Contents
1. Why Kelvin?
When measuring very low resistances — below about 10–20 mΩ — the resistance of the PCB copper traces connecting to the sense resistor is no longer negligible. A 1 mm wide, 10 mm long trace in 35 µm (1 oz) copper has approximately 5 mΩ of resistance. If your sense resistor is 5 mΩ, that trace alone doubles your measured value.
The Kelvin connection (also called 4-wire or force-sense) solves this by separating the current-carrying path from the voltage measurement path:
- Force terminals carry the full load current and contribute trace resistance — but this resistance does not appear in the voltage measurement.
- Sense terminals carry negligible current (the input impedance of a modern current-sense amplifier is typically >100 kΩ), so their trace resistance introduces negligible error.
2. How the 4-Terminal Connection Works
The measured voltage is taken at the points where the sense taps connect to the resistor — inside the force connections. Any resistance in the force traces appears only as a voltage drop in the power path, not in the measurement.
3. 2-Terminal Components: Split Pad Layout
When using a standard 2-terminal current sense resistor (CSS, CSSH, etc.) with a Kelvin layout, you create a split pad at each end of the component:
Pad Dimensions
The split pad at each end consists of two electrically separate copper areas:
- Force pad (outer): Wide — sized to carry the full load current without excessive resistance or temperature rise. For a 2512 package at 5 A, a minimum width of 2 mm is typical.
- Sense pad (inner): Small — just large enough to reliably solder the component termination. Typically 0.5–0.8 mm wide, separated from the force pad by a 0.15 mm gap (minimum manufacturer-recommended clearance).
4. 4-Terminal Components: CSSK Layout
The CSSK series has the Kelvin principle built in at the component level. The outer two terminals are the force connections; the inner two terminals are the sense connections. This simplifies the layout considerably.
Key layout rules for CSSK:
- Do not connect the inner (sense) pads to the force bus — they must be isolated and routed directly to the amplifier input.
- Route the two sense traces as a matched-length differential pair. A mismatch of 1 mm is typically acceptable; more than 5 mm risks differential noise pickup.
- Place a 100 pF ceramic capacitor differentially across the sense inputs to filter high-frequency noise (after a small series resistor of 10–22 Ω).
5. Step-by-Step Layout Walkthrough
Place the sense resistor
Position it on the high-current path as close as possible to the current source or load being measured. Orient the component so that the force current flows straight through it without routing around bends.
Define the force bus
Route the main current path as a wide, low-resistance trace. For currents above 5 A, use multiple copper layers in parallel or increase to 2 oz copper. Keep the current loop compact to minimise inductance.
Create sense taps at the pads
For a 2-terminal component, split each pad into force and sense sections. The sense tap must originate at the edge of the pad closest to the resistor body — not partway along the force trace.
Route the sense pair to the amplifier
Use thin, matched-length traces (0.1–0.15 mm). Route them together as a pair, keeping them away from the high-current traces. Use a minimum of 0.5 mm clearance from any conductor carrying more than 1 A.
Place the sense amplifier close by
The sense amplifier (e.g. INA180, MAX40056) should be within 10–15 mm of the sense resistor. Longer sense traces are more susceptible to noise pickup and impedance imbalance.
Add RC filter at amplifier input
Place a matched pair of series resistors (10–22 Ω) and a differential capacitor (100 pF ceramic, C0G) at the amplifier input pins. This filters switching noise without affecting DC accuracy.
Verify in DRC and review
Check that the sense pads are not accidentally connected to the force bus. Confirm matched trace lengths. Review the current loop area and check that sense traces do not cross any switching nodes.
6. Common Mistakes
✓ Correct
- Sense tap at the resistor pad
- Separate force and sense vias
- Matched-length sense pair
- Sense traces away from force bus
- RC filter at amplifier input
- Wide, low-resistance force traces
✗ Incorrect
- Sense tap partway along the force trace
- Force and sense sharing a single via
- Asymmetric sense trace lengths
- Sense traces routed over switching nodes
- No filtering at amplifier input in a switching application
- Narrow force traces causing heating and additional TCR shift