Why we are connecting capacitor in series in high speed designs like SGMII. If it is to remove DC voltages, why we are not using on other signals like I2C, SPI and so on. What is the major purpose. Can anyone explain in easy way??
The data is differential and encoded to have no DC component. To allow connecting together chips that have different supply voltages at the SGMII interface, or simply to allow them to use their own DC biasing for the transmitters and receivers, the capacitors are there to block DC and can pass the high frequency AC data signals just fine.
Interfaces such as I2C, SPI and so on are not encoded and need to preserve the DC bias component, and usually the devices all need to use same supply voltages too. SGMII uses high speed differential serdes blocks with CML IO interfaces, I2C and SPI use just CMOS IO interfaces.