How to lay out the sensitive circuitry of the I/O module

Proper placement of sensitive circuits in I/O modules is a prerequisite for ensuring their performance. Sensitive circuits, such as high-precision ADCs/DACs, reference voltage sources, and analog front-ends, should be placed in a quiet, clean area on the PCB, away from noise sources such as switching power supplies, digital clock circuits, and relay drivers. Sufficient isolation space should be reserved during placement, and grounding shields can be added if necessary. The power supply for sensitive circuits should be independently led from the main power supply through a π-type filter or LDO, and high-quality decoupling capacitors should be used.

Their grounding should use a separate analog ground plane and be connected to digital ground at a single point. Signal traces should be short and preferably placed on layers with a complete ground plane reference. Crystal oscillators or clock generators should be kept away from sensitive circuits and surrounded by ground lines. If analog and digital circuits must be adjacent, a grounded isolation strip should be placed between them. After placement, a near-field probe can be used to scan the PCB surface to detect the electric and magnetic field noise levels in the sensitive circuit area, and the placement can be further adjusted based on the results.