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标题：PCB接地技术描述：本文源自Maxim官方的一篇深入浅出的论文，旨在探讨混合信号设计中PCB接地的最佳实践。作者Mark Fortunato，作为高级技术员工，详细解析了电流路径、组件布局、信号跟踪以及电源电流管理等关键知识点，以确保电路板在面对交叉干扰等问题时仍能保持良好的信号完整性。标签：PCB硬件接地信号完整性知识点详述：一、电流路径与接地平面在讨论混合信号PCB设计的接地策略时，首要理解的是电流流动的基本原理。电路中的电流总是寻求最小阻抗路径，因此，在设计中应尽量避免在接地平面上切割，以确保电流能够顺畅地流过，减少电磁干扰（EMI）和信号完整性问题。这种简单的方法对于大多数应用来说已足够，无需复杂的“麦加”接地模式，即所有地线连接至单一点的策略。二、组件布局与信号跟踪正确的组件布局和信号跟踪是实现良好接地效果的关键。为了最小化交叉干扰，应将模拟和数字信号区分开，同时确保它们之间的耦合最小。这通常意味着模拟和数字部分应尽可能地远离，且各自拥有独立的接地路径。此外，信号线应尽量靠近地平面，以形成低阻抗回路，从而降低辐射和接收干扰的风险。三、电源电流管理电源电流的处理同样至关重要。当多个混合信号IC并存于同一PCB上时，电源电流的波动可能会导致地电位波动，进而引起信号噪声。为避免这一问题，可以采用局部旁路电容，为每个IC提供稳定的电源，减少电源线上电流变化的影响。同时，合理布局电源层和地层，使用多点接地策略，有助于分散电流，减少地平面的电压降。四、多IC电路的接地扩展当电路涉及多个混合信号IC时，接地策略需进一步考虑。一方面，各IC的地线应尽可能短且直接连接到公共地平面，以减少回路面积；另一方面，通过使用分割地平面或局部接地岛的方式，可以有效隔离不同功能区域，避免信号间的相互干扰。此外，应关注各IC间的共模电流，确保其不会通过地线形成耦合，增加噪声。五、结论 PCB接地技术在混合信号设计中扮演着至关重要的角色。通过遵循电流路径、优化组件布局、精细管理电源电流以及扩展多IC电路的接地策略，可以显著提高电路板的信号完整性和抗干扰能力。然而，每种设计都有其独特性，因此，实践中应根据具体应用场景灵活调整接地方案，以达到最佳效果。参考文献：本文部分内容摘自Maxim官网的TUTORIAL5450，原作者Mark Fortunato在其论文中提供了丰富的实例和理论支持，值得深入学习和研究。

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Maxim > Design Support > Technical Documents > Tutorials > A/D and D/A Conversion/Sampling Circuits > APP 5450

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Keywords: PCB, mixed-signal, layout, ground, grounding, grounds, printed circuit board

TUTORIAL 5450

Successful PCB Grounding with Mixed-Signal Chips -

Follow the Path of Least Impedance

By: Mark Fortunato, Senior Principal Member of Technical Staff

Oct 15, 2012

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

Page 1 of 22

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

. 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. Thus, it 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

Page 3 of 22

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

Page 4 of 22

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