Crosstalk in Terminators-EEWORLD

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In Figure 6.16, adjacent termination circuits cross-couple signal energy between circuit traces. This cross-coupling is more severe than the crosstalk that normally occurs between adjacent transmission lines .

This article will provide actual measurement results of cross-coupling and give some tips on predicting crosstalk in terminated circuits.

Crosstalk in the termination comes from both mutual inductive coupling and mutual capacitive coupling. Inductive coupling is usually larger, and the total coupling is the sum of the inductive and capacitive parts, both of which are proportional to the derivative of the input signal. Our goal is to find a total coupling coefficient, regardless of whether it is magnetic or electric field coupling.

Where: Noise voltage = Peak crosstalk coupled to trace 2
K = Cross-coupling coefficient
R = Impedance, Ω
△V = Drive signal step amplitude, V
T10~90% = Drive signal rise time, S

1. Crosstalk between adjacent solid core resistors

The inductive coupling of adjacent plug-ins of the termination resistor usually follows the above rule. We can use this approximate formula to easily estimate the crosstalk coefficient:

Where Y = the length of the resistor between the two jacks, IN
H = the height of the center line to the ground plane, IN
W = the spacing between the center lines of the resistors, and IN
K = the crosstalk coupling coefficient

Figure 6.17 plots the measured and calculated values of K. The measured value is the actual crosstalk measured from an actual example, and then the K value is calculated using formula (). The calculated value is obtained from the above formula, using the same length and height as in the actual example, but with different spacing.

If the resistors are arranged in a staggered pattern, as shown in Figure 6.18, the overlap length should be used in place of Y in the above equation.

2. Crosstalk between adjacent surface mount resistors

The surface mount resistor itself is relatively close to the circuit board, which can greatly reduce the crosstalk coefficient compared with the solid core resistor. In order to achieve the best effect, make the ground plane layer close to the outer surface of the circuit board and bury the ground layer directly under the surface mount components, which can reduce the parameter H in the above formula and reduce crosstalk.

3. Data of single in-line plug (SIP) termination resistor

These devices may or may not work well, depending on the internal routing. Figure 6.19 shows the common current path of a single ground pin termination resistor, which introduces a large amount of coupling inductance between the resistors in the package.

Table 6.2 lists the typical coupling coefficients of 0.1IN pitch SIP package resistor arrays. Package SIP-A contains 7 resistors, using an 8-pin package, with a common ground pin at one end. In package SIP-A, resistor 7 is the farthest from the ground pin. Package SIP-B contains 4 resistors, using an 8-pin package, each resistor has an independent ground, and all resistors are 50 ohms. The performance of the independent SIP-B package is almost 100 times better than the common ground network.