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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??

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    \$\begingroup\$ Because I2C, SPI, etc. depend on the DC level in the signal. They were never designed to be AC coupled, and have no inherent "DC balance". \$\endgroup\$
    – Dave Tweed
    Jun 26, 2021 at 10:39

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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.

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  • \$\begingroup\$ So its like to shift the voltage inorder to match Voh vol parameters?? why it is only in encoding signals? \$\endgroup\$
    – Selva97
    Jun 26, 2021 at 10:51
  • \$\begingroup\$ I am sorry I don't understand your question, but the DC is deliberately removed as only the AC peak-to-peak voltage is important and each transmitter and receiver may require it's own common mode DC bias point to transmit or receive it. I2C and SPI do use NRZ encoding but it kind if means there is no encoding and preserving DC is important to transmit and receive them. \$\endgroup\$
    – Justme
    Jun 26, 2021 at 11:05
  • \$\begingroup\$ Hi sorry, im just new to it. when i go through the design file of SGMII connection, they used caps in between. I know that capacitor is for removing DC signals, but in high speed designs there is a possibility of only a high frequency noise signal. How can DC interfere in transmission line, and i gone through lot of documents but didnt get clear picture reg it. can anyone suggest good documents or answer for it? \$\endgroup\$
    – Selva97
    Jun 26, 2021 at 11:10
  • \$\begingroup\$ If you have a hypotetical situation where a 1.2V powered SGMII trasmitter is connected to 3.3V powered SGMII receiver and they are not AC coupled, it may not work if 3.3V input expexts 1.65V DC bias to receive it. If you connect a 3.3V powered transmitter to 1.2V receiver without AC coupling, the DC bias can damage the receiver. \$\endgroup\$
    – Justme
    Jun 26, 2021 at 11:17
  • \$\begingroup\$ ok great. i got it. is there any other uses? \$\endgroup\$
    – Selva97
    Jun 26, 2021 at 11:21

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