Current Sense Shunt Layout: Kelvin Connections and Noise

A current sense resistor converts current into a voltage that an amplifier can measure. The conversion depends on the resistance between the two points the amplifier actually connects to, and that is almost never the resistance of the resistor alone.

Kelvin, or Four Terminal, Sensing

A two terminal connection carries the measured current and the sense signal through the same copper. The voltage seen by the amplifier is then the sum of the voltage across the resistor and the voltage across the trace and solder joint on each side.

A Kelvin connection separates the current path from the sense path. The current enters and leaves through the main pads, and the sense connections are taken from separate traces that meet the resistor exactly at the inside edges of the pads.

The result is that the sense circuit measures the voltage across the resistive element only, and the trace and joint resistances drop out of the measurement. The improvement is large where the resistor value is small, because the parasitic terms do not scale with the resistor.

Where the Sense Traces Must Meet the Pad

The sense traces should join the pad at the inner edge of the resistor terminal, which is the boundary between the resistive element and the terminal. Joining anywhere else includes part of the terminal in the measurement.

On a two terminal shunt with wide terminals, the sense connection is usually taken from a point on the pad that lines up with the inside edge. Some packages provide dedicated sense pins, which removes the ambiguity entirely.

The two sense traces should run as a pair and should be routed symmetrically, because any asymmetry adds a voltage that depends on the current distribution in the plane rather than on the resistor.

Return Current and Plane Splits

The current that flows through the shunt returns through the ground structure, and the sense amplifier measures the difference between two points in that same structure. If the return current shares a path with the sense reference, the measured value includes a component of the current’s own drop.

The classical arrangement places the shunt in the path of the current and takes the sense reference from the same terminal of the shunt, so that the return path is excluded from the measurement.

Where the amplifier is referenced to a ground plane that carries the load current, the sense return should be routed as a separate trace back to the shunt rather than left to find its own way through the plane. Our power integrity notes describe how the return path is planned.

<img src="https://www.gopcba.com/wp-content/uploads/2025/08/16-1-1.png" alt="Kelvin sense traces connected to a shunt resistor” />

Noise, Bandwidth and Filtering

The signal from a shunt is small, often tens of millivolts, and it sits in an environment with switching nodes nearby. The differential pair carrying it should be routed close together and away from the switch node, with the loop area kept small.

A differential filter at the amplifier input reduces high frequency noise, and its components should be placed close to the amplifier rather than near the shunt. The filter’s resistor and capacitor values form part of the measurement bandwidth, so they should be chosen with the signal bandwidth in mind.

Where the current has a large alternating component, the shunt inductance matters and a four terminal shunt with a low inductance construction should be selected. Our switching regulator layout notes describe the related question of loop area in the power stage.

Thermal Drift and Self Heating

The resistance of a shunt changes with temperature, and the temperature coefficient is a specified parameter of the part. A shunt that self heats will drift, and the drift appears as a measurement error that correlates with load rather than with the measured quantity.

The power dissipated in the shunt is the square of the current times the resistance, so the drift is worst at high current. The layout can help by providing copper around the shunt to spread the heat, and by keeping it away from other heat sources.

Where the shunt is a four terminal part with a specified thermal EMF, using a low thermal EMF alloy matters as much as the copper layout. Our current capacity notes describe the copper areas required for high current paths.

Layout Checklist for a Shunt

Place the shunt where the current path is well defined and where the sense traces can be short. Keep the sense pair symmetric and routed as a differential pair.

Keep the high current path out from under the amplifier and away from the sense traces. The current path carries a large alternating component in a switching circuit, and it is the main noise source in the measurement.

Provide a Kelvin connection that meets the pad at the inner edge, and provide a separate return for the sense reference. Our component reliability notes describe the tolerance considerations for the sense resistor itself, which set the achievable accuracy.

Verification

The simplest verification is a comparison between the current reported by the measurement circuit and a reference instrument, made at several load points. A constant offset suggests a layout error, while a gain error suggests a resistor tolerance or an amplifier gain error.

Changing the layout of the sense traces and repeating the measurement isolates the layout contribution. A measurement that changes when the sense routing changes was dominated by layout rather than by the resistor.

Recording the measured value and the reference at each load point in the test record makes the calibration traceable and makes a later drift visible as a shift in the same table.

Process Control and Verification

On a design of this kind, current sense is the item that decides how the rest of the board is arranged. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

Process Control and Verification

On a design of this kind, current sense is the item that decides how the rest of the board is arranged. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

FAQ

Is Kelvin sensing necessary for every shunt? It matters most where the shunt resistance is small and the accuracy requirement is tight. For a large shunt in a tolerant application, a two terminal connection may be adequate.

How close should the sense traces run? Close enough that they see the same field, which means routing them as a pair with constant spacing and equal length.

What does gopcb provide for current sensing layouts? We provide Kelvin connection geometry in the layout, symmetric sense routing with filtering placed at the amplifier, copper areas sized for the dissipation, and assembly records for the shunt. Where the accuracy target is demanding, we review the sense path before the design is released.