Like a game of telephone, information contained in an analog signal can be easily distorted on the way to its destination. If not designed properly, noise and distortion can accumulate throughout your signal change, making it difficult for your message to stand out above the noise.
With the high frequencies used in wireless devices and other devices that interface with analog equipment, including embedded systems, there is plenty of opportunity for analog and digital signals on your board to interfere with each other. This can corrupt digital data, create distortions in analog signals, and lead to other signal problems that may not be so easy to diagnose. The right layout and routing help ensure your signals remain clean throughout your device.
Confronting Mixed Signal Crosstalk
Compared to digital circuitry that is run at saturation, analog ICs and signals are much more sensitive to noise. When an analog trace is the crosstalk aggressor against a digital trace, the analog signal will induce a signal that superimposes on a digital signal, but the fluctuation in the digital signal is usually not enough to cause involuntary switching at the input of a logic circuit. One exception to this occurs with switching power supplies, which can cause involuntary switching in nearby digital circuits.
In effect, a digital circuit that is run at saturation filters out low level noise at the input. The converse is not true. When a switching digital IC produces a digital pulse, it can induce a strong burst of current signal in a nearby analog trace. Analog devices are generally run in the linear regime without offset, so the induced noise easily propagates into the output from the analog IC.
High frequency boards require some additional consideration, particularly in wireless devices. For example, an embedded system designed to gather data from external sensors, process converted digital data, and then send this data via a wireless protocol to another device will require some segmentation among different portions of the board.
With the right routing and layout choices, you can segregate different portions of your board and help prevent crosstalk in mixed signal devices. You can also ensure that analog signals at different frequencies and signal strengths do not interfere with each other.
It Starts With a Ground Floor Plan and Stackup
If you are working with a purely analog or mixed signal device, there are a number of important points to consider in your stackup and grounding floor plan. Your device will require some power source, and noise in the output signal from your power source can induce some signal integrity problems. The right stackup will help prevent power integrity problems from turning into signal integrity problems, and the right routing choices will prevent different types of signals from interfering with each other.
First, you should separate your ground planes into digital and analog sections, but leave these two sections connected near the return to the power supply in order to provide a low reactance path back to ground. If you are adding RF signals into the mix, you’ll want to create a third analog ground section for the RF components. You should do the same with the power planes. Do not overlap ground or power planes that carry different types of signals as this will cause radiation at wireless frequencies.
In the RF section, you’ll need to suppress radiation from your RF power plane so that it does not interact with analog signals in other areas of the board. This also prevents radiation from causing problems in other boards as part of a multi-board system. Placing a grounded via fence around the RF power plane provides a good level of suppression as it creates two out-of-phase radiators. If your layer stack will allow, you should place the RF power plane between two ground planes as this will decouple the RF power and ground planes in your board.
Routing Your Signals
Now that your analog, digital, and RF analog sections are separated into different sections, you can route signals in each section using best practices. As high frequency signals can produce transmission line effects in shorter traces, it is best to keep traces as short as possible. You should also use impedance control in your board to ensure traces have consistent impedance throughout the board.
While routing, try to arrange your traces such that the use of vias is brought to a minimum. Each via adds impedance to a trace, and designing vias to have a specific impedance that matches a trace is quite difficult. Any vias should be backdrilled to prevent signal resonance, and extreme care should be taken to ensure that the backdrilling on differential pairs is symmetric.
With vias on RF signal traces, one option to prevent impedance changes is to use two vias in parallel. This provides two benefits. First, it reduces the additional impedance on the trace, which leads to the second benefit. The reduced overall impedance of two vias in parallel increases the lowest resonance frequency of the via pair. Ideally, you should try to increase the lowest resonance frequency so that it is larger than the frequency of the signal on the trace to avoid resonance and re-radiation.
There is plenty more to consider in high frequency circuits, especially in embedded systems with wireless capabilities. However, we would be remiss if we did not say something about components. It’s best to plan for surface-mount components in high speed and high frequency sections of the board. If through-hole components are used, the leftover stub on the pin acts just like the remaining stub on a via. This creates another source of signal reflection that can degrade signal quality. These small stubs also act as resonators that contribute to EMI.
High frequency wireless and embedded systems are complicated beasts that require a mix of design rules, and missing some of these rules can create a number of signal problems in your device. Altium Designer contains all of the tools you need to design and verify the functionality of your next embedded system in a unified environment.
If you’re interested in learning more about Altium Designer, you can contact us or download a free trial and get access to the industry’s best layout, routing, and simulation tools. Talk to an Altium expert today to learn more.
About the AuthorMore Content by Zachariah Peterson