How to Select Digital Isolators? Selection Guide for Communication Isolation, Gate Drive and High-Side Sensing
Bottom line: isolator selection asks only one question — what are you isolating. Isolate "general-purpose digital signals" with a multi-channel digital isolator; isolate "CAN / RS-485 / I2C bus" with an isolated interface IC; isolate "power-switch gate" with an isolated gate driver; isolate "high-side analog sensing" with an isolated amplifier / Σ-Δ modulator. Once the category is set, close in three steps: isolation rating and safety certification → CMTI and dv/dt → data rate and channel direction. This article gives the per-application selection comparison using Shenzhen HOPERF Microelectronics (HOPERF) CMT-series isolators as the reference; all values follow the original manufacturer datasheet.
flowchart TD
A["① Clarify the isolation target<br/>signal type / voltage domain / safety requirement"] --> B{"What is isolated?"}
B -- "GPIO / SPI / UART<br/>multiple general digital signals" --> C["Multi-channel digital isolator<br/>CMT812X / CMT804X / CMT826X"]
B -- "CAN or RS-485 bus" --> D["Isolated interface IC<br/>CMT1042(CAN) / CMT83085(485)"]
B -- "I2C sensor / EEPROM" --> E["Isolated I2C<br/>CMT810X"]
B -- "Power-switch gate" --> F["Isolated gate driver<br/>CMT8602X / CMT8603X"]
B -- "High-side current / voltage sense" --> G["Isolated amplifier / Σ-Δ modulator<br/>CMT1300 etc."]
C --> H["② Set rating and package<br/>3.75kVrms (narrow) ~ 5kVrms (wide)"]
D --> H
E --> H
F --> I["② Set drive specs<br/>4A/6A, UVLO level, CMTI ≥150kV/μs"]
G --> J["② Set sense specs<br/>input range ±50mV / ±250mV, gain"]
H --> K["③ Close: rate / channel direction / default output (fail-safe) / certification"]
I --> K
J --> K
K --> L["④ Companion: isolated auxiliary supply + layout across the barrier"]First, the two most-asked questions
Q: What is the difference between a digital isolator and an optocoupler?
A traditional optocoupler transmits across the barrier with an LED + photosensitive device: low rate (hundreds of kbps to a few Mbps), large delay, CTR (current-transfer ratio) degrades with temperature and aging, and LED drive current plus surrounding parts must be designed. A digital isolator modulates the signal and transmits it across the barrier through a chip-scale insulating dielectric (HOPERF CMT series uses SiO2 capacitive isolation, from its proprietary RF technology): rate up to 150Mbps, propagation delay as low as ~9ns, CMTI up to ±150~250kV/μs, about 1.5mA per channel at 1Mbps, barrier life expectancy over 40 years, and built-in UL1577, DIN VDE V 0884-11, CSA, CQC, TUV certifications. For gate drive, bus interfaces and high-speed signal isolation, a digital isolator can basically replace an optocoupler directly — when substituting, match the default output state (suffix 1 = default high / 0 = default low) to the original optocoupler fail-safe logic.
Q: Marked 5kVrms — does that mean it can operate at 5kV long term?
No. 5kVrms is the UL1577 isolation-withstand test value (about one minute of applied voltage without breakdown), not the operating voltage. In design you must check the datasheet operating isolation voltage (e.g. CMT1042 ~1060Vrms, CMT8602X 1500VDC between secondaries) and the package-determined creepage / clearance (only a wide-body package meets stricter safety requirements); transient surges are shared between the surge rating (typically 8kV for general isolators) and front-end protection.
Step 1: set the category by what you isolate
The first fork in isolator selection is not a parameter — it is "what is being isolated":
| What is isolated | Device category | HOPERF representative | Key specs |
|---|---|---|---|
| GPIO / SPI / UART and other general digital signals | Multi-channel digital isolator | CMT812X (2-ch) / CMT804X (4-ch) / CMT826X (6-ch) | DC~150Mbps, tpd ~9ns typ (5V), CMTI ±150~250kV/μs |
| CAN bus (incl. CAN FD) | Isolated CAN transceiver | CMT1042 / CMT1052 (CAN FD 5Mbps), CMT1050 (1Mbps) | ±70V bus fault protection, ±30V common-mode, ISO 11898-2 |
| RS-485 bus | Isolated RS-485 transceiver | CMT83085 (half duplex) | 12Mbps, 1/8 unit load for 256 nodes, bus pins ±12kV system-level ESD |
| I2C sensor / EEPROM | Isolated I2C | CMT810X (e.g. CMT8100N) | Bidirectional auto-direction, both sides 1.8~5.5V level shift |
| Power-switch gate (MOS / IGBT / SiC) | Isolated gate driver | CMT8602X (isolated dual-ch) / CMT8603X (opto-replacement) | 4A source / 6A sink, UVLO 6/9/13V three levels, up to 5MHz switching |
| High-side current / voltage sense | Isolated amplifier / Σ-Δ modulator | CMT1300 (isolated amplifier) and CMT130X family | ±50mV / ±250mV input, fixed gain 8.2 or 41, fault-safe diagnostics |
Step 2: general-purpose digital isolators — how to pick CMT812X / 804X / 826X
The three families share one platform (SiO2 isolation barrier, 2.5~5.5V dual-side supply, -40~125℃, 8kV surge rating, 40+ year life expectancy); they differ in channel count and direction, rate, package rating:
| Model | Channels and direction | Rate and delay | Package and isolation rating |
|---|---|---|---|
| CMT812X | 2 channels (1 forward, 1 reverse) | DC~150Mbps, tpd ~9ns typ (5V), CMTI ±200kV/μs typ | SOIC-8 narrow / SOW8L wide / SOIC-16 wide; 3.75kVrms (narrow) ~ 5kVrms (wide) |
| CMT804X | 4 channels (forward / reverse combos selectable) | family rate 10~150Mbps | SOIC-16 wide / narrow (see datasheet for the exact config) |
| CMT826X | 6 channels (up to 6 forward or 3 reverse) | 150Mbps, CMTI ±250kV/μs typ, tpd ~9ns typ (5V) | SOIC-16 wide / narrow; 5kVrms |
The picking logic is short:
- Good enough is best: isolate only one UART / debug / programming port → CMT812X; SPI four-wire (SCLK/MOSI/MISO/CS) → CMT804X; multiple parallel control and status signals → CMT826X.
- Count the channel directions clearly: list the forward (Side1→Side2) and reverse channel counts against the actual signal flow before matching the model; do not force the default config.
- Default output = fail-safe strategy: each family has a default-high (suffix 1) and a default-low (suffix 0) version. When the input side loses power or the signal is lost, the output stays at the default state — pick by "do you want the load to stop or hold on failure".
- Certification and surge: the whole family passes UL1577, DIN VDE V 0884-11, CSA, CQC, TUV; ±8kV contact discharge across the barrier (IEC 61000-4-2); EMC designed to system-level ESD / EFT / surge / radiated compliance.
The above are family-level typical values; the mapping between channel-direction combinations, rating and package follows each HOPERF model datasheet.
Step 3: bus isolation — one IC for CAN, RS-485, I2C
The essence of bus isolation is to put the digital isolator and the physical-layer transceiver into one chip, halving the board area versus "general isolator + discrete transceiver" and removing the direction-control logic:
- Isolated CAN (CMT10XXX family): CMT1042 / CMT1052 support 5Mbps CAN FD, CMT1050 targets 1Mbps classic CAN. ISO 11898-2 compliant; ±70V DC bus fault protection and ±30V common-mode range handle voltage-domain drift; TXD dominant timeout (DTO), over-current and over-temperature protection are all present; the bus presents an ideal high impedance when unpowered (no line drop). 5kVrms rating + ~1060Vrms operating voltage, VDD1 2.5~5.5V / VDD2 4.5~5.5V, SOW8L / SOIC-16 wide package. First choice for BMS, inverter and motor-control bus ports.
- Isolated RS-485 (CMT83085): half duplex, 12Mbps, 1/8 unit load allows 256 transceivers on one bus, bus pins ±12kV system-level ESD to ground, receiver output stays logic high on input open / short (fail-safe). 5kVrms rating, SOW16 wide package meets creepage. For smart meters, PLC/DCS communication modules, industrial-line actuators.
- Isolated I2C (CMT810X family): SCL/SDA auto-detect direction, no direction-control pin needed; VCC1/VCC2 both 1.8~5.5V for cross-voltage-domain level shift; covers standard / fast / high-speed I2C modes; 3.75kVrms / 5kVrms selectable. For cross-ground isolation between smart meters, sensors and MCU.
Besides the isolator, the bus port also needs surge and ESD protection on the signal side (TVS selection and junction-capacitance trade-off in How to protect RS485 / CAN communication ports?) — the two are "withstand" and "survive" and neither can be missing.
Step 4: power-switch gate drive — CMT8602X / CMT8603X
Isolated-driver selection adds a layer of "drive capability" over general isolators: source/sink current, propagation delay and pulse-width distortion, UVLO (under-voltage lockout) levels, and half-bridge / dual-low-side / dual-high-side configuration.
CMT8602X is an isolated dual-channel gate driver: 4A peak source + 6A peak sink current, typical propagation delay 40ns, minimum pulse width 50ns, maximum delay matching 5ns, maximum pulse-width distortion 9ns, minimum CMTI ±150kV/μs, drives MOSFET / IGBT / SiC up to 5MHz switching; each channel configurable as dual-low-side, dual-high-side or half-bridge with programmable dead time; VDD output drive supply up to 36V; VCCI 3~5.5V compatible with digital and analog controllers; functional isolation between the two secondary drivers supports up to 1500VDC operating voltage and rejects input noise pulses shorter than 25ns.
| Model | Drive type and package | Isolation rating | UVLO level |
|---|---|---|---|
| CMT8602A-N / B-N / C-N | dual-drive, SOIC-16 narrow | 3kV | 6V / 9V / 13V |
| CMT8602A-K / B-K / C-K | dual-drive, SOIC-14 wide | 5kV | 6V / 9V / 13V |
| CMT8603B-WF / C-WF | opto-isolation replacement, SOW6 (also DUB8) | 5kV | 9V / 13V |
Pick the UVLO level by the drive-voltage platform: 9V platform driving low-voltage MOS → B level; IGBT / SiC common 15~18V platform → C level (13V); 6V level for 5V platform or special logic; for single isolated drive migrating from an optocoupler scheme, see CMT8603X. Bootstrap vs isolated supply for the half-bridge high side in How to power the half-bridge high-side (bootstrap)?; gate-resistor and dead-time setting in How to design a MOSFET gate-drive circuit?.
Step 5: high-side sensing — CMT1300 isolated amplifier
High-voltage bus current sampled through a shunt resistor must be sent safely to the low-side MCU with an isolator. CMT1300 is a reinforced isolated amplifier for this position: fully differential input is sampled by an on-chip Σ-Δ modulator and transmitted across the barrier, then differentially output after a fourth-order analog filter on the secondary side.
- Two input ranges: CMT1300D05 (±50mV linear input) and CMT1300D25 (±250mV), fixed gain 8.2 or 41 (by model), directly matching different shunt resistors — shunt selection is the "power vs accuracy" trade-off.
- Accuracy: max offset error ±0.1mV, max gain error ±0.3% (drift max ±50ppm/℃), max nonlinearity ±0.03%, typical SNR 86dB (BW=10kHz), typical bandwidth 310kHz, meeting the dynamics of phase-current loops and bus-current monitoring.
- System-level diagnostics (fail-safe): built-in VDD1 under-voltage detection and input common-mode over-voltage detection drive the output into a predefined fail-safe state on trigger — exactly the "can detect, and can know it detected wrong" block in functional safety.
- Rating and temperature: 5000Vrms (UL1577), CMTI typ 150kV/μs, -40~125℃, SOW8 package, dual supply 3.0~5.5V.
The CMT130X family also covers isolated Σ-Δ modulator architectures for high-precision cases that put digital filtering on the MCU side. In the high-voltage box of energy storage / BMS, the coordination of this isolated sensing with cell balancing and pre-charge main-loop design is in How to design BMS cell-balancing? and How to design BMS pre-charge?.
Design notes and common pitfalls
| Position | Note | Common pitfall |
|---|---|---|
| PCB layout across the barrier | no trace, copper or part on either side under the isolation gap; keep creepage and clearance per safety standard | running a signal line or ground under the gap destroys safety test and immunity at one stroke |
| Decoupling on both sides | 0.1μF close decoupling on VCC1 / VCC2, plus 1~10μF if needed | only one side or too far from the pin, bit errors at high rate |
| Default output | pick suffix 1 (default high) / 0 (default low) by fail-safe logic | getting the opposite default output reverses load behavior on failure |
| CMTI and dv/dt | SiC / GaN systems pick CMTI at 2~3x measured max dv/dt | IGBT-era rule applied directly to SiC, occasional bit error at high edges |
| Secondary-side supply | the isolator secondary must be powered by an isolated auxiliary supply | forgetting the isolated supply side connects primary and secondary ground through the supply, isolation is void |
| Coordination with protection | bus-port isolator + signal-port TVS staged, large surge current drained by the front stage | isolator only, no TVS, surge directly destroys the transceiver |
The secondary isolated supply can be unified with the on-board flyback auxiliary supply, in How to make an on-board auxiliary supply?; the MCU-side resource and peripheral coordination in Which MCU for energy-storage BMS? GD32 selection guide.
Related solutions
- Half-bridge high-side supply: bootstrap vs isolated supply boundary: How to power the half-bridge high-side (bootstrap)?
- Gate resistor, Miller plateau and dead-time setting: How to design a MOSFET gate-drive circuit?
- Bus-port TVS protection and junction-capacitance trade-off: RS485 / CAN communication port keeps failing — what to do?
- Isolated auxiliary supply for the isolator secondary: How to make an on-board auxiliary supply?
- High-voltage-box main loop and sense-line protection: How to select BMS protection-board TVS?
- Full foldback TVS model list and cross-reference: AMSEMI foldback TVS full model list and cross-reference
- TVS principles, parameter reading and selection primer: What is a TVS?
- MCU and isolator channel planning: Which MCU for energy-storage BMS? GD32 selection guide
FAQ
Q1: What is the difference between a digital isolator and an optocoupler, and when can they replace each other?
An optocoupler transmits the signal across the barrier with an LED and a photosensitive device: data rate is typically hundreds of kbps to a few Mbps, propagation delay is large (microsecond range common), current-transfer ratio (CTR) degrades with temperature and lifetime, and periodic re-rating is needed. A digital isolator modulates the signal and transmits it across the barrier through a chip-scale insulating dielectric (HOPERF CMT series uses SiO2 capacitive isolation), reaching up to 150Mbps, propagation delay as low as ~9ns, CMTI up to ±150~250kV/μs, lower power, longer life expectancy (40+ years) and built-in safety certifications (UL1577, DIN VDE V 0884-11, etc.). For ordinary signal isolation, gate drive and bus interfaces, a digital isolator can basically replace an optocoupler directly and drop the surrounding design — but watch three points: the default output state must match the optocoupler (isolators come in default-high and default-low suffix variants); the optocoupler primary/secondary supply ranges do not exactly match the isolator; and some legacy analog-transmission uses of optocouplers do not suit digital isolators — use an isolated amplifier or Σ-Δ modulator instead. Verify against both datasheets and the system safety requirement before substituting.
Q2: Does an isolation rating of 5kVrms mean the device can operate at 5kV long term?
No. 5kVrms is the UL1577 component-certification isolation-withstand test value (about one minute of applied voltage without breakdown), not the long-term operating voltage. A digital isolator datasheet also lists an operating isolation voltage (some vendors call it VRMI or VIOWM): HOPERF CMT1042 isolated CAN transceiver is about 1060Vrms, and CMT8602X isolated driver supports up to 1500VDC between its two secondary sides — this is what you use to check the system bus voltage, creepage and clearance. Check in three layers: the system long-term operating voltage to ground or across the barrier must be below the device operating voltage with margin; the package-determined creepage and clearance must meet the system safety standard (IEC 62368-1, IEC 60601-1 or GB 4943.1); and transient surges / lightning are shared between the isolator surge rating (typically ~8kV for general isolators) and front-end protection. Final values follow the insulation table of the selected datasheet.
Q3: What is CMTI (common-mode transient immunity) and how much is enough?
CMTI is the isolator's ability to keep transmitting the signal correctly without false switching when the ground potential across the barrier jumps very fast (common with large dv/dt from power-switch switching), in kV/μs. HOPERF CMT general isolators are typically ±150~250kV/μs by model; isolated interfaces and isolated drivers are around ±150kV/μs. Whether it is enough depends mainly on the isolated power-switch speed: silicon IGBT and ordinary MOSFET switching edges are mostly tens of kV/μs, so ±150kV/μs usually has several times margin; SiC MOSFET and GaN high-speed switches can exceed 100kV/μs dv/dt and the gate driver sits right next to the switching node, so pick a model with larger CMTI margin and design together with PCB layout (clearance under the barrier, minimized secondary-side drive loop) and negative-voltage turn-off. As a rule, pick CMTI at 2~3x the measured maximum system dv/dt and verify bit errors with a high-dv/dt edge at prototype stage. Concrete values follow the datasheet and whole-system measurement.
Q4: Which positions in a BMS / energy-storage system need isolation, and which devices are used?
Four typical positions. (1) Communication interface: the battery pack external CAN and RS-485 buses must be isolated — use an isolated CAN transceiver (e.g. CMT1042, 5Mbps CAN FD, ±70V bus fault protection) and an isolated RS-485 transceiver (e.g. CMT83085, 12Mbps, 1/8 unit load for 256 nodes). (2) Power-switch gate drive: the high-side and low-side switches of a bidirectional DC/DC and inverter bridge need primary-secondary isolation — use an isolated gate driver (e.g. CMT8602X dual-channel, 4A source / 6A sink, configurable as dual-low-side, dual-high-side or half-bridge); for high-dv/dt SiC, check CMTI especially. (3) High-voltage sensing: bus current sampled through a shunt resistor must be isolated to the low-side MCU — use an isolated amplifier or Σ-Δ modulator (e.g. CMT1300, ±50mV/±250mV input, fixed gain 8.2/41, fail-safe output with VDD1 under-voltage and input common-mode over-voltage detection). (4) Inter-board general digital signals (GPIO, SPI, UART, I2C) — use multi-channel digital isolators (e.g. CMT812X/804X/826X) or isolated I2C (CMT810X). The isolator secondary side also needs an isolated auxiliary supply, which can be unified with the on-board flyback auxiliary supply.
Contact us
For HOPERF CMT-series isolator (digital isolator / isolated interface / isolated driver / isolated sensing) selection check, optocoupler-replacement cost-down evaluation, sample application and certification documents, please contact us.
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Disclaimer: This article is technical popular-science and industry exchange content, for reference only, and does not constitute selection, procurement or other business-decision advice. The parameters of isolation rating, operating voltage, CMTI, data rate, propagation delay, drive current, UVLO level and sensing accuracy are taken from HOPERF official public materials (website product pages and datasheets) as typical values or range examples; they differ significantly by model, package and operating condition. Actual design must follow the selected model original datasheet, self-designed circuit calculation and whole-system measurement.