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42 DESIGN007 MAGAZINE I MARCH 2018 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 sig- nal transmission. A capacitor is typically placed in series with both differential signal traces to remove com- mon mode voltage differences between ICs or different technologies. An "AC coupling capacitor" or "DC blocking capacitor" basi- cally 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." In Figure 2 (top), the signal fluctuates about the DC offset. After performing a Fourier trans- form 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 com- ponent of a signal acts as a voltage offset, and removing it can increase the resolution of the signal and allow different technologies to com- municate without level shifters. Level shifter ICs can otherwise provide an interface between components that operate at different voltages. However, level shifters introduce delay varia- tion (skew), increase power consumption, and are not suit- able for low supply core volt- ages. 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 param- eter, of the AC coupling capaci- tor, 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 transmis- sion 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, sol- der 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 Figure 2: AC and DC components of a signal transferred to the frequency domain.

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