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4.2 Recommended PCB Design Practices
The device is designed to perform with very few external components required. However, there is no fighting
physics and some filtering capacitors are required for both the supply rails as well as the internally generated
voltage references.
32
31
27
26
25
24
23
SVSS
33
34
22
35
21
Digital Ground Plane
36
37
20
BVSS
Analog Ground Plane
38
BVDD 18
39
VMRC
40
VREFPC
41
VREFNC 15
42
AVDD2 14
43
SHIELD
Analog Power
Analog Power
44
1
2
3
Analog Power
AVDD
AVSS
Do Not Route High Speed Logic Across (Perpendicular to) Plane Split
Digital Power
4
7
8
9
10
11
Analog Power
Figure 33 – Basic Guidelines for Optimal PCB Design (Ground Planes)
Your PCB design should include some of the following features to ensure good separation between the digital
and analog signal environments in your system. Good PCB design practices dictate that the digital and analog
power and ground planes be separated. It is important to maintain these planes at the same basic potential
but care should be exercised to prevent the usual noise of a digital plane from coupling onto the analog plane.
In Figure 33, the electrical connection between the two planes is made at only two points, through Ferrite
bead choked wire. The Ferrite beads act as low pass filters.
As with any mixed signal board design, it is good practice to keep digital signals (especially digital signals with
high edge rates) routed only over areas where digital power and ground planes underlay. Care should be
exercised to never route a high edge rate digital signal perpendicular to a plane split. Doing so will cause a
noise wavefront to launch (left and right) onto both planes along the split.
It is recommended that the digital supply DVDD be bypassed to DVSS using ceramic capacitors. A .1 µF
capacitor in parallel with a .01 µF capacitor is usually sufficient. The capacitor connections to the device
should be made as close as practical to the package to reduce detrimental inductance. This same bypassing
scheme will work sufficiently for BVDD - BVSS, AVDD - AVSS, AVDD2 - AVSS, SHIELD - AVSS pairs as well.
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