Successful PCB Grounding with Mixed-Signal Chips - Follow the Path of Least Impedance

　　By： Mark Fortunato， Senior Principal Member of Technical Staff

　　Abstract： This tutorial discusses proper printed-circuit board （PCB） grounding for mixed-signal designs. For most applications a simple method without cuts in the ground plane allows for successful PCB layouts with this kind of IC. We begin this document with the basics： where the current flows. Later， we describe how to place components and route signal traces to minimize problems with crosstalk. Finally， we move on to consider power supply-currents and end by discussing how to extend what we have learned to circuits with multiple mixed-signal ICs.

　　A similar version of this article appears in three parts on EDN， August 27， 2012， September 11， 2012， and September 17， 2012.

　　Introduction

　　Board-level designers often have concerns about the proper way to handle grounding for integrated circuits （ICs）， which have separate analog and digital grounds. Should the two be completely separate and never touch？ Should they connect at a single point with cuts in the ground plane to enforce this single point or “Mecca” ground？ How can a Mecca ground be implemented when there are several ICs that call for analog and digital grounds？

　　This tutorial discusses proper printed-circuit board （PCB） grounding for mixed-signal designs. For most applications a simple method without cuts in the ground plane allows for successful PCB layouts with this kind of IC. Next， we learn how to place components and route signal traces to minimize problems with crosstalk. Finally we move on to consider power supply-currents and end by discussing how to extend what we have learned to circuits with multiple mixed-signal ICs.

　　Follow the Current

　　Remember that we call a collection of connected electrical or electronic components a “circuit” because currents always flow from a source to a load and then back via a return path—a circle of sorts. Keeping in mind where the current flows， both in the direction intended to do the desired job as well as the resultant return current， is fundamental to making any analog circuit work well. And， yes， all digital circuits are analog circuits; they are a subset for which we assign meaning to only two states. The transistors and other components， as well as the currents and voltages within the circuit， still operate by the same physical principles as other analog circuits. They will induce return currents in the same way as any other circuit.

　　Figure 1. A simple connection is a direct connection from one IC to another.

　　Figure 1 illustrates the simplest of connections in a design： a direct connection from one chip to another. Taken as an ideal circuit in an ideal world1， the output impedance of IC1 would be zero and the input impedance of IC2 would be infinite. Therefore， there would be no current flowing. In the real world， however， current will flow from IC1 and into IC2， or the reverse. What happens to this current？ Does it just fill up IC2 or IC1？ That is a facetious rhetorical question.

　　Actually， there must be another connection between IC1 and IC2 to allow the current flowing into IC2 from IC1 to return to IC1 and vice-versa. This connection is usually ground and is often not indicated in a digital section of a schematic （Figure 1）。 It is at most implied by use of ground symbols as shown in Figure 2A. Figure 2B shows the full circuit for current flow.

　　Figure 2. The simple circuit of Figure 1 with ground implied （2A） and with the ground current path indicated （2B）。

　　Of course， the ICs themselves are not the sources of current. The power supply for the circuit is. To keep things simple， we assume a single power rail and think of the supply as a battery. To be complete， we bypass the supplies to ICs with capacitors.

　　All DC currents ultimately start and end at the power source. Figure 3 shows the complete circuit with DC current flow when IC1 is sourcing the current indicated.