This article shares core PCB layout and routing rules for high-speed converters (e.g., high-performance ADCs) to maintain wide dynamic range and low noise of analog signals in digital environments.
This article shares core PCB layout and routing rules for high-speed converters (e.g., high-performance ADCs) to maintain wide dynamic range and low noise of analog signals in digital environments.
1. AGND and DGND Placement
Analog ground (AGND) and digital ground (DGND) should generally not be split. Splitting increases return current inductance and voltage noise (per V = L(di/dt)), which degrades performance.
- For special cases requiring ground separation (e.g., partitioned layout constraints), connect the split grounds at one or more optimal points (near or under the converter) to avoid performance loss.
2. Power Plane Design
- Maximize copper area for power planes; avoid segmentation caused by excessive traces or vias, which causes voltage drops at converter power pins.
- Never overlap noisy digital power planes with analog power planes to prevent coupling; separate them as much as possible.
3. Power Distribution System (PDS) Design
PDS design cannot be ignored; its goal is to minimize voltage ripple from dynamic current demands.
- Use a stack-up with large inter-plane capacitance (place ground and power planes 2–3 mils apart).
- Add decoupling capacitors (0.001 μF to 100 μF) at power entrances and around the device to maintain low PDS impedance across frequencies.
4. Exposed Pad (E-Pad) Design
The exposed pad under high-speed ICs is critical for grounding and heat dissipation but is often overlooked. Follow three steps for optimal connection:
- Replicate the exposed pad on all PCB layers for low-impedance grounding and heat dissipation.
- Split the pad into a checkerboard pattern to ensure uniform solder bonding.
- Add ground vias for each segment and fill vias with solder paste/epoxy to prevent solder wicking.
5. Inter-Layer Cross-Coupling
Adjacent layers form parasitic capacitors, causing severe coupling even at 40 mil spacing.
- Noisy digital layers easily couple noise into sensitive analog layers/power rails.
- Higher-resolution ADCs (e.g., 14-bit vs. 12-bit) are far more sensitive to coupling noise.
- Avoid overlapping sensitive analog layers with high-noise digital layers during layout.
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