Comparing digital and analog signals with an oscilloscope
(Alt Text: Digital and analog signals on an oscilloscope)
Some of us long for a simpler time when everything could be done in the digital and analog domains. Nowadays, you cannot get away with running a purely digital or analog system in a complicated electronic product. Mixed signal PCBs are the norm, making old layout strategies obsolete.
We live in a nonlinear analog world, and things only really become digital or linear on paper. The differences between analog and digital integrated circuits begin with the signal waveform and it doesn’t end with the right layout choices. Using the right layout strategy can help ensure signals from analog and digital integrated circuits (ICs) remain clean and do not interfere with each other.
Linear vs. Nonlinear ICs
Analog ICs come in the linear and nonlinear variety. The output signal from an analog linear IC will scale in proportion to the input signal. In other words, if you graph the output signal versus the input signal, the curve relating them will be a straight line. This is not the case with nonlinear analog ICs. The curve for the signal output will not be a straight line. The output signal from these ICs typically exhibits saturation at high input voltages.
Two common examples of circuit elements that exhibit nonlinear outputs are transistors and diodes. The output from a diode is an exponential function of the input voltage. With a transistor, the output signal is initially linear at low input voltage but the output soon saturates to a constant output once the input voltage is large enough.
Nonlinear analog ICs can exhibit a nonlinear response in time while still exhibiting a linear response in the output signal. This means that nonlinear effects arise depending on the input frequency, not the input voltage. One common analog integrated circuit that acts nonlinearly on an input AC signal is an amplifier circuit. The slew rate in an amplifier circuit (i.e., the maximum output voltage change per unit time) will limit its response speed. If a high frequency sinusoidal waveform is input to an amplifier with a low slew rate, the output will resemble a triangle wave as the response rate saturates to the slew rate.
Most circuits with nonlinear elements are not so easy to examine analytically, meaning the current and voltage in the circuit must be calculated numerically. Whether you are using analog or digital ICs (or both), a great simulation package can help you model and verify the functionality of your device.
Noise and What it Means For Your PCB Layout
Unfortunately, digital ICs are noisy beasts compared to analog ICs. For a digital signal with a given switching frequency, the power spectrum will concentrate power at many more frequencies compared to an analog signal with the same frequency and amplitude. Add to this the numerous noise sources in digital circuits, and noise figures can be significantly higher in analog vs digital integrated circuits.
Depending on how circuit elements in your IC are mixed, the output signal from an analog IC can exhibit strong nonlinearity. Meanwhile, digital ICs are nonlinear by definition, and logic processing circuits are really piecewise nonlinear. Because noise is unavoidable in any signal, the way in which these circuit elements respond to noise depends on the relationship between the input and output signals (known as the transfer function).
Whether you are working with digital or analog ICs, you should always use best practices for suppressing radiated and conducted EMI, crosstalk, and signal reflections. You can’t completely remove white noise by filtration. For example, if you want to remove white noise from a sinusoidal signal with a bandpass filter, you will only filter out noise at frequencies far from the band center. Using tried-and-true layout practices can help you reduce noise levels so low that they become unmeasurable.
Analog, Digital, and Mixed Signal Layouts
Working with analog and digital ICs on the same PCBs requires separating your board into analog and digital functional blocks. The analog portion of your board should be placed over the analog ground plane while maintaining isolation between grounds across the PCB. The same applies to digital ICs: they should be placed over their own ground plane. Each type of component should only be routed over its own ground plane.
A single ground plane can be separated into digital and analog sections while still being connected. The best place to connect the two planes is near the power source. Noise induced in one plane will be blocked from entering the other plane by the high impedance return path between them. Noisy signals in one plane will be prevented from crossing into the analog plane and coupling noise back into analog signals, and vice versa. Instead, noise induced in either region will have to follow the path of least reactance back to the connected region and ultimately to the power source.
The separation distance between digital and analog ground plane sections in your layout is also important. Even though the two ground planes are flat, they still couple capacitively, so noise in one section can still couple into the other section. Generally, you should opt for a wider separation if space will allow as this will reduce the equivalent capacitance, thus increasing the equivalent impedance between ground planes.
Your analog and digital components can live in harmony
If you are working with mixed signal ICs, you have a problem because you will need to place them across the analog and digital ground sections simultaneously. Older ICs did not completely separate analog and digital pins on a mixed signal IC onto each side of a dual in-line package (DIP). Thankfully, newer ICs do separate pins in this way, allowing you to mount the IC across the two ground planes. The size of the package will limit the separation between your ground plane in this case.
The best PCB design software package makes it easy to work with analog, digital, or mixed signal boards. If your design package includes simulation capabilities, you’ll be able to evaluate potential noise problems in your design and address them before your board comes off the production line.
Cadence offers PCB design and analysis solutions beyond the average CAD tool. With a priority on electronics design innovation, Cadence’s analysis tools that are easily integrated into your layout environment are there to ensure board integrity throughout the design process.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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