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A shunt resistor is a calibrated low-resistance element placed in series with the current being measured. Current through the resistor produces a small differential voltage, and that voltage is read directly by an ADC or amplified before conversion. The resistance may be only a few milliohms, yet its tolerance, temperature rise, terminal geometry and PCB connection can set the accuracy of the complete measurement channel.
Current sense resistor, current sensing resistor and current shunt resistor usually describe the same function in board-level electronics. The selected value must generate enough signal for the amplifier and ADC without causing unacceptable voltage loss or heat. High-current designs also require a defined Kelvin connection so that copper and solder resistance do not become part of the measurement.
What Is a Shunt Resistor?
A current-measurement shunt is connected in the load-current path rather than across the supply. Its voltage drop follows Ohm's law. The measurement circuit reads the voltage between the two intended sense points and calculates current from the known resistance.
A 5 mΩ shunt carrying 10 A develops 50 mV and dissipates 0.5 W. Those two results expose the central tradeoff. Raising the resistance produces a larger, easier-to-measure signal but increases insertion loss and heating. Lowering it improves efficiency while making amplifier offset, PCB resistance, noise and thermoelectric effects a larger fraction of the reading.
| Term | Meaning in a current-sense circuit | Common source of confusion |
|---|---|---|
| Shunt resistor | A known low resistance used to convert current into a measurable voltage | In other circuits, "shunt" can mean any component connected in parallel; a current-measurement shunt is normally in series with the monitored path |
| Current sense resistor | Board-level name for a resistor selected and specified for current measurement | A general low-value resistor may lack suitable tolerance, TCR, pulse rating or terminal construction |
| Sense voltage | The differential voltage measured across the defined sense terminals | It must stay inside the amplifier's differential input range during normal operation and faults |
| Common-mode voltage | The voltage shared by both amplifier inputs relative to circuit ground | A 50 mV shunt signal can sit on a 48 V rail, so small differential range does not imply a low common-mode requirement |
How to Choose Shunt Resistance
Start with the maximum current that must be measured accurately, including any permitted overload range. Then select a full-scale shunt voltage that fits the measurement IC. Tens of millivolts are common in low-loss board designs, while other systems use higher values to improve noise margin. The amplifier data sheet determines the usable differential range, input offset, gain and output headroom.
For a 10 A range and a 50 mV target, the calculated resistance is 5 mΩ. An amplifier gain of 40 V/V produces a nominal 2.0 V output at 10 A. The usable output limit must include supply tolerance, output swing, the ADC input limit and any reference voltage used for bidirectional sensing.
Check the minimum current, not only full scale
The same 5 mΩ shunt produces only 0.5 mV at 100 mA. A 25 µV input-offset error corresponds to 5 mA of input-referred current error, before noise, gain error and ADC uncertainty are added. The full-scale percentage looks small; the percentage error rises rapidly near zero. Wide dynamic range may require a lower-offset amplifier, more gain, multiple ranges or a current monitor with a suitable internal ADC.
High-Side vs Low-Side Current Sensing
A low-side shunt sits between the load return and system ground. Its common-mode voltage is close to ground, which can simplify the amplifier. The shunt also raises the load's local ground by I × R, and any current that returns through another ground path bypasses the measurement.
A high-side shunt sits between the supply and the load. The load remains referenced directly to ground, and supply current can be monitored before it reaches the load. The amplifier inputs must tolerate the rail's normal common-mode voltage, startup behavior, switching transients and reverse conditions. A device powered from 3.3 V can measure on a much higher rail only when its input stage is explicitly rated for that common-mode range.
| Location | Advantages | Design limitations | Typical use |
|---|---|---|---|
| Low side | Low common-mode voltage; simple amplification; convenient bench probing | Introduces ground lift; alternate return paths create measurement error; some ground faults can bypass the shunt | Cost-sensitive loads, local power stages and circuits where a small ground offset is acceptable |
| High side | Preserves the load ground; observes current delivered from the supply; supports supply-path fault monitoring | Requires adequate common-mode range, CMRR, transient tolerance and power-sequencing behavior | Batteries, motor drives, power distribution, servers and automotive rails |
| Bidirectional | Measures charging and discharging or forward and reverse power flow | Needs an output reference, signed digital result or bipolar signal path; zero-current offset becomes critical | Battery management, regenerative drives and bidirectional converters |
Power Rating, Self-Heating and Pulse Current
Continuous heating is calculated from RMS current, not from average current when the waveform varies. A pulsed load with a low average can still create substantial I2R heating. Its peak current can also exceed the resistor's pulse-energy limit or the amplifier's differential input range.
The printed wattage is valid only under the manufacturer's stated mounting and temperature conditions. Surface-mount shunts transfer much of their heat through the terminals into copper. Pad area, copper weight, thermal vias, internal planes and airflow can change terminal temperature and resistance stability. Use the data-sheet derating curve and any terminal-temperature power curve instead of applying a universal safety multiplier.
Short-circuit, motor-stall, capacitor-inrush and hot-plug events need a separate pulse check. Compare peak power, pulse duration, repetition rate and energy with the manufacturer's overload or pulse chart. A resistor that survives one brief event may drift after repeated pulses even when no visible damage occurs.
Two-Terminal and Four-Terminal Shunt Resistors
A two-terminal current sense resistor carries load current and senses voltage through the same physical terminations. Solder, copper spreading resistance and the exact location of the sense-trace connection can therefore change the measured voltage. The error becomes serious when the nominal shunt resistance is comparable with milliohms of interconnect resistance.
A four-terminal resistor separates the high-current terminals from the voltage-sense terminals. The amplifier input current is tiny, so very little voltage is developed along the sense leads. The measured drop is defined closer to the resistive element and excludes most terminal, solder and power-trace resistance.
| Construction | Connection | Accuracy behavior | Selection guidance |
|---|---|---|---|
| Two terminal | Power and sense paths share each end termination | Strongly influenced by pad geometry and the chosen sensing points at very low resistance | Suitable when the error budget permits it and a symmetric Kelvin-style PCB footprint is used |
| Four terminal | Separate current and voltage terminals | Provides a more clearly defined resistance between dedicated sense points | Preferred for high current, low milliohm values and tight production accuracy |
| Busbar or chassis shunt | Bolted current terminals with separate potential terminals | Supports high current but still depends on joint temperature, connection integrity and sense-wire placement | Used for battery packs, industrial DC systems, chargers and large power converters |
Kelvin Connection and PCB Layout
Kelvin sensing routes two dedicated traces from the defined shunt sense points to the differential inputs. Load current must not flow through these traces. On a four-terminal part, connect directly to the sense terminals. On a two-terminal part, take both sense traces from equivalent locations at the inner edges of the pads and keep the current distribution through the resistor symmetrical.
- Place the sense amplifier close enough to keep the differential pair short, while respecting the shunt's hot zone.
- Route the two sense traces together and away from switch nodes, gate-drive loops, inductors and other high-dV/dt or high-dI/dt conductors.
- Do not take either measurement connection from a remote ground pour or from a power trace beyond the intended terminal boundary.
- Keep input-filter resistors equal and close to the amplifier. Mismatch converts common-mode transients into differential error.
- Avoid unequal vias and unequal copper geometry in the two sense paths. Where vias are necessary, use the same structure on both inputs.
- Keep large thermal gradients away from microvolt-level connections; dissimilar-metal junctions can create thermoelectric voltage.
Video: Shunt resistor layout
This Texas Instruments layout demonstration shows how shunt pad geometry and Kelvin routing affect current-sense accuracy.
Shunt Resistor Tolerance, TCR and Self-Heating Error
Initial tolerance defines the resistance range near the stated reference condition. Temperature coefficient of resistance, expressed in ppm/°C, estimates how resistance changes with element temperature. The element temperature is affected by ambient conditions and its own power dissipation, so a board inside a warm enclosure can experience both effects at once.
A 50 ppm/°C component experiencing a 60 °C temperature rise has an estimated 0.30% resistance change from TCR alone. The calculation is a first-order estimate. Component TCR may include terminal effects that are absent from an element-only specification, and resistance can also shift from soldering, mechanical stress, overload history and long-term aging.
Self-heating error should use the actual resistor temperature or the manufacturer's thermal characterization. Ambient temperature alone does not describe an energized shunt. The resistance at normal current can differ from the value measured with a low-current ohmmeter on a cold board.
Building a Current-Measurement Error Budget
A complete error budget follows the signal from current to displayed result. At high current, shunt tolerance, TCR and amplifier gain error often dominate. Near zero current, amplifier offset, offset drift, noise and thermoelectric voltage become much more visible. Fast common-mode transitions add a dynamic error that a DC calculation will not reveal.
| Error source | How it enters the result | Most visible condition | Useful control |
|---|---|---|---|
| Shunt tolerance | Scales every calculated current value | Across the full range | Tighter tolerance or system calibration |
| TCR and self-heating | Changes resistance with temperature | High continuous current and wide ambient range | Low component TCR, thermal design and temperature-aware calibration |
| Amplifier offset and drift | Adds an input-referred voltage independent of current | Low current and zero-current measurement | Low-offset device, zero calibration and adequate sense voltage |
| Gain error | Changes the slope between shunt voltage and output | Near full scale | Specified gain accuracy and one- or two-point calibration |
| Input bias and filter mismatch | Creates unequal voltage drops at the amplifier inputs | Large filter resistance or high temperature | Matched low-value filters and bias-current calculation |
| Common-mode rejection | Converts rail voltage or fast rail transitions into differential error | High-side switching systems | Suitable DC and AC CMRR, symmetric routing and input filtering |
| ADC and reference | Adds offset, gain, quantization, INL and reference uncertainty | High-resolution digital measurement | Allocate ADC codes across the useful range and include reference limits |
| PCB connection | Adds copper, solder and current-spreading resistance to the measured drop | High current and very low shunt values | Four-terminal part or validated Kelvin footprint |
Guaranteed worst-case limits may be added arithmetically when every boundary must be covered. Root-sum-square treatment is appropriate only when the error sources are independent and statistical assumptions are justified. Calibration can remove some initial gain and offset error; it does not automatically remove temperature drift, aging, common-mode transients or layout-dependent changes.
Worked 10 A example
Consider a 5 mΩ, ±1% shunt at 10 A. Its nominal signal is 50 mV and its dissipation is 0.5 W. A gain of 40 produces 2.0 V. If the component TCR is 50 ppm/°C and its temperature rises 60 °C, the first-order resistance change is 0.30%. A 25 µV amplifier offset contributes 0.05% of full scale, because 25 µV is 1/2000 of 50 mV. Before ADC and layout effects, a simple worst-case boundary already reaches approximately 1.35% when the signs align.
At 100 mA, the shunt signal is 0.5 mV. The same 25 µV offset is then 5% of the reading. This is why a current channel that is accurate near full scale may still perform poorly at light load.
Choosing the Amplifier and ADC Interface
A general-purpose op amp can measure some low-side shunts, but its input common-mode range, output swing, offset, bias current, noise and recovery from overload must fit the circuit. The Operational Amplifier Guide covers these non-ideal limits in more detail.
Dedicated current-sense and current-regulation ICs provide a differential front end designed for a stated common-mode range. Selection still requires more than headline voltage and gain. Check:
- input common-mode range during normal operation, startup, shutdown and faults;
- maximum differential input voltage and behavior during an overcurrent event;
- input offset, offset drift, gain error, gain drift and noise;
- DC CMRR and dynamic response to common-mode steps;
- bandwidth, propagation delay and settling time for the required control loop or protection function;
- output swing and reference-pin range for unidirectional or bidirectional current;
- input bias current, filter recommendations and source-resistance limits;
- ADC full-scale range, reference accuracy, resolution, sample rate and input settling.
The conversion for a voltage-output amplifier is commonly:
VREF is zero for many unidirectional circuits. A bidirectional design may place zero current at midscale or another specified reference voltage. Reference accuracy and output headroom must be included in the usable positive and negative current ranges.
Switching Current, Bandwidth and Shunt Inductance
A real shunt contains parasitic inductance. During fast current transitions, the measured voltage includes an inductive term as well as the resistive drop:
The L × dI/dt component can create spikes that do not represent steady current. Low-inductance metal-element construction, compact current loops and correct sense-point placement reduce the effect. Input filters can limit switching noise, but unequal components reduce common-mode rejection and excessive filtering delays overcurrent protection or current-loop feedback.
Use oscilloscope probes with short, symmetric connections and sufficient common-mode rating. A long probe ground lead can add ringing that is absent at the amplifier pins. For high-side measurements on hazardous or high-energy rails, the probe and isolation method must be rated for the circuit.
Shunt Resistor vs Current Transformer and Hall Sensor
| Method | DC measurement | Isolation | Insertion loss | Primary limitation |
|---|---|---|---|---|
| Shunt resistor | Yes; also measures AC and pulses within bandwidth | No by itself | I2R loss | Heat, voltage drop and small-signal accuracy |
| Current sense transformer | No steady DC | Can provide galvanic isolation | Low primary conduction loss | Magnetizing behavior, core reset, burden and low-frequency response |
| Hall-effect sensor | Yes | Available in isolated architectures | Low conductor loss | Offset, drift, magnetic interference, bandwidth and cost |
A shunt is well suited to precise, compact measurement when its power loss and common-mode interface are acceptable. Isolation, very high current or extremely low insertion loss can justify a magnetic method. The system's DC requirement, bandwidth, accuracy, fault energy and isolation rating determine the appropriate technology.
Shunt Resistor Technology and Package Selection
| Construction | Useful characteristics | Points to verify |
|---|---|---|
| Metal strip or metal element | Low resistance, low inductance, good pulse capability and low TCR options | Terminal-temperature rating, PCB copper requirements and resistance-dependent TCR |
| Metal foil | Very tight tolerance and low TCR for precision measurement | Power, pulse energy, package size and cost |
| Thick film | Broad resistance range and economical packages | TCR, overload stability, pulse behavior and excess noise at the required value |
| Wirewound | High power and mechanically robust packages | Inductance, pulse behavior, temperature rise and whether a non-inductive winding is specified |
| Busbar or chassis shunt | Very low resistance, high current and separate potential terminals | Mounting torque, bolted-joint resistance, terminal temperature and ventilation |
Check the complete orderable configuration: nominal resistance, tolerance, TCR grade, terminal style, power rating, pulse curve, operating temperature, automotive or industrial qualification, and packaging. Different suffixes within one series can change electrical grade or termination material.
Shunt Resistor Selection Checklist
- Define minimum, nominal, maximum, reverse and fault current.
- Select high-side, low-side or bidirectional placement.
- Choose a full-scale sense voltage compatible with loss, noise and amplifier input range.
- Calculate resistance, continuous RMS power and maximum instantaneous power.
- Check pulse energy, repetition rate, overload drift and fault-clearing time.
- Include shunt tolerance, component TCR, self-heating and long-term stability.
- Choose two-terminal, four-terminal or busbar construction from the accuracy and current requirement.
- Define Kelvin sense points and symmetric routing before completing the PCB layout.
- Verify amplifier common-mode range, offset, gain, CMRR, bandwidth and differential-input limit.
- Verify ADC range, reference, resolution, sampling and settling.
- Calculate error at full scale and at the minimum current that must be measured.
- Validate temperature rise and measurement accuracy on the production PCB under representative load cycles.
Technical References
- Texas Instruments - Current Sensing Using Nanopower Op Amps
- Analog Devices - AN-105 Current Sense Circuit Collection
- Analog Devices - Optimize High-Current Sensing Accuracy by Improving Pad Layout
- Vishay - Temperature Coefficient of Resistance for Current Sensing
Frequently Asked Questions
What is a shunt resistor?
A shunt resistor is a known low resistance placed in series with a current path. The voltage across it is proportional to current and can be measured directly or amplified before conversion by an ADC.
Is a shunt resistor the same as a current sense resistor?
In board-level current measurement, the terms usually describe the same function. A purpose-built current sense resistor normally provides low resistance, controlled tolerance, specified TCR, suitable pulse behavior and a package designed for predictable current flow.
How do I choose a shunt resistor value?
Choose the desired full-scale sense voltage and divide it by maximum measured current. Then verify power loss, amplifier differential range, offset error, minimum-current accuracy and available standard resistance values.
How is shunt resistor power calculated?
Use P = I2R. For changing current, use RMS current for average heating and check peak power, pulse duration, repetition and energy against the manufacturer's pulse rating.
Should a shunt resistor be placed on the high side or low side?
Low-side placement simplifies common-mode requirements but creates a small ground lift. High-side placement preserves the load ground and monitors supply current, while requiring an amplifier rated for the rail voltage and its transients.
Why does a shunt resistor need a Kelvin connection?
A Kelvin connection separates voltage sensing from the high-current path. It prevents most power-trace, solder and terminal resistance from being included in the measured shunt voltage.
Can a two-terminal shunt use Kelvin sensing?
Yes. Two dedicated sense traces can connect at symmetrical inner pad locations. Accuracy remains more dependent on footprint geometry and current distribution than it is with a true four-terminal resistor.
Why does shunt current measurement drift as the circuit warms up?
Self-heating changes resistance through TCR. Amplifier offset and gain also drift, and thermal gradients can create small thermoelectric voltages. Measure the energized shunt temperature and include the entire signal chain in the error budget.
Can a shunt resistor measure AC and DC current?
Yes. A shunt responds to DC, AC and pulses, but the amplifier bandwidth, sampling system and the resistor's parasitic inductance determine the usable frequency range.
What is the difference between a shunt resistor and a current transformer?
A shunt produces a resistive voltage drop and can measure DC or AC without inherent isolation. A current transformer measures changing current, can provide isolation and does not measure steady DC.
Source Current-Sense Components
Send the exact resistance, tolerance, TCR, power rating, package, qualification and required quantity. Aetrix can also help source current-sense amplifiers and review orderable suffixes for production requirements.
