Lesson 15/1788%
MODULE 15

Optocouplers vs Modern Isolation Technologies

A quantitative face-off: capacitive and magnetic digital isolators, isolation transformers, and fiber optics versus the optocoupler — benchmarked on speed, power, isolation quality, EMI, reliability, aging, and cost, ending with a technology selection decision tree.

Learning Objectives

After completing this module, you will be able to:

Explain how capacitive and magnetic digital isolators work
Benchmark all isolation technologies quantitatively
Weigh EMI emission and susceptibility differences
Apply a structured technology selection methodology
Defend technology choices in design reviews
Where this fits. You now know the optocoupler in depth. Before choosing one, you should know its competitors. This module compares optocouplers with capacitive and magnetic digital isolators, isolation transformers and fibre-optic links, using numbers rather than marketing, and ends with a decision method you can defend in a design review. It builds on Module 5 (families), Module 6 (parameters) and Module 9 (ageing). The standards that certify each technology are in Module 16.
Illustrations and animations are original teaching drawings, not to scale. Values marked representative are typical of common parts; always confirm against the current datasheet or standard you design to.

Every isolator does the same job: move information (and sometimes power) across an insulation barrier that blocks DC and low-frequency current. They differ in what carries the signal (light, electric field, magnetic field) and what the barrier is made of, and those two choices drive almost every performance difference.

Five cards comparing optocoupler, capacitive isolator, magnetic isolator, isolation transformer and fibre-optic link by signal carrier and barrier material
Figure 1. The isolation technology landscape.
TechnologyTypical parts (examples)Sweet spot
Phototransistor optocouplerPC817, TLP185, 4N35Low-speed logic, PLC inputs, SMPS feedback, lowest cost
High-speed / gate-drive optocoupler6N137, TLP2361, ACPL-064L, HCPL-3120, ACPL-332J1–25 Mbps logic, IGBT gate drive
Capacitive digital isolatorTI ISO77xx family, Silicon Labs (Skyworks) Si86xx familySPI/UART/CAN/I2C, many channels, high CMTI
Magnetic digital isolatorAnalog Devices iCoupler ADuM14xx/ADuM12xx; isoPower ADuM5xxxMany channels, integrated isolated power
Isolation transformerEthernet magnetics, gate-drive transformers, flyback transformersAC signals, power transfer
Fibre-optic linkIndustrial plastic-fibre or glass-fibre transceiversVery high voltage, long distance, extreme EMI

Part families are examples of widely used devices; vendors and part numbers change, so confirm current availability and ratings.

A capacitive isolator puts two silicon dies in one package. The transmitter drives a signal across a pair of series capacitors built from thick SiO2 layers on the die; the receiver detects it. A capacitor blocks DC, so the data must be encoded as an AC signal.

On-off keying (OOK)

Used by e.g. TI ISO77xx: one logic state sends a high-frequency carrier, the other sends nothing. The receiver rectifies the envelope. DC-correct by design: a steady HIGH is a steady carrier.

Edge / differential encoding

Other designs send edge information differentially and use refresh or state-keeping logic to hold DC levels.

SiO2 barrier

Very high dielectric strength (hundreds of V/µm), so a thin layer withstands kV. Its wear-out mechanism is TDDB, which vendors model to predict barrier life at the working voltage (Section 8).

Default output

With the input side unpowered, the output goes to a defined default (HIGH or LOW, selected by part suffix). Choose the safe state for your system.

Animation of on-off keying: the data input, a carrier burst across the barrier whenever data is high, the detected envelope and the recovered data output
Figure 2 (animation). On-off keying across a capacitive barrier.
Worked example — replacing four 6N137s on an SPI bus

An isolated ADC uses SPI at 8 MHz: SCLK, MOSI and CS forward, MISO back.

  1. 6N137 max 10 Mbps with tPHL/tPLH up to about 75 ns: round-trip SCLK→MISO delay ≈ 2 × 75 = 150 ns, more than one 125 ns clock period. SPI timing fails unless you slow the clock.
  2. A 3-forward/1-reverse capacitive isolator (ISO7741-class): delay about 10–16 ns per direction, so the round trip is about 30 ns. That fits easily.
  3. Supply: 4 × 6N137 ≈ 4 × (10 mA ICC + 5 mA average LED) = 60 mA; the quad isolator needs a few mA in total.
Design trap — forgetting the second supply. A digital isolator needs VDD on both sides. On a PLC input with no field-side supply, that means adding an isolated DC-DC converter, which can erase the cost advantage.

Magnetic isolators (Analog Devices’ iCoupler is the best-known family) use micro-transformers: planar coils on the die separated by a thick polyimide layer. Because a transformer only passes changes, the input is edge-encoded into short current pulses.

Timing diagram: data input, coil pulses (one pulse for a rising edge, two for a falling edge, plus periodic refresh pulses) and the reconstructed data output
Figure 3. Edge encoding with refresh pulses.
Cross-sections: optocoupler with a thick silicone gap, capacitive isolator with SiO2 capacitors on two dies, and magnetic isolator with coils separated by polyimide
Figure 4. Barrier construction: distance (opto) versus thin, high-quality dielectrics (digital isolators).
Power efficiency

Energy is spent only on edges (plus refresh), so current scales with data rate: very low at low rates.

isoPower

The same micro-transformer technology transfers power (tens to hundreds of mW) with an on-chip oscillator, removing the external DC-DC. The price is efficiency (often well below 50 %) and radiated emissions that need careful layout (stitching capacitance across the barrier).

Magnetic field immunity

The tiny coils are very small loops, so external fields induce little voltage. Datasheets quote immunity in A/m or tesla; check it near welders, large motors or induction heaters.

Missed-pulse recovery

Refresh pulses (about every microsecond) and a watchdog restore the correct state or force the default after loss of input power.

Discrete transformers are the oldest isolation technology and still the only one that moves significant power. As signal isolators they pass AC only, so DC levels need encoding (Manchester, pulse trains) or a DC-free protocol (Ethernet).

Signal transformers

Ethernet magnetics isolate every wired Ethernet port: typically 1.5 kVrms functional isolation (IEEE 802.3). Gate-drive transformers pass PWM pulses to high-side switches without a separate supply.

Power transformers

Flyback and forward transformers provide both power and safety insulation, using triple-insulated wire, tape layers and bobbin creepage. In an SMPS, the optocoupler in the feedback loop and the transformer share the same barrier.

Weaknesses

Size, interwinding capacitance (tens of pF: poor CMTI unless shielded), no DC, magnetic-field sensitivity and custom design effort.

ICM = CIO × dV/dt
  • ICM: common-mode displacement current through the barrier (A)
  • CIO: barrier capacitance (F): ≈ 0.5–1 pF for an opto or digital isolator, 5–50 pF for a transformer
  • dV/dt: common-mode slew rate (V/s)
Worked example — why transformers struggle with CMTI
  1. At 50 kV/µs, a 1 pF barrier passes 1 pF × 50 × 109 V/s = 50 mA for the duration of the edge.
  2. A 20 pF signal transformer passes 1 A, which is enough to disturb nearly any receiver unless the winding is shielded and the return path is designed.

A fibre-optic link is an optocoupler stretched out: an LED or laser, a length of fibre and a photodiode receiver. Because the insulation is the fibre length itself, the isolation voltage is limited only by the fibre surface and routing, and the barrier capacitance is effectively zero.

Where it wins

HV converters (MV drives, HVDC, traction), test halls, pulsed-power systems, and long links between separately earthed buildings.

Media

Plastic optical fibre (POF) for short industrial links (tens of metres, tens of Mbps); glass fibre for long distances and Gbps.

Costs

Transceivers, connectors, a transmitter LED current of tens of mA, and handling (bend radius, dirt on connectors).

Ageing

The transmitter LED ages like an opto LED: the link budget must include end-of-life optical power margin, the same idea as the CTR budget in Module 9.

The table below uses representative values for common parts in each class. Individual grades vary by an order of magnitude, so use it to choose a class, then design with the actual datasheet.

MetricPhototransistor optoHigh-speed optoCapacitive isolatorMagnetic isolatorFibre link
Max data rate~10–50 kbps1–25 Mbps100–150 Mbps25–150 Mbps10 Mbps – Gbps
Propagation delay3–20 µs20–100 ns7–20 ns20–60 ns20–100 ns
PWDµs (load-dependent)5–35 ns< 5 ns< 5 nsns
Supply per channel5–10 mA (LED + pull-up)10–20 mA~1–3 mA~0.5–3 mA (rate-dependent)10–50 mA
CMTI (min)1–15 kV/µs10–50 kV/µs50–150 kV/µs25–150 kV/µsvery high
Isolation ratingto ~5 kVrmsto ~5 kVrmsto ~5.7 kVrmsto ~5 kVrmslimited by distance
Wear-outLED → CTR driftLED → threshold driftTDDB of SiO2polyimide ageingTX LED output
Temperature−30/−55 … +100/+125 °C−40 … +85/+105 °C−40 … +125 °C−40 … +125 °C−40 … +85 °C
Relative costlowestlow–mediummedium, low per channelmedium, low per channelhigh
Three bar charts on log scales comparing max data rate, propagation delay and minimum CMTI for phototransistor optos, high-speed optos, capacitive isolators, magnetic isolators and fibre links
Figure 5. Speed, delay and CMTI on log scales (representative).

Interactive: Technology Benchmark

Toggle technologies and metrics. Scores are 1–5 (5 = best) derived from the representative table above; they are a teaching summary, not vendor data. Deselect metrics that don’t matter to your application and watch the ranking change.

Highest total score (selected metrics)–

Optocoupler power is dominated by the LED, and it is spent whether you send 1 bit/s or 10 kbit/s. Digital isolator power is mostly quiescent plus a term proportional to data rate. The crossover depends strongly on channel count.

Popto ≈ N × VDD × (D × IF + ICC + IPU)    Piso ≈ N × VDD × (IQ + k × fdata)
  • N: channels; VDD: supply (V); D: duty cycle of LED-on state
  • IF: LED current; ICC: detector supply current (high-speed optos); IPU: average pull-up current (A)
  • IQ: quiescent current per channel (both sides); k: dynamic current per Mbps (A/Mbps); fdata: data rate (Mbps)

Interactive: Power vs Data Rate and Channel Count

Representative current models: PC817 at IF = 5 mA with a 1 kΩ pull-up; 6N137 at IF = 10 mA; capacitive 1.6 mA + 0.03 mA/Mbps per channel; magnetic 0.6 mA + 0.1 mA/Mbps per channel; fibre TX 30 mA at 50 % duty + RX 5 mA. Check your part’s supply-current curves.

Lowest-power feasible technology–
Optocoupler total–
Capacitive isolator total–
–

Pro tip For battery or 4–20 mA loop-powered sensors, magnetic isolators at low data rates can draw tens of µA per channel in some low-power variants, which is impossible with an LED. Conversely, for a single 1 Hz status signal, a PC817 driven at 1 mA may be the simplest low-power answer.

Both technologies can be certified to reinforced insulation, but they wear out differently, and the certification standards differ: IEC 60747-5-5 for optocouplers, IEC 60747-17 for capacitive and magnetic couplers (Module 16).

Left: optocoupler CTR falling over operating hours, faster at higher temperature. Right: predicted digital-isolator barrier life falling exponentially with working voltage, with the rated working voltage giving a life well above a typical design life
Figure 6. Two wear-out mechanisms: LED light loss (function) versus TDDB (insulation). Conceptual shapes.
Opto: function ages, insulation robust

The thick polymer barrier has large margin; the LED ages. A degraded opto usually fails safe (the signal stops) rather than unsafe (the insulation breaks down).

Digital isolator: function stable, insulation modelled

No LED to age, so timing and thresholds barely drift. The thin barrier has a voltage-dependent lifetime, and vendors publish TDDB curves showing the working voltage for 20–40+ years with margin.

Surge capability

Both are tested with a surge (VIOSM, 1.2/50 µs). Check the value for your overvoltage category and whether it applies to basic or reinforced insulation.

Distance through insulation

Some end-product standards require a minimum DTI (e.g. 0.4 mm) for reinforced solid insulation, unless the component standard route is used. Thin-film isolators rely on the component certification (Module 16).

EMI cuts both ways: what the isolator emits, and what disturbs it.

AspectOptocouplerCapacitiveMagneticDesign response
Radiated emissionNegligible (no RF carrier)Carrier harmonics can radiate via the barrierLow for data; high for isoPower DC-DCStitching capacitance across the barrier (PCB-embedded), short return paths, vendor layout guides
Common-mode transientsWeakest (low CMTI), especially phototransistorsStrongStrongChoose CMTI ≥ 2× application dV/dt
Magnetic fieldsImmuneImmuneHighly immune; check ratingKeep away from high-current loops anyway
RF immunity (conducted/radiated)Good; slow parts filter naturallyGood; check datasheet EMC resultsGoodFilter inputs; test per IEC 61000-4-3/-4-6 (Module 13)
ESD on I/ODepends on the circuit, not the barrierSameSameTVS on the cable side
Design trap — assuming a digital isolator passes radiated emissions because the evaluation board did. Evaluation boards use four-layer stack-ups with embedded stitching capacitance. A two-layer board with a wide slot can radiate the carrier or DC-DC switching harmonics. Budget for emission pre-scans early.

Digital isolators have taken most high-speed digital sockets, but optocouplers remain the best answer in several places. Knowing them lets you defend an opto choice instead of apologising for it.

Simplicity and cost at low speed

A PC817 and two resistors: no second supply, no decoupling, cents in cost, dozens of second sources.

No active supply on one side

The output side needs only a pull-up, and photovoltaic/PhotoMOS outputs need no supply at all. Ideal for PLC inputs and contact outputs.

Linear / analog

IL300 and HCNR201 servo-linearised couplers give simple, low-cost analog isolation (Module 5).

SSR and TRIAC functions

MOC3041-class zero-cross TRIAC drivers and AQY210/TLP222 PhotoMOS relays have no drop-in digital-isolator equivalent.

SMPS feedback

TL431 + PC817 is the lowest-cost, well-understood loop (Module 10); isolated error amplifiers exist but cost more.

Fail-safe behaviour

An aged LED fades toward “off”; many safety interlocks exploit this with a dynamic (pulsed) signal.

Robust thick barrier

Large DTI and a long track record in field-proven designs simplify some certification arguments.

Wide input tolerance

AC-input (H11AA1) and wide-range inputs with a series resistor accept 5–240 V signals directly.

Use a structured method so the same inputs always lead to the same answer, and so reviewers can check your reasoning. Start with function (analog, switching, digital), then speed, channel count, supplies and environment.

Decision tree: analog to linear opto or isolated amplifier; AC switching to TRIAC opto or PhotoMOS; distance or very high voltage to fibre; over 10 Mbps to digital isolator; three or more channels to multi-channel isolator; isolated power to integrated DC-DC; otherwise an optocoupler, with a list of checks
Figure 7. First-pass decision tree.

Interactive: Decision-Tree Wizard

Answer the questions for your interface. The wizard follows the same logic as Figure 7 with a gate-drive branch, and explains its recommendation.

In a design review, “we always use PC817s” and “digital isolators are modern” are both weak arguments. A defensible choice states the requirements, the alternatives considered, and the numbers that decided it.

Template — isolation technology justification
  1. Requirement: signal type, data rate/bandwidth, channels and directions, insulation class and working voltage, CMTI (application dV/dt × 2), temperature range, lifetime, supplies available on each side, cost target.
  2. Alternatives: at least two technologies, with representative parts.
  3. Comparison: the three to five metrics that matter here, with numbers (speed and delay, power, CMTI, lifetime, cost per channel, BOM impact).
  4. Decision and risks: the choice, its weak points and how they are mitigated (e.g. “opto: CTR ageing covered by kage = 0.5 in the margin calculation”).
  5. Evidence: certificates (UL 1577, IEC 60747-5-5 or -17), test results (Module 13), second sources.
Worked example — 24 V PLC digital input card, 16 channels
  1. Requirements: 16 inputs, 1 kHz max, 24 V field side with no logic supply, basic insulation 50 V to logic (functional in practice, but specified as basic), −25 to +70 °C, lowest cost.
  2. Option A: 4 × quad-channel PC817-type (PC847 class) couplers, field side supplied by the input signal itself, about 3–5 mA per input at the threshold; no field-side supply.
  3. Option B: 2 × 8-channel digital isolators + a field-side DC-DC (or loop-derived supply) + input comparators.
  4. Decision: A, because speed and CMTI needs are trivial, there is no field-side supply and the cost is lowest. Risk: CTR ageing, covered by an end-of-life design at 15 V threshold (Module 10, example 2).
Carrier and barrier

Light/field/magnetism and polymer/SiO2/polyimide explain most differences.

Speed

Digital isolators win above ~1–10 Mbps with ns-level delay and PWD.

Power

Opto power is LED-dominated and rate-independent; isolator power scales with rate and channels.

CMTI

Low barrier capacitance and differential receivers give digital isolators 50–150 kV/µs; fibre is effectively immune.

Lifetime

Opto function ages (LED); thin-film barriers have TDDB-modelled insulation life.

EMI

Optos emit nothing; OOK carriers and isoPower need layout care.

Opto strongholds

Low speed and cost, no receive-side supply, linear, TRIAC/SSR, SMPS feedback.

Method

Function → speed → channels → supplies → environment, then verify with numbers.

Design checklist

Tick items as you review a design. Your ticks are saved in this browser only; use your browser’s print function for a sign-off copy.

Four parts: multiple choice, true/false, a matching exercise and engineering scenarios. Score 80 % on the multiple choice to pass the module.

Part A — Multiple Choice

Question 1 of 10  •  Score: 0/10

Part B — True or False

Score: 0/8

Part C — Match the Term to Its Meaning

Drag each term onto its definition, or click a term and then click a definition.

Placed: 0/6

Part D — Engineering Scenarios

Scenario 1 — Motor-drive encoder interface
An inverter reads an RS-485 encoder at 10 Mbps across reinforced isolation. The SiC stage switches at 60 kV/µs. The original design used a 6N137-class opto (10 Mbps, CMTI 10 kV/µs min) and sees occasional CRC errors.
Analysis. Data rate is at the opto limit (PWD eats the bit time: 100 ns bits vs up to ~35 ns PWD), and the CMTI is 6× below the application dV/dt. Fix: an isolated RS-485 transceiver or a capacitive/magnetic isolator with ≥ 100 kV/µs CMTI and < 5 ns PWD. Power per channel also falls. Re-verify creepage for reinforced insulation at the DC-link voltage.
Scenario 2 — Battery-powered sensor node
A wireless sensor (2 × AA cells) must isolate one 1 Hz status input and a 100 kbps UART pair. The designer proposes three PC817s at IF = 5 mA.
Analysis. Each opto draws ~2.5 mA average (50 % duty) plus pull-up current: roughly 10 mA in total, which drains AA cells (~2500 mAh) in about 10 days. Fix: a low-power dual magnetic/capacitive isolator for the UART (tens of µA at 100 kbps in low-power variants), and for the status input either the same isolator or a pulsed opto (sample 1 ms every second: 5 µA average).
Scenario 3 — The emission surprise
A product using an isolator with integrated isoPower-style DC-DC fails radiated emissions by 8 dB at 300–400 MHz. The layout is two-layer with a 6 mm slot under the part.
Analysis. The integrated DC-DC switches at high frequency; common-mode current across the barrier has no low-impedance return, so the slot and cables become an antenna. Fixes: a four-layer stack with overlapping internal planes forming stitching capacitance across the barrier (keeping creepage and DTI), or a discrete isolated DC-DC with a Y-cap, ferrites on I/O, and following the vendor’s EMC layout guide. Alternatively, an opto-based design without the integrated power stage.

Additional Resources

Vendor technical material

Texas Instruments, Analog Devices and Skyworks/Silicon Labs application notes and white papers on digital-isolator barrier lifetime (TDDB), EMC layout and isoPower emissions; Broadcom, Toshiba, Vishay optocoupler application notes on CMTI and LED ageing.

Standards

IEC 60747-5-5 (optocouplers) • IEC 60747-17 / VDE 0884-17 (magnetic and capacitive couplers) • UL 1577 • IEEE 802.3 (Ethernet isolation). See Module 16.

Keep learning

Module 14 shows where each technology is used by sector; Module 17 includes selection challenges that apply this decision method.