Do your layout with digital and analog in mind

You can implement an ideal layout with your embedded-system design by using four or more layers. You use these layers to properly implement the separation of digital and analog planes as well as to keep the traces with switching signals away from high-impedance analog traces. But this world is far from perfect, and management is probably asking you for a cheaper approach, such as a two-layer board or even the simpler one-layer board for your complex mixed-signal circuits. So, what are the key concerns with this type of board design? They are the thickness of your traces, the use of bypass (or decoupling) capacitors, and the relative position of your chips and traces between the digital and analog systems.

If possible, your power and ground traces should be double or, better yet, triple the width of the standard traces you might be inclined to lay out. But remember that the thinnest width in the trace dictates the thickness of the entire trace. Keep the trace resistance and inductance to a minimum. Figure 1 illustrates this strategy, in which a thermal sensor, a 12-bit ADC, and a microcontroller implement a board layout.

With analog and digital pc-board design, you should keep the noisy and quiet portions of the circuit separate where possible. You achieve this separation by keeping the digital-signal lines and return paths in the ground connector as far away from the analog circuitry as possible. In the single-layer environment, the trick is to keep the analog circuitry at the farthest side of the board with respect to the power connector. This step creates an environment in which the digital circuitry injects a minimal amount of interference into the analog circuitry. In contrast, the digital circuitry can tolerate a great deal of noise before problems start to appear.

Now, you need to pay attention to the placement of bypass capacitors to make your mixed-signal design robust. Both the analog and the digital side of the board require two classes of power-supply bypass capacitors. All devices require a capacitor as close to the package power-supply pins. A common value for this capacitor is 0.1 µF for industrial speeds and 0.01 µF for speeds greater than 50 MHz. You use this class of capacitor next to the analog chip's power pins to redirect high-frequency signals on the power traces to ground. For the digital processors and controllers, you use this capacitor as a mini charge reservoir, facilitating fast switching requirements.

The power-supply source requires a second class of capacitors in the system. The value of this capacitor is usually approximately 2.2 to 10 µF. You should position it near the power-supply source. This larger capacitor also supplies excess low-impedance charge, thereby minimizing the effects of resistance and inductance in the power supplies. In all cases, capacitors should have short leads to reduce parasitic-inductance errors.

When the digital and analog domains are on the same layer, careful layout is critical if you intend to have a successful pc-board implementation. Using wider traces and bypass capacitors is the first step in minimizing extraneous noise in the analog circuits and false triggering on the digital side. The second step is to carefully plan your current-return paths on your ground traces, keeping the analog circuitry as clean as possible.

Author Information

Bonnie Baker is the analog/mixed-signal-applications engineering manager for Microchip Technology's microperipherals division. You can reach her at Bonnie.Baker@microchip.com.