Analog blocks are quite sensitive to noise and below
precautions need to be taken
- Sufficient isolation is to be provided for the Analog layout from the digital logic. This includes both the IO and core logic.
- Normally, the IO power for the Analog block needs to be isolated. Even if the power source is the same, it is important to isolate the power connections to the analog block IOs from the other IOs.
- They need to be isolated at the package source. For very critical blocks, it is needed to isolate even the source and the ground completely. Providing an independent power source may not always be possible and need to be looked at from the architecture side.
- Providing an isolated ground can be even more difficult to implement at a system level. It is better to have a separate mesh/connection from the source of power/ground itself
- The power for the Analog IO interface and a block needs to be separately estimated and accordingly the number of power and ground IOs/bump pads need to be estimated. Since the Analog IOs are sensitive to noise, it is recommended to have a power/ground pad alternatively after each signal IO
- The ESD rail is generally common across all the IOs. However, in rare cases, there may be a need to have a separate ESD rail for the analog IO interface (for very critical and sensitive IOs).
- Even the analog block may have digital logic inside and only part of it may be true analog logic and custom layout. It helps to have this information so we can apply the isolation accordingly.
- There may be core voltage being used in the analog block but it may be for digital logic or analog logic within the block. If it is for analog logic, the supply to it needs to be isolated from the core mesh (used by the core digital logic).
- One power source may be placed close to the analog block and from there, the power routed to the power pins of the block and not shared with other logic. This power routing needs to be of sufficient width (based on the power requirement and the distance from the source) and with 3-4 additional spacing.
- The analog logic area of the block needs to be isolated from the digital logic placement area. The isolation area can be filled with decaps. For an extended width from the isolation, care to be taken to not use high drive cells
- All the power pins of the block are to be connected with the appropriate width and at least the width of the power pin
