Every hardware engineer has experienced the same ritual: a board comes back, you power it up, measure your ADC output, and see noise that bears no resemblance to what your schematic promised. The switching noise from your DC-DC converter has found its way into your analog front end. This is entirely preventable.

Rule 1: Separate Analog and Digital Ground Planes — Correctly

The goal is not two separate polygons — it is to control where high-frequency ground currents flow. Use a single ground plane with a deliberate split joined at one point: the ADC ground pin or a star point near the power connector.

• Place the split under the ADC/DAC device boundary
• Never route a high-speed digital trace across the split
• Connect analog GND and digital GND at exactly one point via zero-ohm resistor

Rule 2: Never Route High-Speed Signals Over Plane Splits

When a high-frequency signal crosses a gap in the reference plane, return current detours around the split — creating a loop antenna that radiates and couples noise into adjacent circuits. On 4-layer boards, this is the most common violation made by engineers who do not think in 3D.

Rule 3: Control Decoupling Capacitor Placement Rigorously

A decoupling capacitor placed 5 mm from the IC provides almost no benefit above 20 MHz. Trace inductance (~1-2 nH/mm) creates a series LC resonance that makes the cap useless above its resonant frequency.

Rule 4: Keep Switching Elements Far From the Analog Domain

Your buck converter LX node transitions at 10-50 V/ns at 400 kHz-2 MHz. Its electromagnetic field couples into any analog signal trace within 5-10 mm. Keep the switch node loop as small as possible and keep it far from your ADC input path.

Rule 5: Manage High-Current Ground Return Paths

The ground plane is not an equipotential surface at high frequencies. Every milliohm of resistance and nanohenry of inductance creates a voltage difference. If your ADC reference ground and motor driver ground are separated by 20 mm, there is a measurable voltage difference at switching frequencies.

Rule 6: Use Guard Rings for High-Impedance Analog Inputs

Amplifier inputs above 1 Mohm are vulnerable to leakage currents and humidity-induced surface conductance. A guard ring — copper at the same potential as the input, surrounding the trace — eliminates surface leakage and shields from capacitive coupling.

Rule 7: Simulate Before You Spin

Run SI simulation and power integrity analysis before ordering. Tools like HyperLynx, Cadence Sigrity, or Saturn PCB Toolkit let you model decoupling networks, plane resonances, and trace impedances. A simulated board spin costs nothing. A respun PCB costs time and money.