Digital Isolator Selection: Working Voltage, CMTI, and Channel Direction

Table of Contents

• Define the Barrier from the System Requirements
• Draw the Channel Map Before Choosing the Part
• Evaluate CMTI Under the Conditions That Matter
• Include Delay, Power Sequencing, and Default Outputs
• Release an Exact Configuration, Not a Family Name

A digital isolator can pass logic signals across a galvanic barrier, but choosing one requires two parallel reviews: whether the insulation is suitable for the application and whether the signals remain correct through every operating state. A headline isolation voltage answers neither review on its own.

When the isolated output must source and sink gate current and protect a power switch, use the isolated-gate-driver selection workflow. A logic isolator and a gate driver can share insulation concepts without being interchangeable functions.

For procurement, the approved item must include the full ordering code and package. For engineering, it must include the channel map, timing budget, power behavior, and insulation evidence used in the design. Keep those records together so that a suffix change cannot bypass the original reasoning.

Define the Barrier from the System Requirements

Identify the circuits on each side and why they are isolated. The purpose may involve safety, functional separation, or ground-potential differences. The applicable equipment requirements, operating voltage, transients, environment, and intended service conditions determine what evidence is needed.

TI’s digital-isolator selection brief distinguishes working, repetitive, transient, and withstand-voltage parameters. A short-duration withstand test is not permission to operate continuously at that voltage. Likewise, a peak value and an RMS value cannot be compared as if their units and stress conditions were identical.

Ask the design owner to document the required insulation category and relevant standards before selecting a package. Component approvals are inputs to the equipment assessment. PCB spacing, connectors, power conversion, contamination assumptions, and other paths across the barrier can still determine whether the complete system meets its requirements.

Review item	Evidence to compare
Operating stress	Working and repetitive-voltage ratings under applicable conditions
Transient stress	Relevant transient or surge ratings and waveform definitions
Package separation	Creepage, clearance, and package-specific documentation
Approval scope	Certificate or datasheet coverage for the exact device and package
System implementation	Board layout, power barrier, and equipment-level assessment

Do not select the smallest package first and assume that a high headline rating resolves all spacing questions.

Draw the Channel Map Before Choosing the Part

Write each signal on the barrier drawing and mark its direction. A UART may need one channel each way; a particular SPI implementation may need several forward signals and a return signal. Additional chip selects, interrupts, or fault outputs can change the required count.

The ADuM1201 manufacturer page specifies two channels with 1/1 directionality. The ADuM1400 page specifies four channels with 4/0 directionality. These are different signal maps, not interchangeable amounts of “isolation.”

The catalog entries for ADUM1201CRZ and ADUM1400BRWZ therefore illustrate two distinct configurations. Do not infer the highest speed listed on a family page for every ordering grade. The ADuM1400 family ordering and electrical tables distinguish speed grades within the family.

A useful procurement comparison has separate columns for channel direction, package, speed grade, output behavior, and approval scope. A single “four-channel isolator” description hides too much to control substitutions.

Evaluate CMTI Under the Conditions That Matter

Common-mode transient immunity describes signal operation while the voltage between the two ground domains changes rapidly. It is not the data rate of the digital signal and is not a substitute for insulation voltage ratings. TI’s selection brief treats CMTI as a separate parameter for this reason.

For a first estimate of the application’s stress, relate the change in common-mode voltage to its transition time. An explicitly hypothetical 800-V change in 20 ns corresponds to 40 kV/µs. This calculation is not a device qualification result; it gives the engineer a stress estimate to refine using actual waveforms and measurement conditions.

Compare the guaranteed minimum and test conditions rather than only a typical marketing figure. Review polarity, supply conditions, temperature coverage, and the logic state used in the specification. Then validate the relevant states in the actual circuit, including startup and switching conditions that may not appear during steady operation.

Avoid probing hazardous or floating circuits with an unsuitable oscilloscope connection. The measurement plan needs equipment and procedures appropriate to the system’s voltage and isolation requirements.

Include Delay, Power Sequencing, and Default Outputs

An isolator adds propagation delay and may introduce pulse-width distortion or channel-to-channel skew. In a clocked interface, evaluate the entire path, including the outbound clock, the peripheral’s response, and any return signal. Maximum data rate alone does not establish setup and hold margin.

Power sequencing deserves its own test cases. Define what the receiving circuit must see when one side is unpowered, an input is floating, an enable changes state, or the controller resets. Compare that requirement with the exact device’s documented default behavior. A safe output for one actuator may be an unwanted command for another.

Also establish how the isolated side receives power. A signal isolator does not automatically supply isolated power. If a separate converter crosses the barrier, its rating, layout, and fault behavior belong in the same system review.

For measurement and control applications, the sensor signal-conditioner sourcing discussion provides adjacent signal-chain context. The isolation decision still needs its own electrical and safety evidence.

Release an Exact Configuration, Not a Family Name

Before accepting an alternate, retain the channel diagram, timing assessment, applicable certificates or approval references, and the test report for the board configuration. Record any assumptions about supply voltage, switching waveform, environment, and service conditions.

Use the Analog Devices catalog to locate sourcing candidates, then verify specifications with the manufacturer documentation. Catalog availability is not a guarantee of functional equivalence or system certification.

The final approval should tell receiving and manufacturing exactly which code may be used and tell engineering what would trigger a renewed assessment. A change in package, channel direction, output default, or qualification scope should remain visible even when the commercial description still says “digital isolator.”