Differential Crosstalk and Spacing Between Differential Pairs
Crosstalk is a fundamental aspect of signal integrity, both in single-ended and differential trace. The spacing between signal lines with each routing configuration is defined using typical rules of thumb, which can be easily defined as design rules in your PCB design software. One rule of thumb for defining spacing between each trace in a differential pair is the “5S” rule, sometimes called the “5W” rule in application notes and other PCB design guidelines.
The 5S rule states that the spacing between two lines in a differential pair should be a factor 5 larger than the width of each trace in the pair. When dense routing is required for multiple differential pairs, crosstalk between differential pairs becomes an important consideration, and we need a way to analyze the spacing between multiple differential pairs. As it turns out, this is a function of the height of the pairs to the nearest ground plane. Let’s dig into this more deeply and see how we can determine the right spacing between differential pairs to prevent differential crosstalk.
What is Differential Crosstalk?
As its name implies, differential crosstalk is the differential mode analog of single-ended crosstalk, referring to forms of crosstalk between differential pairs, or to the crosstalk generated on a single-ended trace by a differential pair. The two types of crosstalk found between single-ended pairs (NEXT and FEXT) also occur between differential pairs. Strong differential crosstalk can be induced capacitively and inductively, depending on the frequency and geometry of the structure.
The total field seen at some lateral distance away from the pair is the sum of the fields from the two pairs. Because the two ends of a pair have some spacing between them, the total field seen at some lateral distance away from the differential pair is not equal to zero. Furthermore, the strength of the electromagnetic field away from the two traces is larger when the two pairs have a larger spacing.
This motivates formulating some rule that is used to define the spacing between two differential pairs. From the above discussion, and simply by knowing that the field strength decreases as you move away from the pair, one would naturally formulate the following layout requirements for differential pairs:
- The spacing between two differential pairs should be proportional to the spacing between each trace in the pair.
- The spacing between two differential pairs should be in some way proportional to the distance between each pair and its reference plane (if any is present).
Let’s take a look at the following geometry for two differential pairs and determine the differential mode crosstalk between these them. You’re probably thinking that the whole point of differential pairs is noise suppression; while this is true for common mode noise, the difference in field strength between the two traces in the victim pair will produce different levels of noise in each pair, appearing as differential mode noise at the receiver.
Calculating Differential Crosstalk Strength in a First-Order Model
In the above discussion, there is one other aspect that was not considered: the height of the trace above its reference plane and the exact arrangement of traces in the pair. Similar considerations could be made for stripline differential pairs. Here, we would like to quantify the strength of differential crosstalk as a function of geometry. The approach shown here follows closely the approach shown by Douglas Brooks. This is normally done by defining a crosstalk coefficient from a circuit model. The problem with these models is that they fail to account for the field strength at the victim trace as a function of distance between the aggressor and victim.
In the above model, we can define a crosstalk coefficient C as a function of the trace spacing S and the height above the reference plane H. It is convenient to define the crosstalk coefficient as a function of the ratio (S/H). In this case, the single-ended crosstalk coefficient between two traces separated by a distance S with opposite polarity is:
Here, k is a proportionality constant which is related to the signal rise time on the aggressor line, the transfer function of the victim line, and the dielectric constant of substrate. At sufficiently high frequencies and with nearly resistive loads, k will be a constant that is proportional to the signal rise time. As we will see soon, the value of C can be used to define the ratio of common mode to differential crosstalk noise generated on the victim trace for a given ratio (S/H). The differential receiver will eliminate the common mode noise, so we would like to minimize the differential mode noise.
The differential crosstalk is defined by calculating sums and differences in crosstalk coefficients. For the arrangement shown above, the crosstalk between one differential pair and one trace in the victim pair is just the sum of their coefficients. Note that, for any spacing value, simply take the scale transformation S → S(1+x). The differential crosstalk is just the difference in crosstalk coefficients for the victim traces:
If we plot this as a function of x for various values of (S/H), we find that the spacing between two pairs can be reduced when the traces are closer to a ground plane. The image below shows such a plot for k = 1; increasing k just moves these curves up the y-axis. This is done to satisfy a given requirement on differential crosstalk. For example, suppose you require a differential crosstalk coefficient of 0.002; if the traces are farther from the nearest ground plane, then a larger spacing is required to ensure you meet this design goal.
Also, take a look at what happens when (S/H) = 0.5; the maximum crosstalk coefficient does not always occur when x = 0. Depending on your design goal, you could put traces closer together and see the same level of differential crosstalk as when the traces are farther apart.
You might be wondering: what about trace width? The trace width is important as it determines the single-ended and differential impedance, capacitance, and inductance. If you take the approach I described in an earlier article for single-ended traces, you’ll be minimizing the inductance and capacitance of an individual trace simultaneously. In an upcoming article, I’ll adapt the approach shown both here and in the previous article for optimizing differential impedance while constraining the differential crosstalk coefficient between two differential pairs.
You can define any trace spacing requirements you derive as design rules when you work with Altium Designer®. This will help you optimize your routing for low common mode and differential crosstalk throughout your board. The stackup manager also allows you to design your layer stack from a range of standard materials, helping to ensure signal integrity and power integrity.