UK What a digital isolator is
A digital isolator is an integrated circuit that copies a logic signal from one side of an insulating barrier to the other. The two sides have separate supplies and separate grounds, and there is no conductive path between them. Inside the package there are usually two or more silicon dies, one for each side, joined only by an insulating layer of silicon dioxide or polyimide.
Signals cross that layer by an electric field, through tiny on-chip capacitors, or by a magnetic field, through tiny on-chip transformers. That is the difference from an optocoupler, which uses light from an LED. Digital isolators appeared in the early 2000s and have since replaced optocouplers in most new serial, CAN and SPI interfaces.
Each channel is one way, so a part might have four channels arranged three forward and one back, which suits an SPI bus. Bidirectional versions exist for I²C, and dedicated parts handle USB.
How digital isolators work
A capacitor or transformer cannot pass a steady logic level, only changes, so the isolator has to encode the signal. Two methods are common. In on-off keying, a high-frequency carrier is sent while the input is high and stopped while it is low. In edge encoding, each rising or falling edge sends a short pulse, and a refresh circuit resends the level now and then so the output cannot drift. The coupling element varies by manufacturer.
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Capacitive. A pair of capacitors with silicon dioxide as the dielectric carries the signal, usually differentially to reject noise. Silicon dioxide is a very stable insulator, and capacitive parts handle high data rates at low power.
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Magnetic (transformer). Planar coils on the chip, separated by a thick polyimide layer, carry pulses across. Transformers are naturally differential, which helps them shrug off fast ground shifts.
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Giant magnetoresistance. A coil on one side changes the resistance of a magnetic sensor on the other. It is a less common method, used for very fast or very low-power parts.
Digital isolator vs optocoupler
Both provide galvanic isolation, but they behave differently in a circuit. The table compares a typical general-purpose part of each kind; fast optocouplers and specialist isolators exist at both extremes.
| Feature | Digital isolator | Optocoupler |
|---|---|---|
| Crosses the barrier by | Electric or magnetic field | Light from an LED |
| Typical data rate | Tens to over 100 Mbit/s | Below 1 Mbit/s for standard parts; fast types reach tens of Mbit/s |
| Power | Low, because energy is sent only on edges or as a small carrier | Higher, because the LED draws current while it is on |
| Timing | Short, closely matched delays between channels | Longer delays that vary more between parts |
| Ageing | Little change over life | LED output falls over years, so designs allow a margin |
| Input | Logic level, typically 2.5 V to 5.5 V | LED current, so it can be driven from a wide range of field voltages through a resistor |
| Component standard | IEC 60747-17 | IEC 60747-5-5 |
That last input row is why optocouplers have not disappeared. A 24 V digital input on an industrial computer still often uses a photocoupler, because its LED can take a field signal through a simple resistor, while a digital isolator needs the signal brought down to logic level first.
Reading a digital isolator datasheet
Most selection mistakes come from a handful of lines on the datasheet.
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Channel count and direction. Count the signals in each direction, including chip selects, enables and fault lines, not just data.
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Default output state. When the input side loses power, the output goes to a fixed high or low. Many families offer both, set by the part number, so pick the one that leaves the far side safe.
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Isolation ratings. VISO is the one-minute withstand test from UL 1577, often 2.5, 3.75 or 5 kV rms. VIOWM and VIORM are the working voltages the barrier holds for its whole life, and VIOTM and VIOSM cover transients and surges. Basic or reinforced is stated against IEC 60747-17.
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Common-mode transient immunity. CMTI, in kV/µs, is how fast the two grounds can move relative to each other before data is corrupted. Motor drives and inverters need high figures, often 100 kV/µs or more.
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Supplies on both sides. Each side needs its own supply. Where only one exists, an isolated DC-DC converter or an isolator with integrated isolated power provides the other.
Where digital isolators are used
In industrial and embedded systems, digital isolators sit wherever a processor meets wiring that leaves the enclosure. Common examples are isolated RS-485 and CAN transceivers, SPI links to analogue-to-digital converters in data acquisition, gate drivers in motor drives and inverters, and PLC backplanes. Many manufacturers now combine the isolator, the line transceiver and an isolated power supply in one package.
Several of our CAN bus I/O modules use exactly that kind of part: each CAN port has its own combined isolator and transceiver, isolated 500 V port to port and port to host. Our serial I/O modules include cards that isolate every RS-232, RS-422 or RS-485 port separately, some to 2500 V RMS. Our guide to isolated digital inputs explains how isolation is applied on the slower 24 V side, where photocouplers remain common.
Quick answers on digital isolators
Digital isolators: common questions
Is a digital isolator the same as an optocoupler?
No, but it does the same job. Both pass signals across an isolation barrier. An optocoupler uses light, while a digital isolator uses on-chip capacitors or transformers, which makes it faster and lower in power.
Does a digital isolator need power on both sides?
Yes. Each die needs its own supply. An isolated DC-DC converter, or an isolator with built-in isolated power, provides the far-side supply when there is only one.
Can a digital isolator pass analogue signals?
Not directly. Digitise the signal first with an ADC and isolate the digital link, or use an isolated amplifier or isolated modulator designed for analogue measurement.
What is reinforced isolation?
The highest grade of isolation. It gives protection equal to two independent layers of basic insulation, and is needed where a single fault must not expose people to a dangerous voltage.
Why do digital isolators have a default output?
For a known state on power loss. Without power on the input side the output cannot follow the signal, so it settles high or low by design, and the system can treat that as a fault.
We specify isolated serial, CAN and digital I/O on the industrial computers and I/O modules we supply as standard, matched to the grounding and voltages on site.