10.1 Layout Guidelines

Proper grounding and proper routing of all signals is essential to ensure accurate conversion. Each ground layer should be a single unified ground plane, rather than splitting the ground planes into analog and digital areas.

Because digital switching transients are composed largely of high frequency components, the skin effect dictates that the total ground-plane copper weight has little effect upon the logic-generated noise. Total surface area is more important than the total ground-plane volume. Coupling between the typically-noisy digital circuitry and the sensitive analog circuitry can lead to poor performance that can be impossible to isolate and remedy. The solution is to keep the analog circuitry well separated from the digital circuitry.

High-power digital components must not be located on or near any linear component or power-supply trace or plane that services analog or mixed-signal components because the resulting common return current path could cause fluctuation in the analog input ground return of the ADC which causes excessive noise in the conversion result.

In general, assume that analog and digital lines must cross each other at 90° to avoid digital noise into the analog path. In high frequency systems, however, avoid crossing analog and digital lines altogether. The input clock lines must be isolated from all other lines, both analog and digital. The generally-accepted 90° crossing must be avoided because even a same amount of coupling causes problems at high frequencies. Best performance at high frequencies is obtained with a straight signal path.

Coupling onto or between the clock and input signal paths must be avoided using any isolation techniques available including distance isolation, orientation planning to prevent field coupling of components like inductors and transformers, and providing well coupled reference planes. Via stitching around the clock signal path and the input analog signal path provides a quiet ground reference for the critical signal paths and reduces noise coupling onto these paths. Sensitive signal traces must not cross other signal traces or power routing on adjacent PCB layers, rather a ground plane must separate the traces. If necessary, the traces should cross at 90° angles to minimize crosstalk.

Isolation of the analog input is important because of the low-level drive required of the ADC12J1600 and ADC12J2700 devices. Quality analog input signal and clock signal path layout is required for full dynamic performance. Symmetry of the differential signal paths and discrete components in the path is mandatory and symmetrical shunt-oriented components should have a common grounding via. The high frequency requirements of the input and clock signal paths necessitate using differential routing with controlled impedances and minimizing signal path stubs (including vias) when possible.

Layout of the high-speed serial-data lines is of particular importance. These traces must be routed as tightly coupled 100-Ω differential pairs, with minimal vias. When vias must be used, care must be taken to implement control-impedance vias (that is, 50-Ω) with adjacent ground vias for image current control.