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Beyond Design: AC/DC is Not Just a Rock Band

Positioned at our usual table, directly in front of the stage at the local pub in Melbourne, Australia one Friday night in 1972, the boys and I laughed as a school boy, guitarist Angus Young, set up equipment and tuned a guitar. We assumed he was just one of the roadies, and were gobsmacked when the band unexpectedly fired up. High-voltage is not the word—more high-wattage, deafening—you could feel the sound as your ears distorted. The slick, gritty, blues-based lead riffs of the budding guitarist were insane. Little did we know that AC/DC's raucous image, with wild solo riffs, would make them one of the world’s top heavy-rock bands. We willingly endured this every Friday night for weeks on end. Fortunately, the venue was also a target-rich environment of eligible young ladies. In this month’s column, I will discuss AC coupling (or is it DC blocking?) of high-speed serial links as my taste in music has matured over the years.

Figure 1: AC/DC fires up at an early gig (source: Kat Benzova).

SERDES (serializer/deserializer) serial links are used to provide high-speed, high-bandwidth data transmission over differential signals and minimize the number of I/O pins and interconnects. And although it saves the PCB designer routing numerous parallel traces, implementing high-speed serial links can be challenging. Any small discontinuities in the physical geometries, along the transmission path, can significantly degrade the signal. This degradation includes loss of amplitude, reduction of rise time, and increased jitter. As a result, one must be able to identify these discontinuities, in the high-speed channel, and mitigate their impact to improve the performance of the signal transmission.

A capacitor is typically placed in series with both differential signal traces to remove common mode voltage differences between ICs or different technologies.  An “AC coupling capacitor” or “DC blocking capacitor” basically refers to the same thing. Any capacitor placed in series with the signal path tends to pass the high-frequency, AC portions of the signal, while simultaneously blocking the low-frequency DC portions. Since these capacitors couple transmitter to receiver, I prefer to use the term “AC coupling.”

Figure 2: AC and DC components of a signal transferred to the frequency domain.

In Figure 2 (top), the signal fluctuates about the DC offset. After performing a Fourier transform on a signal that consists of both AC and DC components, the DC component will be at 0Hz and the AC signal will be at its associated harmonic frequencies (bottom).

AC coupling is useful because the DC component of a signal acts as a voltage offset, and removing it can increase the resolution of the signal and allow different technologies to communicate without level shifters. Level shifter ICs can otherwise provide an interface between components that operate at different voltages. However, level shifters introduce delay variation (skew), increase power consumption, and are not suitable for low supply core voltages. AC coupling is needed to maintain the correct DC bias for receivers. If the transmitter has 0V DC bias and is of the same technology, then AC coupling does not have to implement.

The most important parameter, of the AC coupling capacitor, is the relative geometry with respect to the substrate. The capacitors are placed in series with high-speed traces and as such, the capacitor body becomes a section of transmission line.  The equivalent series inductance (ESL) of a capacitor, critical for bypass and decoupling applications, becomes negligible for AC coupling applications because the transmission line has inherent inductance anyway.  Instead, the thickness of the stackup outer dielectric, trace width, land size, solder thickness and cover-layer thickness of the capacitor all interact together in the area of the capacitor. 

In a well-matched interconnect, it does not matter where an AC coupling capacitor is placed.  What does matter is how well the capacitor transition is designed, how low the reflectivity is, and whether it is placed near other channel discontinuities. Far away from other discontinuities is best.

AC coupling removes the common mode level and allows the receiver to set its own bias point. This is especially useful for rack-to-rack systems where the common mode cannot be well controlled. It also has the advantages of allowing: 

• VTT referenced and GND referenced systems to work together
• A single SERDES channel to cover multiple standards
• Newer (restricted supply) devices to work with legacy devices
• The ability to hot-swap and protection from external shorts

However, AC coupling capacitors are common sources of impedance discontinuities in high-speed serial channels. Typically, narrow trace width and close trace spacing are used to construct the 100Ω differential transmission line pair. However, as these narrow trace pairs are routed into the surface mount lands of a capacitor, the sudden widening of the copper, as they join with the capacitor lands, causes an abrupt impedance discontinuity. The effect of this discontinuity appears as excess capacitance because the surface mount lands, of the capacitors, act as a parallel plate with the reference plane beneath.

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