Isolation Amplifier Circuit Design

An isolation amplifier measures a signal that is referenced to a potential the measuring circuit cannot share. It is used to read a current in a motor phase, a voltage on a high side switch or a signal from a sensor at the end of a long cable.

What the Barrier Provides

galvanic isolation removes the conductive path between the two grounds, so a potential difference between them produces no current. That is what allows a measurement on a high side shunt to be read by a controller referenced to earth.

The barrier also protects the controller from a fault. A short in the power circuit that raises the local ground above the controller is contained, provided the barrier is rated for the voltage that appears.

The isolation is not perfect, and the capacitance across the barrier couples a current that changes with the rate of change of the potential difference. That coupling is what the common mode transient specification describes.

Barrier Technologies

A capacitive barrier gives a high data rate and a short propagation delay, and its isolation capacitance is a few picofarads per channel. It is the usual choice for a signal path that carries a modulated or a digital signal.

A transformer barrier gives a similar performance with a lower capacitance in some designs, and it is used where the transient immunity has to be highest. A magnetic isolator works on the same principle with a different implementation.

An optical barrier has the lowest coupling capacitance and the longest propagation delay, and its current transfer ratio changes with age. It is used where the measurement is slow and the immunity matters more than the timing.

Common Mode Transient and Its Effect

common mode transient immunity is specified as a rate of change of the barrier voltage in kilovolts per microsecond. In a motor drive with a fast switching node the real waveform can exceed ten kilovolts per microsecond.

A transient that exceeds the rating produces a wrong output, which appears as a spike in the measured current. The spike coincides with the switching edge, which is the diagnostic clue.

Symmetrical layout of the two sides of the barrier and a low capacitance across it reduce the coupling. The package is part of the answer, which is why the manufacturer’s recommended layout should be followed rather than approximated.

Measuring a Current with a Shunt

A shunt in the phase of a motor is a low value resistor, often a milliohm, carrying tens of amps and sitting on a node that swings hundreds of volts. The amplifier measures a few tens of millivolts across a resistor whose own temperature coefficient matters.

Kelvin connections to the shunt are mandatory, and the trace from the shunt to the amplifier has to be short. The common mode voltage moves at the switching rate, and a long trace adds inductance that turns the transient into a differential error.

A shunt that is placed in the low side of the bridge has a much smaller common mode swing and is easier to measure. Where the low side is not available, the isolated amplifier is the only practical answer.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/pl149813980-oem_fast_turn_custom_pcb_rigid_enig_printed_circuit_board_quick_turn.webp" alt="Isolation barrier and shunt resistor layout on a measurement board” />

Accuracy and Drift

An isolated amplifier has an offset, a gain error and a non linearity, and its offset drift is larger than that of an ordinary precision amplifier. The drift is quoted over the temperature range and it appears as a scale error on the current measurement.

A chopper stabilised input stage inside an isolated part reduces the offset drift considerably. The switching of the chopper produces a small ripple at the output, which the filter removes.

Gain error is often calibrated at production by injecting a known current and recording the reading. The calibration removes both the gain error and the offset, and it leaves the drift as the residual error.

creepage and Layout of the Barrier

creepage and clearance across the barrier are set by the working voltage and by the pollution degree of the environment. The package provides part of the distance, and the board layout provides the rest.

No trace may cross under the package, and no copper plane may extend from one side to the other. A ground plane that continues under an isolated part is a common mistake and it destroys the isolation.

A slot in the board under the barrier increases the surface distance where the package does not provide enough. The slot is only useful if it is wide enough to break the creepage path effectively.

Supply and Reference Design

Each side of the barrier needs its own supply, and the isolated side needs a supply that is itself isolated. A small transformer or an integrated isolated converter provides it, and its isolation rating has to match the amplifier.

The reference of the isolated side is the same node as the shunt return, and that is the point at which the measurement is made. Bringing the reference from somewhere else introduces the voltage drop of the connection into the measurement.

Decoupling on both sides is required, and the capacitors must be placed within the respective domains. A capacitor that bridges the barrier is a short circuit for the coupled transient and defeats the purpose.

Applications and Their Requirements

A motor phase current is the most demanding application, because the common mode moves at the switching rate and the current itself is a pulse width modulated waveform that has to be measured accurately while the switch is on.

A high side voltage measurement is easier, because the common mode is relatively static and changes only when the switch alters state. The isolation rating still has to cover the full bus voltage with a comfortable margin.

A sensor at the end of a long cable is the third case, and here the isolation breaks a ground loop rather than protecting against a high voltage. The accuracy requirement is often higher and the transient requirement much lower.

Verification and Faults

Verify the channel with a known current and, if possible, a common mode step applied to the isolated side. The step should not appear in the output, and any residual indicates a coupling path.

Measure the output with the shunt current at zero and the switching node active. A reading that follows the switching indicates a common mode or a layout problem rather than a sensor problem.

A part that fails after a fault on the power side has usually been subjected to a transient beyond its rating. The release checks that keep such a barrier intact are collected in our PCB design release checklist, the assembly points in judging PCB quality, and the layout measures for two separate domains in our guide to mixed signal board design.

FAQ

What is common mode transient immunity? The rate of change of the barrier voltage the part can withstand without producing a wrong output, quoted in kilovolts per microsecond.

Can a plane cross under an isolated part? No. It would bridge the barrier and destroy the isolation, even though the package appears to provide it.

How accurate is an isolated amplifier? Good after a production calibration. Its offset drift is larger than a precision non isolated part, so it is calibrated rather than relied upon.