PCB layout mistakes that cause noise problems in analog circuits

Mixed-signal printed circuit boards—where sensitive analog sensors coexist with high-speed digital processors—frequently suffer from unexpected noise problems during prototype bring-up.

Common Layout Pitfalls

1. Splitting Ground Planes Improperly

A common myth in hardware design is that splitting analog ground (AGND) and digital ground (DGND) always reduces noise. In modern high-speed designs, split planes often create large return loop inductances when signals cross the gap.

2. Poor Decoupling Capacitor Placement

Decoupling capacitors must be placed as close as physically possible to the IC power pins, with short, wide traces to minimize parasitic inductance.

3. Routing High-Speed Digital Traces Near Sensitive Analog Inputs

SPI clocks and switching regulators generate high dv/dt noise. Keep these at least 3x trace-width away from analog traces or isolate them with ground shielding.