On-Chip Design For Better Noise Control

By Dr. Glen Dash, Ampyx LLC

ICs can be designed to produce fewer emissions without compromising performance. In fact, good on-chip EMC design can go a long way towards eliminating other undesirable noise related problems, such as "ground bounce." And we are not talking about frequency-modulated clocks here. What we are talking about are techniques that work to lower emissions without changing the clock frequency.

Before discussing how these techniques work however, we should review two main causes of unwanted emissions from digital designs. Here a clock driver is shown driving a distant load. The clock driver, like all integrated circuits, has certain internal characteristics including its internal inductance (L0) and output resistance (R0) that affect its performance. In application, a designer may add a series resistance (RS), usually to match trace impedances. Other impedances present in a circuit will be the impedance of the load (Z2), the trace impedance Z1 and the impedance of the return plane, Z3.