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 series of three articles comprises a basic tutorial on 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. Below we will learn about this method and that the basic principles used here can be extended to handle more complex and difficult applications.

We begin this Part 1 of the article with the basics: where the current flows. In Part 2 we will learn how to place components and route signal traces to minimize problems with crosstalk. In Part 3 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 world,1 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.

Figure 3 IC1 sourcing DC current

For high-frequency signals (“high” largely determined by the bypass capacitance and power-source impedance) the current starts and ends with the bypass capacitor. Figure 4 shows the high-frequency signal current flow.

Figure 4 IC1 sourcing the high-frequency signal current

It is important to remember that an output is not always the source of currents. For example, consider the case where an output from IC1 is connected to an input of IC2 which has a pullup resistor to V DD . Figure 5 shows transient (high frequency) current flow for this situation with the current coming from C2 through the pullup in IC2 over to the low-side FET in IC1, which is on, and then through the ground lead of IC1 to the ground lead of C2. While IC1 is the “driving” device, sinking current at its output pin by shorting it to ground with a FET, the current source is from C2 through IC2.

Figure 5 IC2 sourcing the high-frequency current.

If the output pin of IC1 in Figure 5 stays low for a long time, then the static current that will be drawn will come directly from the power source (Figure 6 ).

Figure 6 IC2 sourcing DC current.

To this point in our discussion of the basics, the model has been somewhat simplistic. We conveniently divided signals into low frequency and high frequency as if there were a well-defined boundary between the two.

The truth is that both paths are always involved. In Figure 6, at the initial transition of the IC1 output to the low state, the current comes from the bypass capacitor at IC2. This is because the output of IC1 is “demanding” a near-instantaneous current from the input pin of IC2, which pulls this current from its power pin.

We placed a bypass capacitor at IC2 with very short connections to its power and ground pins precisely to supply the fast current demands. The power source cannot provide this transient current as it is not very close to the IC and, thus, has substantial resistance and, more importantly, inductance between it and the power pin of IC2.

This is the whole reason for placing bypass capacitors at the ICs: to supply the transient (high-frequency) currents that the power supply cannot. As the transient settles out, more and more current comes from the power source and less and less comes from the bypass capacitor.

We simplify this concept further by saying that the DC current comes from the power source and the AC current comes from the bypass capacitor(s). We know, of course, that it is a bit more complex than this explanation.

As we consider more dynamic situations, we find that all the currents flow through a combination of the above four paths. The common path in either direction starts with the power pin of the sourcing component (IC1 or IC2), proceeds through that component and through the interconnect to the other component (IC2 or IC1), and then through the second component to its ground pin.

How the current completes its circuit from ground to the power pin of the sourcing component depends on the speed of the signal. The DC current will all return to the ground lead of the power source; it will flow from the power lead of the power source to the power pin of the sourcing component. High-frequency signal current will instead return to the ground lead of the sourcing component's bypass capacitor, which also supplies the current to the power pin.

In reality, both paths are always involved, with the DC path dominating for low-frequency signals. Keep in mind that even if a digital signal transitions at a slow rate (for example, a 1Hz square wave), the state transitions that cause the transient currents are just as fast as with a much higher frequency signal. They simply do not occur as often.

Of course, we are dealing with a good design here, so the bypass capacitors and the IC power and ground pins are very close. Proper bypassing like this makes a designer's job much easier. We can usually just think of the bypass capacitor and the IC as one entity when considering the flow of signal currents across a PCB.

Notice, finally, that the power current for high-speed AC signals travels a very short distance from a bypass capacitor to the IC that it is bypassing. The paths through the ICs themselves, of course, are short. The vast majority of the distance of the current loop is in the interconnect from the output of one chip to the input of the other and the ground return path.

Review Figure 4 and Figure 5 and consider what happens if the ICs are separated by a greater distance. The bypass capacitors stay close to their respective IC, and all the distance is added to the interconnect and the ground return. For higher-speed signal currents, this is where we will see problems…if they occur.

Digital and analog supplies and grounds

In the circuit diagrams above we have not identified the ICs and signals as digital or analog. IC1 could be an op amp with the low-side FET as the lower part of an output stage; the pin on IC2 could be the input to an analog-to-digital converter (ADC). IC1 could just as easily be a microcontroller with a push-pull output for a standard I/O pin; the IC2 input could be a control pin on a digital-to-analog converter (DAC).

We mention ADCs and DACs as these are typically the components that cause concerns with grounding for both the analog and digital signals.

Analog circuits tend to work with signals that vary in a smoother, continuous fashion and for which small changes in voltage or current are meaningful. Digital circuits tend to transition abruptly from one state to another, generating pulses of currents; they tend to have a wide window of voltage which maps to a single state. It is these relatively large, sharp pulses of digital current during transitions that can overwhelm the precise analog signals if the two are not adequately separated from each other.

The path of least impedance

It is such a well-understood principal that current flows in the path of least resistance that the concept has made its way into everyday language. Unfortunately, this is only true for DC currents. A more complete and accurate way of stating the principle is that the current flows in the path of least impedance.

For DC, only the resistive part of impedance matters. In the case of a solid ground plane, the proverbial straight line is the path of least resistance. In fact current will flow in more indirect paths as well.

The amount of current through any particular path will be inversely proportional to the distance because the ground-plane resistance per unit length is very uniform. Therefore, the most current will flow in the straight-line path of least resistance, and progressively less current will flow through paths that deviate more and more from that straight-line path. For simplicity we will indicate DC currents as flowing in the straight line path, with the understanding that there is a fairly wide spread of currents with the largest current moving along that straight line.

For the signals that matter most to us here, the AC signals of some speed, we have to consider the reactive portion of impedance as well.

For a PCB with a ground-plane layer adjacent to the signal layer, we have a well-defined impedance that is determined by the geometry of the trace, the thickness of the board layer that separates the trace from the ground plane, the board material, and the frequency of the signal. All the mathematical details for these givens are beyond the scope of this article. Fortunately, it is not necessary to grind through all the math in order to use the concepts and get good results. The references at the end of this article cover the details well.

Consider our original very simple example of a single trace between two ICs (Figure 1). This time we show them positioned on a PCB with the trace taking an indirect route (Figure 7 ).

Figure 7 Simple indirect trace

Assume now a solid ground plane with the ground connection at each IC near the trace connection point. The return currents have to go from the ground connection of one IC to the ground connection of the other. Since we have a solid ground plane, the path of least resistance, and thus the path of DC current, will be a straight line (the blue arrow in Figure 8 ). At high frequency the mutual inductance between the trace and the ground plane beneath it make the ground path of least impedance directly under the trace (the red trace in Figure 8).

Figure 8 Ground-return current paths show the path of least resistance (blue) and path of least impedance (red).

But what is “high frequency?” A rule of thumb2 is that frequencies of a few hundred kHz and above have return currents which follow the path under the signal trace. The actual frequency above which we consider to be “high” is determined by the trace, board geometries (trace width, space between layers), and board material (dielectric constant). For the return current to follow the trace, in most common cases we need not worry about exactly what frequency this is.

Mathematical treatments of this phenomenon are extremely complex and, to this author, very confusing. Fortunately, Dr. Bruce Archambeault has published on this matter3 and has graciously provided the following figures which visually demonstrate this subject far better than a page full of equations can ever do.

Figure 9 shows the geometry of an example “U”-shaped trace over a ground plane.

Figure 9 Physical geometry for this example (Drawing courtesy of Dr. Bruce Archambeault)

Dr. Archambeault then ran electromagnetic simulations for signals of different frequencies to see by what paths the current would flow. The forward signal currents for each case, of course, are constrained to the trace. However, the return ground currents can flow anywhere on the ground plane.

Figure 10 shows how the currents for a 1kHz signal flow. The ground current primarily flows directly from the load to the source in a straight line, as indicated by the narrow yellow line. A small amount of the ground current flows along the signal path (light blue), while even smaller amounts flow in between these two paths as indicated by the darker blue color of much of the plane.

Figure 10 1kHz ground current flows from load to source in a straight line (Drawing courtesy of Dr. Bruce Archambeault)

Figure 11 shows current for a 50kHz signal flowing primarily along the signal trace (the wide green line following the path of the trace) and, to a lesser extent, directly from load to source (the fainter, wide, green line from the two ends of the trace) and in between. The middle area is light blue and not dark blue, indicating minimal current flow.

Figure 11 50kHz ground current flows everywhere. (Drawing courtesy of Dr. Bruce Archambeault)

Finally, Figure 12 shows the current paths with a 1MHz signal. Virtually all the return ground current is flowing along the path of the signal trace.

Figure 12 1MHz ground current follows the signal trace. (Drawing courtesy of Dr. Bruce Archambeault)

As one would expect, return current does spread out on the plane wider than the trace itself. The distribution of current for these higher frequencies is given by the following equation.4

Where:

J(x) is the current density;

I is the total current;

w is the trace width;

h is the board layer thickness (the height the trace is above the plane);

x is how far from directly under the trace we measure the current, as shown in Figure 13 .

Figure 13 Cross section of board

It is important to note that Equation 1 is independent of frequency (again, assuming that the frequency is high enough, as discussed above). When we evaluate Equation 1, we get a Gaussian-looking distribution with a peak directly under the center of the trace.

If we sum the current between x = -h to x = h, we find that 50% of the total current is in this range. Further, 80% of the current is between x = -3h and x = 3h. As one would expect intuitively, the thinner the board layer (i.e., the closer the trace is to the plane), the tighter the current distribution will be.

What's next?

With these basic principles of current flow understood, we are now prepared to apply them in more complex circuits. In the next installment of this article series we learn how to combine these basic principles to see where the currents flow in real circuits. We will see how to use this knowledge to create PCB layouts that avoid common grounding problems.

Part 2: Design to minimize signal-path crosstalk.

Citations

In an ideal world,this would be all that we would need to understand. The ideal world does not exist or, at least, the one we are dealing with seems not to be ideal. Ott, Henry W., Electromagnetic Compatibility Engineering , John Wiley and Sons, Hoboken, NJ, 2009. ISBN 978-0-470-18930-6, p 393. Archambeault, Bruce, IEEE® EMC Society Newsletter, Fall 2008, Issue 219, “Part II: Resistive vs. Inductive Return Current Paths,” pp 81-83. Ott, p 392

References

Ott, Henry W., Electromagnetic Compatibility Engineering , John Wiley and Sons, Hoboken, NJ, 2009. ISBN 978-0-470-18930-6 Johnson, Howard W., Ph.D. and Graham, Martin, Ph.D., High-Speed Digital Design: A Handbook of Black Magic , Prentice-Hall, Upper Saddle River, NJ, 1993. ISBN 0-13-395724-1 Stoehr, Martin, “Avoid PC-Layout 'Gotchas' in ISM-RF Products,” Maxim Integrated Products application note 4636.

About the Author

Mark Fortunato hasspent much of his life trying to make those pesky electrons go to the right place at the right time. He has worked on products ranging from speech recognition systems to consumer electronics to millimeter wave instrumentation to LED light bulbs (ones that dim properly, mind you). He spent much of the last 16 years helping customers tame analog circuitry. Mr. Fortunato is currently Senior Principal Member of Technical Staff in the Communications and Power Solutions Group at Maxim Integrated Products. When not wrangling electrons, Mark likes to coach youth sports, read nonfiction books, watch his tween-aged son play lacrosse and his adult son play music. In general he strives to live a life of integrity. Mark regrets that he never did get to meet Jim Williams or Bob Pease.

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