Current Sensor Shunt Resistor Sizer
Find a shunt resistance that fits your current range, voltage-drop budget, and power allowance. Add amplifier and ADC limits to check signal headroom, bidirectional operation, and low-current measurement error.
Set the measurement requirement
DC or known-RMS periodic current, an external resistive shunt, and an optional linear voltage-output amplifier. Example values are not device specifications.
Your shunt design results
Start with a design example, then replace its inputs with your requirements.
Which constraint sets the value?
| Constraint | Nominal resistance ceiling |
|---|
Design checks
Amplifier & ADC signal range
At your evaluation current
Current-to-signal reference
| Current | Shunt voltage | Instantaneous loss | Nominal output |
|---|
Nominal values, not measured data. Negative current reverses shunt voltage but still produces positive dissipation. Output values are unclipped mathematical demands.
The summary includes inputs, resistance ceilings, modeled checks, and assumptions.
Measure the element—not the power trace
A shunt creates a small differential voltage in the load-current path. Separate sense connections pick up that voltage close to the resistive element, avoiding unnecessary power-trace drop in the measurement.
Shunt construction, mounting, and Kelvin connections: Texas Instruments — Shunt Resistor Selection for Isolated Data Converters.
Balance signal strength against loss
Start with the maximum drop
A higher resistance gives more signal at the same current, but also removes more voltage from the load. The tool uses maximum resistance—including the entered tolerance and temperature bound—to keep peak drop within your limit.
V shunt = I × R
R nominal,max = V drop,max / (I peak × resistance upper factor)Use RMS for average heating
For a constant-resistance model, average loss follows RMS current squared. A 10 A rectangular pulse with a 25% duty cycle and zero off-current has 5 A RMS, not 2.5 A RMS. Peak dissipation still needs a separate pulse-capability review.
P average = I RMS² × R
P peak = I peak² × R
I RMS = I peak × √duty (zero-off rectangular pulses)Fit the entire signal range
Match the amplifier’s guaranteed output swing to the ADC input range. In bidirectional sensing, leave room both above and below the zero-current reference. At zero current, offset can still demand a voltage outside the valid window.
V output = V reference + G × (I × R + V offset)
Usable window = amplifier / ADC range overlap, minus your marginsSignal-range sizing: Texas Instruments — Current-sense amplifier current range and shunt selection. The signal-versus-loss trade-off: Analog Devices — High-Side Current-Sense Measurement.
Low current makes offset more visible
The same input offset becomes a larger percentage error as the measured current falls. Reducing shunt resistance saves power, but makes that offset more significant relative to the signal.
Drift bound = |TCR| × |ΔT| × 10⁻⁶
R high = R nominal × (1 + tolerance) × (1 + drift)
R low = R nominal × (1 − tolerance) × (1 − drift)
I indicated = (V output − V reference) / (G nominal × R nominal)
The tool evaluates resistance, gain, and offset corners, then reports the largest absolute current error at your evaluation current. At zero current, percentage error is undefined; the absolute offset-related error remains meaningful.
This is an uncalibrated analog bound. It excludes reference error, ADC gain/offset/INL, quantization, noise, input bias current, thermoelectric effects, and common-mode errors. The TCR bound is a first-order model, not a prediction of actual temperature.
Confirm more than resistance and watts
- Resistance and TCR: specify nominal value, tolerance, reference temperature, and the temperature range over which the TCR applies.
- Mounting and derating: check the resistor’s datasheet conditions for PCB copper, terminal temperature, airflow, and allowed continuous power.
- Pulses and faults: review pulse duration, repetition, surge energy, and short-circuit conditions against the actual resistor and input-protection ratings.
- Connections: choose suitable terminals and route sense traces separately from load-current copper. Check the manufacturer’s footprint guidance.
- Amplifier fit: verify common-mode voltage, supply, gain, offset across temperature, output load, and required bandwidth.
- Procurement: provide the exact part number, package, grade, quantity, and accepted alternatives. A calculated E-series value alone is not a complete purchase specification.
Check the topology separately
Low-side current sensing
The shunt sits in the load return. Its voltage raises the load’s local ground above the supply return. Check that this shift is acceptable for communications, control signals, and other return paths.
High-side current sensing
The shunt sits in the supply path. The amplifier must handle the bus common-mode voltage and relevant transients. A small differential shunt voltage does not imply a low voltage at either sense pin.
Bidirectional current sensing
The output reference maps zero current into the usable input window. Both current directions must fit. This tool uses a symmetric ±peak current range; it does not model current-output or sign-bit sensing architectures.
Topology background: Analog Devices AN-105 — Current Sense Circuit Collection.
Shunt resistor sizing questions
Why not always choose the smallest shunt resistance?
A smaller resistance reduces drop and power loss, but also reduces signal voltage. Input offset, noise, and ADC resolution can then dominate at low current. Evaluate the current range and error target together.
Why does the tool select a lower value than voltage divided by current?
Automatic sizing allows for the entered resistance tolerance and temperature bound. It also considers RMS power and, when enabled, amplifier gain, offset, and usable output range. It rounds down to a feasible preferred value instead of rounding up past a limit.
Can a 1 W resistor always dissipate 1 W?
No. The rating applies under specified conditions. Temperature, PCB copper, terminal mounting, and cooling affect usable power. The allowed-use percentage is only an input for screening; verify the actual derating curve and resistor temperature.
Why does a unidirectional design fail at zero current?
An amplifier may not swing exactly to ground. Input offset can also demand a negative output when the reference is zero. Check the permitted low output voltage, use a supported reference arrangement, or accept and characterize a near-zero dead zone. Reducing the shunt value does not fix a reference-position problem.
Does the ADC result tell me the smallest current I can accurately measure?
No. Current per LSB is only the ideal nominal conversion step, using ADC span divided by 2 to the power of its resolution. Noise, effective resolution, ADC errors, reference accuracy, and the analog front end determine useful measurement capability.
Can I use this for a digital current monitor with an internal ADC?
Disable the external amplifier check and enter the monitor’s allowed linear shunt-input magnitude as the maximum drop, subject to your load-loss limit. Verify its separate positive and negative input ranges, calibration, internal gain, and measurement specifications in its datasheet.
Does a passing result approve a high-current or high-voltage design?
No. The tool checks only the entered algebraic limits. It does not validate isolation, creepage, fault handling, terminals, PCB ampacity, thermal behavior, protection, or regulatory compliance. Bench verification and equipment-specific design review remain necessary.
Source your current-sensing components
Send YURUNOX your shunt, current-sense amplifier, ADC, or protection-device part numbers. Include resistance, tolerance, TCR, power rating, package, quantity, and required grade.