Ground Split: Splitting Analogue and Digital Ground on a PCB

Where a design carries more than one ground, the question of how the two are joined becomes the most consequential decision on the board. Joining them in the wrong place produces a noise problem that no amount of filtering will fix.

Why the Split Exists

A digital return carries a current made of short pulses, and a sensitive analogue return carries a small continuous current. If the two share a conductor, the digital current develops a voltage across the shared impedance, and that voltage appears in the analogue measurement.

The split is therefore a way of keeping one current out of the other’s path. It is not a way of creating two separate grounds, because the two must be connected at some point for the system to work. Our mixed signal notes describe the arrangement.

Where the Two Grounds Meet

The single connection between the grounds should be placed where the current that flows through it produces the least disturbance. In a system where a converter supplies both sections, that point is at the converter’s own ground pin, because the heavy currents return there by design.

Joining the grounds at the analogue input instead forces the analogue reference to carry the digital return current, which is exactly the situation the split was intended to avoid.

The Split as a Slot in the Plane

On a board with a ground plane, the split is a slot, and a slot is only acceptable if nothing crosses it. A trace that crosses the slot has no return path directly beneath it, so the return current detours around the end of the slot and forms a loop.

The loop radiates and receives. It is the single most common cause of both an emission failure and an immunity failure on a mixed signal board, and it is invisible unless the routing is overlaid on the plane. Our power integrity notes describe the plane design.

Bridging and Stitching

Where a trace must cross the split, a bridge is provided: a narrow connection between the two halves of the plane directly beneath the crossing, or a capacitor that gives the return current a path at the frequency of interest.

A capacitor bridge works over a limited band, because its impedance falls with frequency. Above the point where the impedance is low enough, the bridge is effective; below it, the return current still detours. The capacitor value should be chosen for the lowest frequency that matters.

Star Points and Their Limits

A star ground connects several returns at a single point, which is the ideal arrangement for a low frequency system. The problem is that a single point is a physical location, and every conductor that reaches it has an inductance.

Above a few megahertz the inductance dominates and the star point no longer behaves as a single point. That is why high frequency designs use a plane rather than a star, and why the choice depends on the frequency content rather than on the topology.

Grounding for Connectors and Shields

A cable shield has to be connected to the chassis rather than to the signal ground, because the shield carries a common mode current that should be diverted before it reaches the circuitry. Connecting a shield to the signal ground injects the current into the very node that must remain quiet.

The connection should be made with a low inductance path, which means a wide contact or many vias rather than a single wire. The inductance of the connection determines how much of the common mode current actually flows through it.

Measuring Whether the Split Is Working

The test is a measurement of the noise on the analogue reference while the digital section is active. A split that is doing its job shows a small correlated signal; one that is not shows the digital switching pattern on the analogue node.

The measurement requires a probe with a short ground lead, because a long lead picks up the field around the board and reports it as noise. The probe technique is part of the measurement rather than an optional refinement.

Documenting the Intent

The location of the single connection between grounds should be marked on the layout and recorded in the design notes, together with the reason for the split. Without that record, a later modification will often bridge the grounds somewhere convenient.

The record should also state which nets are allowed to cross the split and where the bridges are. That list is what allows a change to be evaluated. Our design review notes describe where the item is checked.

Process Control and Verification

The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record.

Verification and Records

The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released. Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one.

Points to Confirm at First Article

A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

Documented Process Control

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting. Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.

FAQ

Should analogue and digital grounds be separate planes? They should be separate areas of the same plane, joined at one point, rather than two planes that are only connected through the power supply. Two planes joined elsewhere create a loop that is worse than a single plane.

How wide should the bridge be? Wide enough that its impedance is small compared with the impedance of the path it replaces, which usually means a connection several times wider than a normal trace. The capacitor bridge used at a crossing is a different solution to the same problem.

What does gopcb check on a mixed signal design? We check for traces crossing the ground split, the location and width of the bridge, the connection point between the two ground areas, the shielding of any connector that carries signals between them, and the continuity of the plane beneath the fast digital nets.