This article explains the key differences between analog and digital circuits in a PCB layout. The goal is to help everyone understand how grounding and signal integrity can affect both types of circuits. To illustrate this, I designed two simple circuits using ORCAD: one is a basic transistor-based analog amplifier, and the other is a digital oscillator circuit. Let's dive right in. The first diagram shows both circuits with a single-point ground connection. This is just for clarity so that you can easily distinguish the two circuits. On the left, you'll see the analog amplifier, while the right side represents the digital oscillator. During simulation, the current in the digital part was very small, so I connected it to the first stage of the transistor to make it more visible. Here are the waveforms from the two circuits: [Image 1] As you can see, both waveforms look clean and undistorted. The red waveform is from the analog amplifier, and the green one comes from the oscillator. You might wonder why the red waveform doesn’t look as strong. That’s because the power supply in the simulation has some limitations — it’s not a real power source, so the signal is weaker than expected. Both circuits work fine with a single-point ground. There’s no interference or issues between them. Now let’s look at what happens when we don’t use a single-point ground. [Image 2] In this setup, I replaced the trace resistance on the PCB with two resistors to simulate the effects more clearly. Some of you might think, “Why are these resistors so large?” Well, I used them to make the effect more visible. In reality, PCB trace resistance is much smaller, but at high frequencies, the impedance of the wire can be significant compared to inductive effects. This setup simulates a digital circuit connected after the analog one. Now, let’s check the waveforms again. Are they the same? [Image 3] No, they’re not. The analog amplifier’s waveform is now distorted due to interference from the digital circuit. Here’s a closer look: [Image 4] This distortion is quite severe. Let me show you another waveform where the digital circuit is slightly farther from the power supply. The interference isn’t as bad here. [Image 5] Even so, the digital circuit is still affected. Let me zoom in again for better visibility. [Image 6] As you can see, the simulation results clearly show that the analog circuit is being disturbed by the digital signals. This highlights the importance of proper grounding and layout design when mixing analog and digital components on the same PCB. Understanding these differences helps engineers avoid common pitfalls and ensures better performance in real-world applications.

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