Why is it necessary to distinguish between analog ground and digital ground in circuit design?

The sensor unit circuits we typically design include both digital signal processing circuits (such as an MCU) and analog circuits (including the front-end sensor and its signal amplification). Simply put, digital ground is the common reference terminal for the digital circuitry, i.e., the reference terminal for digital voltage signals; analog ground is the common reference terminal for the analog circuitry, the voltage reference terminal (zero potential point) for analog signals.

Since digital signals are generally rectangular waves with a large number of harmonics, if the digital ground and analog ground on the circuit board are not separated at the connection point, the harmonics in the digital signal can easily interfere with the waveform of the analog signal. When the analog signal is a high-frequency or high-voltage signal, it will also affect the normal operation of the digital circuit. Analog circuits deal with weak signals, but digital circuits have higher threshold levels, so their power supply requirements are lower than those of analog circuits. In systems with both digital and analog circuits, noise generated by the digital circuits can affect the analog circuits, degrading their small-signal performance. To ensure signal integrity and avoid mutual interference, the analog ground and digital ground must be separated.

In schematic design, the ground plane of the digital area is labeled DGND, and the ground plane of the analog area is labeled AGND. Then, in PCB design, the ground plane is divided into digital ground and analog ground, with a large distance between them. The digital ground and analog ground should be grounded at a single point, either directly or through component isolation.

1. Direct Connection. As shown in Figure 1, the two are connected at a single point through a wide copper foil. This method is suitable for low-frequency systems or systems that are not sensitive to noise.

2. Component Isolation Connection. As shown in Figure 2, this connection method uses a ferrite bead or a 0-ohm resistor to connect the components. This is a primary connection method. The equivalent circuit of a ferrite bead is similar to a band-stop notch filter, suppressing noise only at a specific frequency. If the frequency range of the noise is known, a ferrite bead is the best choice. A 0-ohm resistor acts as a very narrow current path, effectively limiting the loop current and suppressing noise. Resistors have attenuation effects across all frequency bands (even a 0-ohm resistor has impedance), making a 0-ohm resistor the best choice when the noise frequency range is uncertain.

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