PCB Design
Should you split a ground plane? AGND, DGND and one bridge
A split ground plane divides a board's ground into a noisy area, for motors and logic, and a quiet one, for sensors and analog circuits, joined at exactly one point. Every current needs a way home, so the quiet side must join somewhere; join it twice and the noisy current takes the second road, through the quiet ground. Put the single bridge where signals cross the split, usually under the ADC.
Keep the noise home
A motor's 2 A returns through the noisy ground. The sensor's ground carries only its own milliamp, so it stays still and its readings stay true. Any lift on a ground is just Ohm's law on shared copper: whatever share of the motor's current crosses the quiet side's resistance raises the sensor's ground by I × R.
One bridge, not two
Bridge the two grounds at the supply only, and the motor's current has only its own way home: the sensor's ground lifts by tens of microvolts. Add a second bridge by the motor and you've made a loop, and about half the motor's current now returns through the quiet ground:
Keep only the bridge by the motor and it's worse: the sensor's ground now hangs off the noisy one, lifted about 98 mV. And with no bridge at all, the sensor has no way home and stops working.
Live experiment
One bridge, not two
A motor returns 2 A through the noisy ground and a sensor returns its milliamp through the quiet one. Open and close the two bridges: both closed lifts the sensor's ground about 50 mV, the supply bridge alone a few hundredths of a millivolt, the motor bridge alone 98 mV, and neither stops the sensor.
“One bridge” is lesson 6 of 6 in Gerber Gulf, PCB Design World 3. Free with an account. Lessons unlock in order, so you’ll do the 19 before it first.
Bridge where the signals cross
At DC, current shares its paths by resistance, and a bridge near the supply works like a star point. But a fast signal's return current follows its own trace, as close beneath it as it can. Data from an ADC crosses the split at the ADC, so the bridge belongs right under it. Put the only bridge at a far corner and the return current of a 20 MHz data line detours round the quiet plane, shaking it on the way.
Split, or one solid plane?
Many designers now skip the split: one solid plane, with noisy parts on one side, quiet parts on the other, and traces routed so that return currents stay apart by themselves. A split helps when that isn't possible, and hurts when any trace crosses the gap away from the bridge. ADC datasheets often say to tie AGND and DGND together at the chip; follow that, whichever way you lay out the plane.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“One bridge” is lesson 6 of 6 in Gerber Gulf, PCB Design World 3. Free with an account. Lessons unlock in order, so you’ll do the 19 before it first.
Or start from the very first lesson, no account neededPractice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: Under the ADC, where analog and digital meet. Signals cross the split at the ADC, and their return currents need to cross right under them.
- Why not “Anywhere, as long as there's one”? Where it goes matters: the bridge should sit where signals and their return currents cross.
- Why not “At the far corner, out of the way”? At a far corner, return currents crossing the split detour round the plane to reach it.
- Why not “Under the motor”? Under the motor, the noisy current would cross into the quiet side at its worst point.
Show the answer
Answer: Floats: its current has no way home, so it can't work. Every current needs a loop. Cut the quiet ground off completely and nothing on it flows.
- Why not “Gets quieter still”? With no way home the sensor draws no current at all, so it doesn't work, quiet or not.
- Why not “Draws more current”? With no way home it can't draw any current, let alone more.
- Why not “Works the same”? Every current needs a loop. Cut the quiet ground off and the sensor's loop is open.
Show the answer
Answer: Makes a loop that pulls the noisy current through the quiet ground. Two joins give the noisy current a second road home, straight through the quiet area.
- Why not “Blocks the motor's current”? The motor's current still flows, now split between two roads home.
- Why not “Changes nothing”? It changes a lot: about half the motor's current now returns through the quiet ground.
- Why not “Halves the noise”? A second bridge adds noise to the quiet side: it shares the motor's current with the quiet ground.
“One bridge” is lesson 6 of 6 in Gerber Gulf, PCB Design World 3. Free with an account. Lessons unlock in order, so you’ll do the 19 before it first.
Questions people ask
Should I split analog and digital ground?
Often not. A single solid plane with careful placement, analog parts in one area and digital in another, usually works as well or better. Split only when big noisy currents can't otherwise be kept away from sensitive circuits, and then join the halves at one point under the converter.
Where should AGND and DGND be connected?
At one point, normally under or right beside the ADC or DAC that straddles them, which is where signals cross and where its datasheet usually says to join them. Never at several points, which makes a ground loop.
What happens if a trace crosses a split in the ground plane?
Its return current can't flow directly beneath it and has to detour to the nearest bridge, making a large loop. The loop radiates, picks up noise and adds inductance, so ringing signals and failed emissions tests are common results.
Related topics
- What is a ground plane on a PCB?A whole layer of copper for ground: milliohms for steady current, about a nanohenry for fast current.
- Star grounding explainedGive each load its own return to one point, so a big current never flows through a sensitive circuit's ground.
- Kirchhoff's laws explained: KCL and KVLWhat flows into a junction flows out, and the drops round a loop add up to the rise.
- What is a decoupling capacitor?Why every chip has a capacitor beside its power pins, how big, and why it must be close.