TECH & NEWS

Kelvin Sensing: Preserve the Measurement Path

A current-sense resistor may occupy only a small corner of a robot controller or industrial drive. Yet the voltage measured across it can influence protection, current regulation and system diagnostics. Specifying its resistance in the BOM is only the beginning. The PCB pickup points, component terminals and manufacturing handoff must preserve the intended measurement path.

Consider an illustrative DC example, not a PCBDOG test result. A 20 A current through a 1 mΩ shunt produces 20 mV. If the measurement accidentally includes another 0.1 mΩ of series resistance, it includes another 2 mV: a 10% increase relative to the intended signal. This simplified calculation excludes temperature effects, amplifier error and dynamic behavior. Its purpose is to show why apparently small unwanted resistance deserves attention.

What Kelvin sensing actually separates

Kelvin sensing uses a measurement connection separate from the main load-current connection. The sense input draws much less current than the load, allowing the circuit to avoid measuring some voltage drops along the power path. However, a true four-terminal shunt and a two-terminal resistor with a carefully designed four-pad PCB footprint are different arrangements. The selected component’s documentation must guide the implementation.

Analog Devices’ published investigation shows that solder-joint resistance and pad layout can affect low-value shunt measurements. It also describes an assembly consideration: an asymmetric pad arrangement can pull a component during reflow. The lesson is practical: electrical accuracy and repeatable placement should be reviewed together, rather than treating the footprint as a purely mechanical detail. [1]

There is no universal pickup-point shortcut

Do not copy a generic Kelvin sketch and assume its pickup geometry suits every shunt. Vishay’s component-specific discussion explains how terminal construction and current entry affect the resistance included in the measurement. The manufacturer’s recommended connection is therefore part of the design definition. [2]

Texas Instruments also recommends Kelvin connections and short, balanced sense routing in its current-sensing layout discussion. Its treatment of ground paths highlights another limitation: transient-related ground movement can disturb measurements. Separating sense connections does not automatically solve every noise, reference or amplifier problem. [3]

Turn the design into an unambiguous build package

The following are engineering handoff recommendations, not claims that one layout or inspection method guarantees a particular accuracy.

First, identify the exact shunt manufacturer part number, terminal mapping and approved footprint revision. A resistance value and package size alone do not establish equivalence. If sourcing proposes an alternative, have the responsible engineer review the terminal arrangement, electrical characteristics and land pattern before approval.

Second, make the load-current path and sense pickup locations explicit in the design review. Keep the fabrication data, assembly drawing and component placement information consistent with the approved revision. If an assembly review suggests changing pads, solder-mask openings or paste apertures, route the change back for engineering approval. A seemingly convenient footprint change may affect what the circuit measures.

Third, distinguish assembly inspection from functional verification. A board may pass a continuity check while still measuring current incorrectly. Define the relevant test current, polarity, measurement points, reference instrument and acceptance limits. Where the application requires it, include agreed temperature or operating-load conditions. These decisions belong in the project’s test plan before production testing is configured.

Fourth, make results traceable to the actual build. Record the approved BOM and board revision, any accepted component substitution and the test procedure used. That gives engineering and procurement a common basis for investigating a deviation without assuming that every symptom originates in assembly.

Where PCBDOG fits

PCBDOG supports PCB fabrication, PCBA, SMT and DIP assembly, component sourcing, testing, programming, assembly and Box Build. For a current-sensing board, a useful starting point is the approved fabrication and assembly package together with the shunt specification and project-specific test requirements. Manufacturing and test details can then be discussed against that defined scope; this article does not promise an accuracy level or approve an unreviewed design.

AI needs chips, PCBs, PCBAs and human engineering collaboration to operate in the physical world. PCBDOG manufactures the PCBs and PCBAs that bring AI into the physical world, from robotics and edge devices to intelligent machines. Preserving an approved measurement path supports the hardware implementation; it does not by itself improve a model’s intelligence.

Planning a current-sensing PCB or PCBA build? Share your board files, BOM, selected shunt part number, operating-current range and test requirements. The most useful first question is simple: exactly which voltage drop is this circuit intended to measure?

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