Digital Iso
A digital isolator carries logic-level signals across a galvanic barrier without LEDs. Analog Devices' iCoupler family (e.g. ADuM1100) and Silicon Labs' Si86xx use micro-transformers or capacitors fabricated on the same die as the I/O drivers, supporting data rates from a few Mbit/s to 150 Mbit/s with 5 kV isolation.
A logic edge on the input side modulates an on-chip oscillator that drives a tiny planar transformer (in iCoupler) or a coupling capacitor (in Si86xx) etched into the silicon. The receiver detects the modulated burst, decodes the polarity, and re-drives the output. Because the path is purely electromagnetic, nothing degrades over life and propagation delays match unit-to-unit within nanoseconds.
In plain terms
Two walkie-talkies sealed inside a single plastic shell: instead of shouting through a window (LED → photodetector), they whisper to each other over a built-in radio link that never wears out.
Why designers use it
- Replace an opto-coupler at 5–10× the speed without the LED-aging issue.
- Carry SPI, I²C, or UART across an isolation barrier as a multi-channel package (e.g. ADuM1401).
- Cross a kV-class barrier in a medical ECG front-end or a motor drive's gate-control path.
- Fit four isolated channels in a 16-pin SOIC where four discrete opto-couplers wouldn't.
Best for
- SPI/I²C iso
- Motor drives
- Medical front-ends
Key specifications
- Isolation voltage: 2.5 kV – 10 kV (Reinforced)
- Speed: 1 Mbit/s – 150 Mbit/s
- Channels: 1 – 6
- Quiescent current: 0.5 mA – 5 mA per channel
- CMTI: 75 – 150 kV/µs
When not to use it
- When DC-coupled transmission is needed across a barrier carrying a static logic level for milliseconds — some capacitive isolators require periodic edges and revert to default if the line stays static.
- When mains-line surge withstand specifications must exceed 10 kV — transformer-isolator or beefier opto-pair is the safer choice.
Common mistakes
- Forgetting that the part needs separate decoupling capacitors on each side of the barrier — a missing 100 nF on the secondary Vₒₒ causes erratic glitches that look like data corruption.
- Mixing channel directions on a part that has fixed-direction channels (e.g. ADuM1410 has 4-in-0-out) — cross-coupling never works because there's no driver on the wrong side.
Where you will find it
- A Tesla Model 3 traction-inverter uses Silicon Labs Si8261 isolated gate drivers to fire its IGBT modules: the digital isolator's 5 kV barrier separates the 800 V battery rail from the 12 V control logic, and its 60 ns propagation delay enables 20 kHz PWM with deadtime measured in tens of nanoseconds.
- A bedside hemodialysis machine uses ADuM1401 quad isolators on the SPI bus connecting the patient-side blood-pressure ADC to the system MCU: the four-channel device replaces eight opto-couplers, runs at 25 MHz, and meets IEC 60601-1 patient-applied-part isolation requirements.
- A grid-tied solar inverter's MPPT controller uses a Si8421 to send PWM commands from its DSP to a high-side gate driver on the 800 V boost stage: replacing the previous opto-coupler eliminated the CTR-aging recall the manufacturer had been seeing in 5-year-old field units.
A short history
Digital isolators use on-chip magnetic or capacitive coupling—rather than an LED and phototransistor—to transfer logic signals across a high-voltage isolation barrier. Analog Devices (iCoupler technology using micro-transformer coils) and NVE Corporation (GMR-based isolators) introduced commercial digital isolator ICs around 2000. Silicon Labs entered the market with capacitive-coupled isolators (Si86xx series) in 2004. Digital isolators offer significantly higher data rates (up to 150 Mbit/s), longer lifetimes, and tighter timing than opto-couplers, replacing them in motor drives, isolated power supplies, and industrial communications.
Good to know
- Digital isolators use either tiny on-chip transformers (Analog Devices iCoupler) or capacitive coupling (TI ISO77xx) — both operate at GHz carrier frequencies invisible to the user.
- Where an optocoupler tops out around 50 Mbit/s with high power, digital isolators handle 150 Mbit/s at one-tenth the operating current.
- Reinforced isolation (10 kV impulse, 1.5 kVrms working) is the gold standard for medical-mains barriers — IEC 60601-1 demands it.