1. Signal Ground: The Reference Plane for Circuits

Signal ground is the reference potential for all analog and digital signals. Its main role is to provide a stable return path and avoid signal distortion or crosstalk caused by unstable reference voltage.

Key Points

• Signal ground is a low-impedance reference plane, not just a single point. Use a full ground plane to reduce return impedance;
• Separate analog ground (AGND) and digital ground (DGND). Analog signals are sensitive to noise, while digital signals introduce high-frequency noise;
• Return paths of high-speed signals must be tightly coupled with signal traces, following the shortest return path rule.

Critical Practices

• Connect analog and digital ground planes at a single point (0Ω resistor, ferrite bead, inductor) to prevent digital noise from entering the analog section;
• Maintain a full ground plane under high-speed differential signals (USB, Ethernet). Never split the plane;
• Route weak sensor grounds separately and connect to analog ground near the sensor, away from digital and power devices.

Real Example

In a high-precision analog acquisition board, the sensor’s analog ground and MCU’s digital ground were connected via a 0Ω resistor at a single point. The analog ground plane was fully copper-poured and kept away from the crystal oscillator. Analog ripple dropped from 50mV to 5mV, greatly improving accuracy.

Common Mistakes

❌ Do NOT directly connect analog and digital ground over a large area – high-frequency digital noise will severely interfere with analog signals;

❌ Do NOT run high-current power traces across the signal ground plane, which creates voltage drops and unstable reference potential.

2. Power Ground: The Return Path for Power Systems

Power ground provides the return path for power ICs and power devices. The core requirement is low impedance and high current capability to match output power.

Key Points

• Power ground copper must be wide enough. Higher power requires larger area/width to avoid voltage drop;
• Power ground works with decoupling and filter capacitors to form short filtering loops;
• Different voltage grounds (12V, 5V, 3.3V) can share a main ground plane. Keep the plane continuous whenever possible.

Critical Practices

• Pour large copper areas for power grounds of LDOs and DC-DC ICs, connected to thermal pads for low impedance and heat dissipation;
• Widen power ground traces for power devices (MOSFETs, power resistors) to ≥2mm or use full copper;
• When sharing a ground plane, place filter capacitors near power outputs to contain noise locally.

Real Example

In a 12V-to-3.3V high-power supply board, the DC-DC ground used large copper connected to the thermal pad. 1000uF + 100nF capacitors were placed close to the output. The power and signal grounds shared a full plane. Output ripple was controlled within 20mV with low thermal rise.

Common Mistakes

❌ Narrow power ground traces cause voltage drops and unstable output;

❌ Long loops between power ground and filter capacitors greatly reduce filtering performance.

3. Split Ground: Isolation for Complex Circuits

Split ground divides the ground plane into separate regions by function to isolate noise. It is used in mixed high-noise, high-precision, and multi-power designs. Unnecessary splitting breaks continuity and causes interference.

Key Points

• Applications: AC/DC mixed boards, high/low frequency boards, high-power/low-signal boards, medical/industrial precision boards;
• Rule: Avoid splitting unless necessary. Keep the ground plane full;
• Split regions must be connected at single or multiple points to ensure uniform potential.

Critical Practices

• Use clearance or slots ≥2mm to separate ground regions;
• For AC/DC boards: fully split AC and DC grounds, connect via safety capacitors or isolation transformers;
• For RF/high-frequency: local split ground connected to low-frequency ground via ferrite bead;
• Use stitching capacitors for signals crossing splits to maintain return paths.

Real Example

An industrial RF control board (220V AC, 5V DC, 433MHz RF) used three split grounds: AC, digital, and RF. AC and DC were connected via safety capacitor; digital and RF via ferrite bead. RF ground was locally poured and isolated. The board passed EMC testing with packet loss below 0.1%.

Common Mistakes

❌ Unnecessary splitting on simple consumer boards breaks ground planes and causes crosstalk;

❌ Unconnected split regions create large voltage differences and damage components;

❌ Narrow isolation slots risk shorting between regions during manufacturing.

4. Guard Ground: Shield for High-Speed & Sensitive Signals

Guard ground surrounds high-speed or weak signals with grounded copper connected at multiple points. It shields signals from external noise and provides a short return path.

Key Points

• Applications: clocks ≥10MHz, RF, weak analog, differential signals (RS485, CAN);
• Rule: Guard copper must be close to signals and reliably grounded at multiple points;
• Single-ended and differential signals use different guarding methods.

Critical Practices

• Single-ended guard: Ground copper on both sides, gap ≤30mil, add GND via every ~500mil;
• Differential guard: Couple differential pairs tightly, guard outside – no copper between pairs;
• Connect guard to analog or high-frequency ground, not noisy power or digital ground;
• Use vias ≥0.8mm for low-impedance grounding.

Real Example

In a 100MHz clock board, double-sided guard ground was used with 20mil spacing and vias every 400mil. Radiation dropped from 60dBμV/m to 30dBμV/m, fully meeting EMC requirements.

Common Mistakes

❌ Single-point guard grounding results in high impedance and poor shielding;

❌ Copper between differential pairs destroys coupling and causes distortion;

❌ Large gap between guard and signal greatly reduces shielding effectiveness.

5. Single-Point Ground: Noise Isolation for Low-Frequency Circuits

Single-point ground connects all circuits to one common point to eliminate ground loops and crosstalk. It is ideal for circuits below 1MHz and NOT suitable for high frequency.

Key Points

• Applications: low-frequency analog, high-precision acquisition, multi-module low-frequency systems;
• Rule: Uniform potential across all modules, no ground loop voltage drop;
• Two types: star (high precision) and tree (multi-module).

Critical Practices

• Star: One central point, each module connects independently with no crossing traces;
• Tree: Main point feeds sub-module points, suitable for industrial control boards;
• Ground traces must be short and wide;
• Can be combined with a full ground plane in low-frequency designs.

Real Example

A 500kHz high-precision acquisition board used star single-point grounding at the center via. Sensor, op-amp, and ADC grounds connected separately with no crossings. Accuracy reached ±0.01V, far exceeding industry standards.

Common Mistakes

❌ Using single-point ground above 1MHz causes long return paths and radiation;

❌ Thin or long ground traces create voltage drops and uneven potential;

❌ Crossing star ground traces create hidden loops and defeat isolation.

6. General Grounding Rules & Self-Checklist

(1) General Design Rules

• Full ground plane first: Avoid unnecessary splits;
• Shortest return path: For all signals and power;
• Isolate but unify: Connect split regions to maintain uniform potential;
• Low-impedance high-current ground: Wide copper for power grounds;
• High-frequency shield, low-frequency single-point: Choose based on frequency.

(2) Grounding Checklist

7. Grounding Selection by Application