Current sensing

Intent

• Turn a current into a voltage with a shunt, and know what it costs the circuit.
• Choose between the low side and the high side for the shunt.
• Keep the pads and the solder out of the reading with a Kelvin connection.

Theory

The shunt

By Ohm’s law a shunt turns the current into a sense voltage [Alexander & Sadiku 2017, Ch. 2]:

$$V_\mathrm{SENSE} = I_\mathrm{SENSE} R_\mathrm{SENSE}$$

$V_\mathrm{SENSE}$

voltage across the shunt

$I_\mathrm{SENSE}$

current through the shunt

$R_\mathrm{SENSE}$

sense (shunt) resistor

Voltage across a current-sense resistor✓ verified by 7 tests

Assumptions, convention and source

Assumptions

• the shunt is a linear resistor: its resistance does not change with the current

Notes

the signal the amplifier sees, and the burden: the voltage the shunt takes from the circuit it measures; the shunt dissipates the current times this voltage

Source

Alexander & Sadiku 2017, Ch. 2 (Basic Laws: Ohm's law)

Derivation

step-by-step sympy derivation

Equation id

sense.burden · equations.yaml entry

That voltage is the amplifier’s signal, and also the burden: the voltage the shunt takes from the circuit it measures. A larger shunt gives a larger signal and a smaller share of error from the amplifier’s offset [Texas Instruments 2023], at the price of more burden and more dissipation (conduction loss).

Low side or high side

On the low side the shunt sits in the ground return, so the load’s ground rises above the supply’s by the sense voltage. Low-side sensing suits cost-sensitive designs that can tolerate that ground disturbance, and high-side sensing is preferable when it cannot be tolerated [Texas Instruments, System Trade-offs for High- and Low-side Current Measurements]. On the high side both of the shunt’s terminals sit near the supply voltage, so the amplifier must accept that voltage at its inputs: current-sense amplifiers are specified for low-side use, high-side use or both [Texas Instruments 2023] [Analog Devices, LTC6101/LTC6101HV].

Pads, solder and the Kelvin connection

With a shunt of a few milliohms or less, the resistance of the solder becomes a substantial part of the sense element’s resistance, and adds to the measurement error [O'Sullivan 2012]. If the sense connections sit beyond the pads, the amplifier sees the current through the shunt and the pads together, and the reading is high by the same fraction at every current:

$$\varepsilon_\mathrm{pad} = \frac{R_\mathrm{pad}}{R_\mathrm{SENSE}}$$

$\varepsilon_\mathrm{pad}$

error from pad resistance

$R_\mathrm{SENSE}$

sense (shunt) resistor

$R_\mathrm{pad}$

pad and solder resistance

Gain error from pad and solder resistance in a shunt's sense path✓ verified by 7 tests

Assumptions, convention and source

Assumptions

• a two-terminal shunt whose sense connections share the pads and solder joints with the load current (no Kelvin connection)

Notes

the sense connections see the current through $R_\mathrm{SENSE} + R_\mathrm{pad}$, so the reading is high by this fraction, which grows as the shunt's value falls; a Kelvin connection takes $R_\mathrm{pad}$ out of the sense path

Source

O'Sullivan 2012, solder resistance of milliohm shunts; Kelvin and split-pad sensing; derived here; Analog Devices, LTC6101/LTC6101HV, Kelvin connection of the inputs to the sense resistor

Derivation

step-by-step sympy derivation

Equation id

sense.pad_error · equations.yaml entry

A Kelvin connection takes the pads out of the sense path: a four-terminal shunt, or a two-terminal one on a four-pad (split-pad) layout [O'Sullivan 2012]. The LTC6101 data sheet recommends Kelvin connections to the sense resistor in all but the lowest-power applications [Analog Devices, LTC6101/LTC6101HV, Applications Information].

The amplifier, its offset, the output’s range and the filter before an ADC are worked through in the sense chain tool.

Worked example

A small shunt without a Kelvin connection:

Shunt example

Symbol	Value	How it is obtained
$I_\mathrm{SENSE}$ current through the shunt	10 A	given
$R_\mathrm{SENSE}$ sense (shunt) resistor	1 mΩ	given
$R_\mathrm{pad}$ pad and solder resistance	100 µΩ	given
$V_\mathrm{SENSE}$ voltage across the shunt	10 mV	Voltage across a current-sense resistor
$\varepsilon_\mathrm{pad}$ error from pad resistance	0.1	Gain error from pad and solder resistance in a shunt's sense path

At the full current the shunt gives only 10 mV, and pads and solder of 100 µΩ in its sense path make the reading 0.1 high.

Try it

▶ Open in the equation explorer: Gain error from pad and solder resistance in a shunt's sense path
Preset from the example bench-sense

Sweep the shunt’s value: the same pads cost a larger share of a smaller shunt. A Kelvin connection sets $R_\mathrm{pad}$ to zero at any value.

Bench exercise

Pass a known current through a low-value shunt on a board. Measure the voltage across it twice: once at the ends of its pads, once at the shunt’s own terminals (or its Kelvin pads). Compare each with the current times the shunt’s rated value, and estimate the pads’ resistance from the difference. Then move the shunt from the ground return to the supply side and watch the load’s ground: it no longer moves with the current.

Gotchas

• A low-side shunt lifts the load’s ground by its sense voltage [Texas Instruments, System Trade-offs for High- and Low-side Current Measurements]: a signal the load sends to a ground-referenced circuit moves with the current.
• Sensing beyond the pads adds their resistance to the shunt’s, a fixed fraction of every reading [O'Sullivan 2012] (pad error).
• A high-side shunt puts the supply’s voltage on the amplifier’s inputs: use an amplifier specified for high-side sensing [Texas Instruments 2023].

Go deeper

• Technical article System Trade-offs for High- and Low-side Current Measurements (SSZTA51) — Texas Instruments · retrieved 2026-09-24
  A short comparison of placing the shunt in the ground return or on the supply side.
• Technical article Optimize High-Current Sensing Accuracy by Improving Pad Layout of Low-Value Shunt Resistors (Analog Dialogue 46-06) — M. O'Sullivan, Analog Devices · retrieved 2026-09-24
  How the solder of a low-value shunt enters the reading, and the four-pad layout that keeps it out.

Quiz

1. A 2 milliohm shunt carries 5 A. What is its sense voltage?

   1. 2.5 mV
   2. 10 mV
   3. 100 mV
   4. 0.4 mV
   Answer and explanation

   Correct answer: B Ohm's law: 5 A times 2 milliohm.

2. What is the burden of a shunt?

   1. Its power rating
   2. The amplifier's offset
   3. The voltage it takes from the circuit it measures, its sense voltage
   4. The error of the ADC
   Answer and explanation

   Correct answer: C The sense voltage is the signal and also the voltage the measured circuit loses across the shunt.

3. What does a low-side shunt do to the load?

   1. It raises the load's supply voltage
   2. Nothing: the shunt is in the ground return
   3. It isolates the load from ground
   4. It lifts the load's ground above the supply's by the sense voltage
   Answer and explanation

   Correct answer: D The ground return passes through the shunt, so the load's ground moves with the current.

4. When is high-side sensing preferable?

   1. When the lowest cost matters most
   2. When the current is very small
   3. When the load's ground must not move
   4. Never: low-side sensing is always better
   Answer and explanation

   Correct answer: C High-side sensing keeps the shunt out of the ground return, at the price of an amplifier that works at the supply's voltage.

5. A 1 milliohm shunt is sensed beyond its pads, which add 0.05 milliohm. How far off is the reading?

   1. 5 % low
   2. 5 % high, at every current
   3. 0.05 % high
   4. It depends on the current
   Answer and explanation

   Correct answer: B The pads' resistance over the shunt's, the same fraction at every current.

6. What does a Kelvin connection do?

   1. It lowers the shunt's resistance
   2. It cancels the amplifier's offset
   3. It filters the switching noise
   4. It senses the voltage at the shunt's own terminals, so the pads and solder stay out of the reading
   Answer and explanation

   Correct answer: D Separate sense connections carry almost no current, so the pads' resistance drops no voltage in the reading.