Galvanic Isolation

Why no DC path matters

Take a simple bench measurement. A 24 V DC industrial controller drives a motor, and you'd like to record the motor-current shunt voltage on a USB-connected oscilloscope. Both instruments have grounded chassis through their AC power cords. When you connect the scope probe, you've just tied two separate "grounds" together. The motor's switching transients can produce 50 millivolts between the two ground points; a 50 millivolt current driven through a one-ohm shunt is a 50 millivolt error riding on a millivolt-scale signal. The waveform is unreadable, and trying to clean it up with averaging or filtering won't help, because the noise is conducted, not radiated.

The cure is galvanic isolation. Insert a barrier that lets the shunt signal cross to the scope without the two grounds being tied together. The motor side floats at whatever potential the load wants; the scope side stays at house earth. With no DC path, no ground-loop current can flow, and the noise floor drops by 60 to 100 dB. The same logic applies on a much larger scale to high-voltage gate drivers: a half-bridge inverter switching 600 volts can't be controlled by a 3.3 V microcontroller without a barrier that floats with the high-side switch.

Safety is the other big driver. A patient ECG electrode is in direct contact with the patient's chest, and any leakage current from the mains-powered amplifier could electrocute them — IEC 60601 medical standards require 5 kV barriers between the patient-applied parts and the supply. A wall-plug power supply must keep mains voltage off the USB-C connector regardless of single faults — UL 60950 demands "reinforced" insulation, two redundant barriers each rated for the worst-case transient. Industrial PLCs, motor drives, and electric-vehicle chargers all live or die by their isolation strategy. The barrier is the part of the design that the certification engineer will spend the most time inspecting.

The three isolation methods, in detail

Each method exploits a different physical phenomenon to carry signal across an insulating gap.

Method	Physical mechanism	Barrier material	Typical rating	Bandwidth	Best for
Transformer	Magnetic flux coupling between coils	Air gap + insulating tape	2.5 – 5 kV	DC blocked; 50 Hz – 1 GHz pass	Power, mains supplies, Ethernet
Opto-isolator	LED → photons → photodetector	Glass, silicone, polyimide	2.5 – 5 kV	DC – 50 MHz	Cheap digital, slow analog
Capacitive (integrated)	Displacement current through SiO₂	On-die SiO₂ dielectric	5 – 10 kV	DC – 150 Mbps	Fast multi-channel digital
Inductive (integrated)	Tiny on-die transformer	Polyimide between metal layers	5 – 8 kV	DC – 150 Mbps	ADuM digital isolators
Hall-effect	Magnetic field sensed by Hall element	Air gap	Hundreds of V	DC – 200 kHz	Current sensing
RF / microwave	Antenna pair across barrier	Insulating substrate	Up to 10 kV	Microwave	Specialty mm-wave isolators

The transformer is the oldest and still the only practical choice for non-trivial power transfer. The constraint is that it inherently blocks DC: only changing flux induces voltage in the secondary. A 50 Hz mains transformer wants a big iron core (hundreds of grams to hundreds of kilograms) because at low frequency the magnetic flux must be enormous to develop appreciable secondary voltage. A switched-mode supply running at 100 kHz can use a 10-gram ferrite core for the same wattage — the high frequency means less flux per cycle to move the same power. Audio output transformers handle 20 Hz to 20 kHz with vacuum-tube amplifiers driving 8-ohm speakers; Ethernet pulse transformers handle 1 to 100 MHz isolating physical layers; planar transformers on multilayer PCBs handle 1 MHz at 1 kW for compact server power supplies. The same magnetic principle, scaled from 1 kg to 10 mg by frequency.

The opto-isolator dates to the 1970s and is the cheapest option for slow digital signals. An infrared LED faces a phototransistor across a glass capsule or polyimide film. Drive the LED with milliamps; the phototransistor switches accordingly. Bandwidth is the bottleneck — the phototransistor's storage time limits speed to about 100 kHz for cheap parts, 50 MHz for high-end ones. Two systemic weaknesses limit its adoption in new designs. LED aging: the optical output drops by 30 to 50 percent over 100,000 hours, eventually below the threshold where the receiver triggers. And CTR (current transfer ratio) varies by 3:1 from part to part and 2:1 with temperature, requiring designers to leave huge margins. For new boards, integrated isolators have largely replaced discrete opto-isolators outside of cost-driven consumer applications.

Capacitive and inductive integrated isolators arrived in the mid-2000s and now dominate new designs. A capacitive isolator uses two ~100 fF capacitors on opposite sides of a 10-to-20-micrometer SiO₂ insulating film deposited on silicon. SiO₂ has dielectric strength around 700 V per micrometer, so a 20-micron film survives 14 kV peak transients — far better than air or polyimide. An on-chip modulator translates the input signal up to gigahertz frequencies that pass easily through the small capacitor; a demodulator on the other side recovers it. Silicon Labs (Si8xxx), Texas Instruments (ISO7xxx), and Analog Devices (ADuM, magnetic instead of capacitive) ship hundreds of millions of these parts per year. Six channels of 150 Mbps isolation cost about a dollar, on a 5×5 millimeter package.

Worked example: isolating an industrial USB scope probe

A real design: turn a non-isolated 100 MHz USB oscilloscope into an isolated current-probe front-end for measuring motor-drive shunts.

Requirements:
  Signal:              ±1 V analog, DC to 1 MHz
  Common-mode:         ±500 V (worst-case motor-drive bus)
  Patient-rated?       No (industrial), so 5 kV barrier suffices
  Power across barrier? Yes (the probe-side amp needs its own supply)

Topology:
  1. Isolated DC-DC: 5 V at 100 mA across the barrier
     Transformer-based (push-pull modulator) for the watts
     Output regulated by linear LDO on probe side
  2. Analog signal across the barrier:
     Option A: Sigma-delta modulator on probe side (e.g., AMC1306):
       1 MHz signal → 1-bit serial stream
       Stream crosses via capacitive isolator (8 kV)
       Digital sinc³ filter on scope side reconstructs analog
     Option B: ISOlated linear amplifier (e.g., AMC1100):
       Differential analog in, differential analog out
       Common-mode rejection: 108 dB at DC
       Bandwidth: 60 kHz (insufficient for 1 MHz — Option A wins)
  3. Galvanic-isolation barrier:
     Reinforced: 5 kV / 60 seconds; 2 kV continuous working
     UL 1577 listing required for industrial certification

Final choice:
  Transformer-isolated DC-DC + capacitive sigma-delta data path
  Total barrier voltage: 8 kV peak
  Signal bandwidth: 1 MHz at 16-bit equivalent resolution
  Cost: ~$8 in parts vs $200 for a dedicated isolated scope front-end

This is a representative industrial topology. The two-stage architecture — isolated power plus isolated data — is universal in modern designs because no single chip simultaneously transmits 100 milliwatts across an 8 kV barrier and recovers a 16-bit, 1 MHz analog signal. Decoupling the two lets each stage be optimized independently and replaced when the next-generation part appears.

Where galvanic isolation appears

• Mains power supplies. Every wall-plug, USB charger, and laptop adapter has a 3-to-5 kV barrier between AC input and DC output.
• Medical instrumentation. ECG, EEG, pulse oximetry, defibrillators — 5 kV patient isolation, regulated by IEC 60601.
• Motor drives & EV chargers. Gate drivers for IGBTs and SiC FETs need isolated control signals; high-side bootstrapping isn't enough above ~600 V.
• Industrial PLCs. Each I/O channel isolated from the CPU and from neighboring channels, keeping a single ground fault from killing the rack.
• Audio. Ground-loop hum elimination, balanced XLR transformers, isolating turntables and tape machines.
• Test & measurement. Differential probes, isolated thermocouple front-ends, floating-input multimeters.
• Solar and battery storage. String-level isolation between hundreds-of-volts PV strings and AC inverter output stages.

Common misconceptions

• Optical means slow. Modern integrated photonic isolators reach 100 Mbps; the "slow" label dates to discrete opto-isolators.
• One barrier is enough. Safety standards usually require "reinforced" insulation — equivalent to two redundant basic barriers in series.
• Capacitive isolators block DC. The barrier itself blocks DC, but the on-chip modulator/demodulator pair lets DC information pass.
• Higher voltage rating is always better. Higher-rated parts are bigger, costlier, and slower; pick the rating from the certification target.
• Isolation removes noise. It removes ground-loop noise but adds the isolator's own conversion noise; pick a good part.
• You can isolate after the amplifier. If the amplifier's reference is wrong, the signal is corrupted before it reaches the barrier; isolate the reference, not just the output.