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AM3871: Analog and Digital

Part Number: AM3871

Hi TI experts,

In some situations, it is required to separate Analog and Digital ground when using mixed signals. Based on what is written in the AM3871 specification, I see that this is not required.

1. I want to understand why this requirement does not apply to this processor? Explain, please.

I mean a similar scenario:

2. I noticed that the DDR_VREF pins, as well as some USB-bus pins, such as USB0_DP, USB0_DM and others, have an analog signal type (A). Why is it that way?

3. I cannot understand why a digital interface such as USB is marked in the documentation as an analog signal type (A)? Explain, please.

4. Does this mean that they require the presence of an analog ground plane under the tracks for return currents where these signals pass?

5. Is a solid reference plane needed for the power polygons on the inner layers? Does the gap in the plane on the adjacent inner layers if it intersects the polygon for example analog power on the adjacent inner layer?

  • Hi,

    1. The AM3871 specification covers only requirements on the processor side. It does not imply that separation cannot be used throughout the board.
    2. These are I/O buffer types.
    3. See above.
    4. No.
    5. It's always better to have a solid GND plane on an entire layer, however this is not always possible. Gaps in refernece planes concern mostly signal integrity on high-speed traces.
  • 1. Accepted.

    2;3;4 - Please look at the pictures in the app. Even if these are buffers of the I/O type, this does not negate their belonging to the analog signal type (A), to which they are assigned in the documentation. Therefore, I have questions related to their belonging to the analog part of the processor.

    a. Still, the digital or analog signal type is USB0_DP, USB0_DM, USB0_ID, USB0_CE, USB0_VBUSIN?

    b. If analog, as far as I know from various notes. The currents of the analog signals must pass in the region of the analog plane below them. Is compliance with this requirement necessary for these signals and why?

    c. Should I allocate analog ground under these USB signals as shown in Figure 3.?

    "Analog signals must be routed only in the board’s analog section, and digital signals must be routed only in the board’s digital section, with both on all layers. Under these conditions, the digital return currents do not flow in the analog section of the ground plane and remain under the digital signal trace.
    Figure 4 compares a split plane and a partitioned plane.
    The only problem with partitioning is that it is difficult when analog signals are improperly routed into the board’s digital section, or vice versa (Figure 5). So for any PCB layout, the important points are to use a single ground plane, partition it into analog and digital sections, and apply discipline in routing."

    5. Accepted.

  • a: You are confusing analog signals with analog type I/O cells. These I/O cells are constructed as analog type (opposed to digital type, e.g. CMOS), but they do not belong to a different signal class. They all have a common digital GND.
    b: There is no such requirement.
    c: No.
  • I have a few more questions:

    1. How to determine whether a signal or group of signals is analog class or analog cell (ultimately digital)?

    2. To which analog or digital ground should the signals having "_VSSA" postfix be connected? (Note: HDMI, USB0 / 1, and VDAC relative to their respective VSSA.)

    3. Why do I usually use analog power for high-frequency components DDR_VREF, USB, HDMI and others?

    4. Why do I usually use LDO, rather than DC-DC converters, to obtain analog voltage in PMIC? Does this mean that for parts of the processor with analog power, I have to use LDO instead of DC-DC?

    5. Do PLL_DDR and other "PLL_" signals belong to analogue class?

    6. Are signals of the type DEVOSC_MXI, DEV_CLKIN, DEVOSC_MXO related to the analog class?

  • 1, 5, 6: This device does not have any "analog class" I/O signals.
    2: See section 4.2.27 in the device datasheet.
    3, 4: These power supplies need to be clean, with no spikes and ripples.