AGND, DGND and Chassis Ground: The Three Grounding Questions Everyone Asks

Grounding advice has a reputation for contradicting itself. Split the plane; don't split the plane. Ground the chassis; don't make loops. Part of the problem is that three different questions get asked as if they were one. They have different answers, and knowing which one you're facing is most of the job.

1. The chip has AGND and DGND pins. Do I connect them?

ADCs, DACs and some sensors have separate AGND and DGND pins, and the names make it sound like an instruction to keep two grounds apart. It isn't. The pins are separate inside the package so the analog and digital currents don't share the chip's own tiny bond wires.

For most converters, Analog Devices' guidance (their tutorial MT-031 is the classic reference) is to connect both pins to the same low-impedance ground plane, right at the chip, with the shortest connections you can manage. The digital currents these chips draw are small, so tying DGND to the analog ground does far less harm than separating them, which would put the chip's two grounds at different voltages.

Always check the converter's own datasheet and evaluation board layout first. That's the exception that matters.

2. Should I split the ground plane into analog and digital?

Usually not, and this is the one people find hardest to believe. A return current follows the path directly under its own trace. Cut a slot in the plane and the current has to detour around the gap, making a loop that radiates and picks up exactly the noise you were trying to avoid.

A real mixed-signal layout. Top: analog left, digital right, the converter on the boundary, and one chassis point (a plated hole with 1 nF and 1 MΩ to ground) by the USB connector. Bottom: one ground plane, unbroken under the analog side.

What you separate is the parts, not the plane. Put the analog circuitry on one side, the digital and switching circuitry on the other, with the converter on the boundary. Route analog traces only over the analog area and digital traces only over the digital area. The plane stays whole, and the noisy return currents stay on their own side because that's where their traces are.

The partition only works if traces respect it. A single clock line routed through the analog area brings its return current with it. If a signal has to cross, cross at the converter.

3. Should circuit ground connect to the metal chassis?

Usually at one point, near the connectors where cables enter. The chassis is a shield and the place static discharge and cable noise end up, and the goal is to send that energy to the chassis where it comes in, without it travelling through your circuit first.

• One connection, close to the I/O connectors.
• Cable shields bonded to the chassis at the entry, ideally all the way round, not via a long pigtail wire.
• If you need DC isolation between circuit and chassis, a capacitor at that point, often with a high-value resistor in parallel to bleed off static charge.
• Not several scattered connections, which push current through the enclosure between them.

For mains-powered equipment in a metal case there's a safety requirement on top: the chassis is bonded to protective earth as your product's safety standard specifies. That's not a layout choice, and it's never the thing to remove to fix a hum.

Bonus: the isolated supply

An isolated DC/DC converter exists so that its two sides don't share a ground. Connecting them on the board throws away the reason you bought it. Draw the isolation barrier as a clear line, with no copper, pour or trace crossing it, and keep the spacing the converter and your safety requirements ask for. IPC-2221's conductor spacing table is a floor; safety standards often want more.

A short layout check

1. Continuous ground plane under analog and digital areas, no slots under fast traces.
2. Converter AGND and DGND to the plane right at the chip.
3. Analog traces over the analog area only, digital over digital.
4. Decoupling at every power pin with a short path to the plane.
5. Circuit ground meets the chassis once, near the connectors.
6. Nothing crosses an isolation barrier except the isolator.

The full guide, with a diagram of the partitioned board and the chassis connection, is on pcbeditor.com.

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Which grounding rule have you had to break, and why? The exceptions are the interesting part.