Tying Digital and Analog Grounds with a Resistor

Upon examining the schematic of a motor driver board, I observed that the digital and analog grounds are connected via a 3.9 ohm 1/4 watt resistor. However, I’m curious to understand the rationale behind using a resistor instead of a 0-ohm resistor or a direct copper connection.

The analog ground is linked to a 24V supply off the board, while the digital ground connects to a 5V supply off the board. Each power connector is equipped with multiple decoupling capacitors. Notably, the resistor is not located near any of the connectors or their corresponding capacitors.

My initial assumption was that this setup might be aimed at noise suppression, although I’m skeptical about its effectiveness in achieving this goal.

It is probably aimed at noise reduction and I’ve seen this done several times in the past. It’s effectiveness is related to what is causing any problems and what steps have been taken. The ones I remember were on boards with optical transceivers which may have had unique problems. I’ve seen ferrite beads used this way also.

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It’s possible that a shorted resistor could function as effectively as a 3.9-ohm resistor, but this isn’t always the case. Several factors need consideration. Firstly, does the analog circuit have an external ground connection? If both the analog and digital sides make external ground connections, using a resistor to bridge the two planes is logical. This setup ensures digital currents in the digital plane are less likely to traverse the analog plane, reducing the risk of analog noise pickup due to digital currents.

If the 3.9-ohm resistor were shorted, there could be a possibility of some digital current crossing into the analog ground plane, potentially passing through a sensitive area before returning to the correct grounding point. With the 3.9-ohm resistor in place, the digital current would be less likely to follow this path, as it would prefer the “easier” route to the power grounding point. Additionally, the resistor’s inductance, due to its leads, might further aid in this.
It’s also worth considering that the 3.9-ohm resistor might actually be an inductor, which the original poster may have misidentified. This is a common mistake as some inductors can resemble resistors.

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