AN 958: Board Design Guidelines

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ID 683073
Date 1/28/2022
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5.7. AC Versus DC Coupling

AC coupling refers to the use of a series capacitor on a signal to block the DC signals from going through. DC coupling refers to the case where this capacitor is not present and the signal passes through without any interruption. In AC coupling, a DC restore circuit is generally required after the capacitor to ensure that the common mode voltage requirements of the receiver are met. In Intel devices with transceivers, the DC restore circuitry is built into the device. In that case, external DC restore circuitry is not necessary. DC coupling works only in cases where the output common mode voltage of the transmitter is in the required range of the input common mode voltage of the receiver.

The advantage of AC coupling is that it allows chips with different common mode voltages to interface with each other. The disadvantage is that it requires an extra capacitor, which can add some jitter or other degradation if not properly selected.

If you are certain that the common mode voltage requirements of the receiver are a subset of the common mode voltage output of the transmitter, use DC coupling. If you are in a borderline case, or if the requirements are not satisfied, then use AC coupling.

In choosing the value of the coupling capacitor, consider what happens if the capacitor is too big or too small. If the capacitor is too big, it can significantly slow the signal down and can also respond poorly to fast changing input signals because of the long charge and discharge times. If the capacitor is too small, it presents a fair amount of impedance and can increase attenuation and change the characteristic impedance of the path. A good balance between these two conflicting requirements is a 0.01 μF capacitor, which Intel uses for its 3.125 Gbps transceiver designs.

When selecting components, use the smallest size possible, because smaller size components have smaller size pads, which reduce the discontinuity. Intel has used 0402 components (40 mils x 20 mils) in its designs.

You can design the DC restore circuitry in a variety of ways. Intel typically uses a simple resistive voltage divider (refer to Figure 89). Be sure to use precision resistors (0.1% or 1%) for differential signals, so that the restored DC levels on the positive and negative signals are very closely matched. In Figure 89, the DC restore circuitry restores the DC level to 3.3 * 78.7/(140 + 78.7) = 1.1875 volts.

Figure 89.  AC Coupling

Transceiver devices have DC biasing on the high-speed transceivers inputs and reference clock inputs designed for the 1.5-V PCML standard, so AC coupling is not required. This saves components and board space. If you are using other I/O standards such as LVPECL or LVDS, then you need to AC-couple them, because their common mode voltage is different from the 1.5-V PCML common mode voltage. External biasing networks are not needed, because the common mode is generated internally in the device.

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