Current Sense / Shunt Resistor Calculator

Two modes: Size a shunt (R = V_sense / I_max) or analyze an existing shunt. Calculates amplifier output V_out = V_shunt × gain, ADC utilization, power dissipation, and optional offset error and current resolution.

Current Sense / Shunt Resistor

Mode:
Required inputs
Optional inputs

Formula & Theory

Formulas used

  • Mode 1 (Size): Rshunt = Vsense_target / Imax
  • Vshunt = Imax × Rshunt
  • Pshunt = Imax² × Rshunt
  • Vout_amp = Vshunt × gain
  • ADC utilization = Vout_amp / Vref × 100%
  • Optional: Ioffset_error = Vos / Rshunt
  • Optional: Current resolution = Vref / (2bits × gain × Rshunt)
  • Optional: Drift = |tempco × ΔT| / 106 (first-order; excludes self-heating unless ΔT reflects it)
  • Optional: Current error (tempco) = Imax × drift

Worked Example

Sizing a shunt for a 10 A load, targeting an 80 mV sense voltage:

  • Target current: Imax = 10 A
  • Target sense voltage: Vsense = 80 mV
  • Calculated shunt resistance: Rshunt = 80 mV / 10 A = 8 mΩ
  • Shunt power at full scale: Pshunt = 10² × 0.008 = 0.8 W
  • With a 50 V/V amplifier: Vout_amp = 80 mV × 50 = 4.0 V — this exceeds a 3.3 V ADC reference and will saturate. Dropping gain to 40 V/V gives Vout_amp = 3.2 V, a 97% ADC utilization with headroom for transients.

At 0.8 W, a standard 1206 chip shunt (often rated 0.25–0.5 W) would overheat — this calls for a dedicated power shunt package (e.g., 2512 or a metal-strip shunt rated ≥ 1.5 W after derating). At currents like this, use a 4-terminal (Kelvin) connection: the amplifier's sense leads tap the shunt at dedicated sense pads, not the current-carrying pads, so trace and contact resistance don't add error to the measurement.

Assumptions & Limitations

Common Mistakes

Next step: choose real parts

Use the calculated shunt resistance, power dissipation, and amplifier output to narrow your part selection. Key datasheet specs to verify are listed below for each component category.

1. Current Sense Amplifier

Gain Fixed-gain parts (e.g. 20, 50, 100 V/V) should bring your calculated Vout_amp to 70–95% of Vref at full-scale current — check the calculator's ADC utilization result before picking a gain.
Common-mode voltage range Must cover your rail voltage, including transients — high-side sensing needs a CMV range that extends above the supply.
Offset voltage Vos Directly sets your current measurement floor — use the calculator's optional offset-error field to check this against your low-current accuracy needs.
Search current sense amplifiers — Digi-Key ↗ Search current sense amplifiers — Mouser ↗

2. Shunt / Current Sense Resistor

Power rating Compare the calculator's Pshunt result against the rated power with headroom — derate to 50% of rated power for reliability, more in enclosed or hot environments.
Tolerance & TCR Tighter tolerance (0.5–1%) and low temperature coefficient of resistance (TCR, ppm/°C) matter most for measurement accuracy — use the calculator's optional tolerance and tempco fields to estimate total error.
4-terminal (Kelvin) package At currents above a few amps, use a 4-terminal shunt so the amplifier senses voltage at dedicated pads rather than through the current-carrying contacts — this removes trace and solder-joint resistance from the measurement.
Search shunt resistors — Mouser ↗ Search shunt resistors — Newark ↗
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Frequently Asked Questions

What is a shunt resistor in current sensing?

A shunt resistor (or current sense resistor) is a low-value precision resistor placed in series with the load. The voltage drop across it is proportional to current: V = I × R. A current sense amplifier amplifies this small voltage for an ADC or comparator. Shunt values typically range from 0.1 mΩ to 100 mΩ depending on the current range and acceptable power loss.

How do I choose the sense voltage target?

The sense voltage at full-scale current (V_sense = I_max × R_shunt) sets the SNR and power loss trade-off. A higher sense voltage improves noise immunity and ADC resolution but increases shunt power dissipation and burden voltage. Typical targets are 20–100 mV. Precision amplifiers like the INA190 support sense voltages down to a few millivolts, but below 10 mV noise becomes significant.

What amplifier gain should I use?

Gain is chosen to bring V_sense × gain close to V_ref (the ADC reference voltage) at full-scale current — typically 80–95% utilization. Higher gain improves resolution but reduces headroom for transients. If your sense voltage is 50 mV and V_ref is 3.3 V, a gain of 50 V/V gives V_out = 2.5 V (76% utilization). Fixed-gain current sense amps (INA180, INA190) are preferred over op-amp circuits for their CMR and accuracy.

How do I interpret ADC utilization?

ADC utilization is the fraction of V_ref used at full-scale current: utilization = (V_shunt × gain) / V_ref. Below 30%, you are wasting ADC codes and resolution. Above 100%, the amplifier output clips at the ADC reference and the reading saturates. A utilization of 70–90% is a good target — leaving headroom for transients while maximizing resolution.

Why does shunt power dissipation matter?

The shunt resistor dissipates P = I² × R_shunt as heat. In high-current designs (10 A+), even a 1 mΩ shunt at 100 A dissipates 10 W — requiring a heat sink or power-rated shunt package. Always verify the shunt's power rating against calculated dissipation. For precision measurement, the self-heating of the shunt changes its resistance (temperature coefficient), introducing measurement error.