PCB Technical - Signal integrity research: understanding the critical length, what is a ground bounce?

Understand the critical length

Many people are very vague about the concept of the critical length of lines on the PCB, and even many people don’t know this concept at all. If you design a high-speed circuit board but don’t know this concept, you can be sure that the final circuit board may not work stably., But you are at a loss and have no way of debugging.

Critical length is very confusing in the industry. Some people say that it is 3 inches, and some people say that it is 1 inch. I have heard many other opinions, most of which are caused by incorrect understanding of this concept. Many people say that if the trace is too long, it will cause signal reflection, and if the trace is short, it will not cause reflection. This statement is very wrong, mixing several concepts together like a mash. So what is the critical length, how much is it, and why should we pay attention to the critical length?

The best way to understand the critical length is to analyze it from a time perspective. It takes a certain time for the signal to be transmitted on the PCB trace. The transmission time on the ordinary FR4 board is about 6 inches per nanosecond. Of course, the speed of the surface trace and the inner trace is slightly different. Signal reflection occurs when there is a sudden change in impedance on the trace, which has nothing to do with the length of the trace. However, if the trace is very short, the reflected signal has returned to the source before the source signal has risen to a high level, and the transmitted signal will be submerged in the rising edge, and the signal waveform will not change much. If the trace is very long, the signal at the transmitting end has reached a high level, and the reflected signal reaches the source end, then the reflected signal will be superimposed on the high level position, causing interference. Then there is a critical value for the length of the trace. If it is greater than this value, the return signal is superimposed on the high level, and the reflected signal is submerged by the rising edge if it is less than this value. This critical value is the critical length. Note that this definition is very inaccurate, because only one reflection is considered. This is just for the purpose of understanding the concept.

So what is the precise definition? In practice, reflections occur multiple times. Although the time for the first signal reflection to return to the source is less than the signal rising edge time, the subsequent multiple reflections will also be superimposed on the high-level position, causing interference to the signal waveform. Then, a reasonable definition of the critical length should be: the trace length that can control the interference of the reflected signal within a tolerable range. The signal round trip time of this length is much shorter than the signal rise time. The empirical data found in the experiment is that when the time delay of the signal on the PCB trace is higher than 20% of the rising edge of the signal, the signal will produce obvious ringing. For a square wave signal with a rise time of 1ns, when the PCB trace length is 0.2*6=1.2inch or more, the signal will ring seriously. So the critical length is 1.2inch, about 3cm.

You may have noticed, it's the signal rise time again! Once again, the signal rise time occupies an important position in high-speed design.

What is a ground bomb

The so-called "ground bounce" refers to the change of the internal "ground" level of the chip relative to the "ground" level of the circuit board. Taking the "ground" of the circuit board as a reference, it is like the "ground" level inside the chip is constantly beating, so it is vividly called the ground bounce. When the device output terminal has a state transition to another state, the phenomenon of ground bounce will cause glitches on the logic input terminal of the device.

So how did the "ground bomb" come about?

Parasitic parameters. The ground bounce is caused by the inductance on the pin.

We can use the following figure to explain intuitively. The different positions of switch Q in the figure represent the two states of output "0" and "1". Suppose that due to the circuit state change, switch Q turns on RL low level, the load capacitor discharges to the ground, as the load capacitor voltage drops, its accumulated charge flows to the ground, forming a large current surge on the ground loop. As the discharge current builds up and then decays, this current change acts on the inductance LG of the ground pin, so that a certain voltage difference will be formed between the "ground" of the circuit board outside the chip and the ground inside the chip, as shown in the figure VG . This kind of internal reference ground potential drift of the chip caused by output conversion is ground bounce.

The output of chip A changes, producing ground bounces. This has an impact on the input logic of chip A. The receiving logic compares the input voltage with the ground voltage inside the chip to determine the input. Therefore, from the receiving logic, it is as if the input signal itself is superimposed with the same noise as the ground bounce noise.

Nowadays, the scale of integrated circuits is getting bigger and bigger, the switching speed is constantly increasing, and the ground bounce noise will affect the function of the circuit if it is not well controlled. Therefore, it is necessary to deeply understand the concept of ground bounce and study its laws.