Isolation Amplifier Current Sensing Design

Measuring current in a motor drive, a battery system or a mains connected converter means placing a sensing element at a potential that would destroy the controller connected to it. An isolation amplifier solves that problem by transferring the measurement across a barrier, and the way the barrier is laid out on the board decides whether the isolation holds up in service.

Why Isolation Is Needed in Current Sensing

The sensing element sits in the power circuit, where the potential can swing by hundreds of volts at the switching rate. The controller sits on a low voltage ground, and any direct connection between the two would carry fault current through the signal path and destroy the controller at the first switching event.

galvanic isolation breaks that connection while allowing the measurement to pass. It also removes the ground loop that would otherwise exist between two circuits at different potentials, and it allows the controller to be referenced to earth while the power stage floats. In equipment that a user can touch, the same barrier also provides the safety insulation required by the applicable standard.

Isolated current sensing is used in motor phase measurement, battery management, solar inverters and switched mode supplies. In each case the requirement is the same: measure the current accurately, reject the large common mode voltage that the sensing point carries, and keep the barrier intact for the life of the product.

How an Isolation Amplifier Works

The input side of the device converts the small voltage across the sensing element into a form that can cross the barrier. Some parts use a sigma delta modulator and transfer a digital bitstream across a capacitive barrier, while others modulate an analogue signal onto a carrier and transfer it through a transformer. Both approaches preserve the measurement while breaking the DC path.

The output side reconstructs the signal, filters the modulation artefacts and presents a clean analogue or digital output to the controller. The accuracy of the reconstruction depends on the timing of the modulation, so the clock quality and the supply decoupling on both sides matter for the noise and distortion performance.

Some devices provide a single ended output and others provide a differential one. A differential output is preferable where the controller is some distance away, because it halves the sensitivity to common mode noise and allows a differential input converter to be used. The choice influences both the layout and the calibration of the channel.

Common Mode Voltage and Rejection

common mode voltage is the potential of the sensing point relative to the controller ground, and it is the parameter that decides which device can be used. It is not a static number in a switching circuit: it moves with the switching node, often at several volts per nanosecond, so the device must tolerate both the static level and the rate of change.

Rejection of that common mode signal is specified as a transient immunity figure, expressed as kilovolts per microsecond. A device with poor transient immunity will produce an output glitch at every switching edge, and the glitch appears in the measurement as an offset that changes with duty cycle. Where the current is used in a control loop, that glitch can destabilise the loop.

A differential measurement of the voltage across the sensing element rejects part of the common mode by itself, but only if the two input paths are matched. Any difference in capacitance or resistance between the two inputs converts common mode into differential signal, so the input filtering has to be symmetric and the traces have to be routed together.

High Side versus Low Side Sensing

high side sensing places the element between the supply and the load, where it measures the current the load actually receives. It requires a common mode range that includes the supply rail and, in a motor phase, a rapidly switching node. The advantage is that it detects a short to ground, which low side sensing cannot distinguish from a normal measurement.

Low side sensing places the element between the load and ground, where the common mode is close to zero and a simple amplifier can be used. Its drawback is that it changes the ground potential of the load slightly, which can disturb other circuits, and it misses faults that occur before the element.

In a three phase drive, phase current measurement normally uses a sensing element in each of two or three legs, each with its own isolated or high voltage capable amplifier. The arrangement requires careful attention to the timing of the sampling relative to the switching edges, because sampling during a transition produces a corrupted reading regardless of the amplifier choice.

Supply, Grounding and the Barrier

An isolated amplifier needs supplies on both sides, and each side must be decoupled locally. The isolated side supply is usually derived from an isolated converter, whose own transformer has to meet the same creepage requirement as the amplifier. Treating that converter as a mere power component is a common cause of a board that fails its isolation test.

The safety barrier on the board is defined by the package and by the copper around it. No trace, plane or via should cross the boundary except through the device itself, and the creepage measured around the package has to satisfy the applicable standard for the working voltage. The clearance rules for dense boards, described in via to trace clearance, apply to the barrier as well.

Keep the barrier clear of mounting hardware and board edges. A metal standoff or a screw placed across the line defeats the isolation in a way that no electrical test of the bare board will detect, so the mechanical drawing and the layout have to agree on where the boundary lies.

Layout of the Isolated Front End

The input side of an isolation amplifier is a precision analogue circuit, and the layout around it follows the usual rules: short connections to the sensing element, a symmetric differential pair, and a solid reference plane that does not carry switching current. The sensing element itself should be a four terminal part where accuracy matters, so the load current path is separate from the measurement path.

Keep the input network away from the switching node. The voltage across the sensing element is small, and capacitive coupling from a node that swings hundreds of volts at high speed will inject more error than the amplifier contributes. A tight differential layout over a continuous plane, with the filtering capacitors matched, is the practical answer.

The output side is a mixed signal area, and the partition described in mixed signal board design applies to it. Keep the digital lines that carry the measured value away from the analogue output, and where both are used, provide separate returns that meet at one point under the controller rather than at several places along the trace.

Verification and Common Faults

Verify the isolation first, then verify the accuracy. Applying the rated isolation voltage between the two sides and measuring the leakage current confirms that the barrier is intact, and it is a test that should be applied to every unit rather than only to a prototype.

Verify the accuracy with the current path energised and the common mode voltage present at its worst case, including the maximum rate of change. A device that is accurate at zero common mode may shift its offset when the switching node moves, and that shift is the error that matters in a control loop. Testing at several duty cycles makes it visible.

An output that shows a spike at every switching edge usually indicates an asymmetric input layout or insufficient transient immunity. The path that the disturbance takes can be found with the method described in EMI suppression design, starting from the switching node and following the coupling paths towards the input network.

A channel that reads correctly at low current but compresses at high current usually has a saturation problem rather than an isolation problem. Check the output swing, the supply rails and the gain setting, and confirm that the sensing element is not heating enough to change its resistance at high current.

FAQ

Do I need an isolated supply for the input side? Yes if the sensing point floats. The isolated supply is part of the barrier, and its isolation rating and creepage must meet the same standard as the amplifier.

Can an isolation amplifier replace a current transformer? For DC and low frequency measurement, yes, because a transformer cannot transfer DC. For high frequency measurement a current transformer or a Hall sensor may be simpler.

How do I reduce noise on the isolated measurement? Start with the input network: matched filtering, a tight differential layout and a four terminal sensing element. Then check the isolated supply, because noise on it crosses the barrier with the signal.