a higher DC resistance can be tolerated. With the topology in the above
figure, the filter inductor DC current rating must be at least 30 mA and the maximum IR drop from the
board power plane to the VCCDA ball should be 100 mV or less (corresponds to a series resistance equal
to or less than 3.3 Ω. This larger dc resistance tolerance improves the damping and the filter response.
3.26.1. Filter Specification
The low-pass filter specification with the input being the board power plane and the output measured
across the filter capacitor is defined as follows for the filter topology shown in the above figure.
• pass band gain < 0.2 dB
• DC IR drop from board power plane to the DPLL VCCDA ball < 100 mV (and a maximum DC
resistance < 3.3 Ω)
• filter should support a DC current > 30 mA
• minimum attenuation from 100 kHz to 10 MHz = 10 dB (desired attenuation > 20 dB)
• a magnetically shielded inductor is recommended
The resistance from the board power plane to the filter capacitor node should be designed to meet the
filter specifications outlined above. This resistance acts as a damping resistance for the filter and affects