In high-speed analog signal chain design, printed circuit board (PCB) layout needs to consider many options, some more important than others, some depending on the application. The final answer varies, but in all cases, the design engineer should try to eliminate the error of best practice and not overestimate every detail of the layout. This article recommended for you today will start from the bare pad and talk about decoupling and layer capacitance, layer coupling, and separated grounding in turn.

Bare pad
  The bare pad (EPAD) is sometimes overlooked, but it is very important to maximize the performance of the signal chain and to fully dissipate the device.
    The bare pad, which ADI calls pin 0, is the pad underneath most devices today. It is an important connection through which all the internal grounding of the chip is connected to a central point below the device. I don't know if you have noticed that many converters and amplifiers lack ground pins because of the bare pads.
    The key is to properly secure (i.e., solder) this pin to the PCB for a strong electrical, gas and thermal connection. If this connection is not strong, confusion can occur; in other words, the design may not work.

Achieve optimal connectivity
  There are three steps to achieving optimal electrical and thermal connections with bare pads
     1, where possible, the bare pad should be replicated on each PCB layer, the purpose of doing so is to form a dense thermal connection with all ground and ground strata, from the rapid heat dissipation. This step is relevant for high-power devices and applications with a high number of channels. In electrical terms, this will provide good equipotential connections for all ground layers. The bare pad can even be replicated in the bottom layer, which can be used as a place to decouple the heat dissipation ground point and mount the bottom side radiator.
     2, the bare pad is divided into multiple identical parts, like a chessboard. Use a wire mesh cross grate on an open bare pad, or use a solder stop layer. This step ensures a solid connection between the device and the PCB. During reflow assembly, it is not possible to determine how the solder paste will flow and ultimately connect the device to the PCB. Connections may exist, but they are unevenly distributed. You may only get one connection, and the connection is small, or worse, located around the corner. Dividing the bare pad into smaller sections ensures that each area has a connection point, resulting in a more secure, evenly connected bare pad.
    3, should ensure that all parts have holes connected to the ground. Each area is usually large enough to hold multiple holes. Before assembly, it is important to fill each pass hole with solder paste or epoxy to ensure that the bare pad paste does not backflow into these pass holes and affect the proper connection.

Decoupling and layer capacitance
    Sometimes engineers ignore the purpose of using decoupling and simply scatter many capacitors of different sizes on the board, allowing lower impedance power supplies to be connected to the ground. But the question remains: How much capacitance? Much of the literature suggests that many capacitors of different sizes must be used to reduce the impedance of a power transmission system (PDS), but this is not entirely true. In contrast, the PDS impedance can be reduced simply by choosing the right size and the right kind of capacitance.
    Layer coupling
    Some layouts inevitably have overlapping circuit layers. In some cases, this may be a sensitive analog layer (such as power, ground, or signal), with a high-noise digital layer below it.
    This is often overlooked because the high noise layer is in another layer - below the sensitive modeling layer. However, a simple experiment shows that this is not the case. Take a layer as an example, inject signals at either layer. Another layer is then connected and the adjacent layer is cross-coupled to the spectrum analyzer.

Separated ground
  The most common question posed by analog signal chain designers is: Should the ADCs be separated into AGND and DGND? The short answer is: it depends. The detailed answer is: Usually not. Why not? Because in most cases, blind separation of ground strata only increases the inductance of the return path, it brings more harm than good.