Which Types of Opto-Isolators Are Right For Your PCB Signal?

 Opto-isolators, also called optocouplers, provide indispensable galvanic isolation between circuit sections in a vast array of electronics applications. These clever devices incorporate an LED and phototransistor or photodiode enclosed in a package that blocks current but allows light transmission. This enables opto-isolators to transfer signals between circuits while preventing noise, voltage issues, and ground loops.

With an array of opto-isolator types available, it can prove challenging to select the optimal isolator to carry and protect your signals on a printed circuit board (PCB). Important aspects that factor into the decision include signal type, voltage levels, speed, linearity, power limitations, package style, isolation ratings, and cost.

This guide takes an in-depth look at key opto-isolator classifications, technologies, and specifications to consider. It arms PCB designers with essential knowledge to expertly match opto-isolator characteristics to application requirements. After reviewing isolator types and traits, a decision flowchart guides appropriate device selection for optimal balancing of critical parameters like bandwidth, power, isolation ability, package size, and budget.

Isolator Classifications

The first step toward finding the right opto-isolator lies in understanding basic classifications based on signal type, output configurations, package style, and features.

Digital Versus Analog Opto-Isolators

The most fundamental isolator distinction involves support for either digital or analog signals:

Digital Opto-Isolators

• Transfer binary on/off signals
• Fast switching speeds
• Lower linearity needs
• Example: 6N137 high-speed digital isolator

Analog Opto-Isolators

• Transfer continuous variable signals
• Linear transfer response
• Wider bandwidth when needed
• Example: IL300 linear analog isolator

So first determine whether you require isolation of a digital trigger or pulse, or an analog waveform like audio or sensor measurement.

Output Configurations and Modes

Another vital differentiator lies in output stage and mode variations:

Transistor Output

• NPN or PNP transistor output stage
• Can drive higher loads than diode
• Examples: 4N25, TLP2767, ACPL-C87x

Diode Output

• Output light generates current
• Lower load driving capability
• Example: OPI1277A

Digital Compatible

• Functions like digital output
• Fast edge speeds for pulse response
• Examples: HCPL-07xx, HCPL-3700

Linear Analog Output

• Linear transfer function
• Accurate reproduction of analog signals
• Examples: IL300, ACPL-C79x

Operational Amplifier Output

• Integrated op amp output stage
• Can handle very low outputs
• Example: HCPL-7800

Comparator Compatible

• Built-in comparator for analog signals
• Adjustable reference levels
• Example: ACPL-C870

So in terms of output, consider driving capability, impedance, modes like linear versus digital compatibility, reference customization needs, and downstream circuit integration requirements.

Packaging Choices

When exploring opto-isolators, you will encounter some common package options:

DIP

• Dual in-line package
• Through-hole mounting
• Example circuits: 4N35, TLP621

SMD

• Surface mount device
• Saves space versus DIP
• Examples: FOD819C, TLP291–4

SMT Flat Lead

• Surface mount with flat leads
• Allows visual inspection
• Example: EL30515T

SOIC

• Small outline integrated circuit
• Analog chips with high pin counts
• Example: ACPL-P346

SIP/DIP

• Single/dual in-line package
• Through-hole single channel
• Example: HCPL4502, HCPL-3600

High Density Housings

• Shrink package size
• Tradeoff reduced creepage/clearance
• Examples: SO-6, SSOP-6, TSSOP-6

Device package drives board space efficiency, visual inspection ability, through-hole versus SMT, and creepage/clearance needs.

Key Opto-Isolator Specifications

Beyond classifications, opto-isolator types differ significantly across critical electrical and performance specifications. These parameters can make or break suitability for an application, so carefully matching device traits to design requirements proves essential.

Breakdown Voltage

A prime specification called breakdown voltage or isolation voltage indicates the voltage level at which the isolator insulation breaks down, allowing current to flow between input and output. Common levels include:

• 1kV to 5kV for signal level isolation
• 15kV to 25kV for power isolation

Higher breakdown voltages provide greater immunity to transients and noise but increase package size and cost.

Propagation Delay

This defines the time taken for the output to respond to an input change. Fast pulse response requires low propagation delay — common levels in nanoseconds include:

• 1ns to 10ns: high speed digital
• 10ns to 500ns: standard digital
• 500ns to 5μs: slower analog isolators

To prevent output signal distortion, propagation delay must be shorter than the input signal’s rise/fall time.

Bandwidth or Response Time

Bandwidth indicates the range between highest and lowest signal frequencies that can pass through the isolator. This trait directly impacts response time. For analog signals, higher bandwidths allow faster rise/fall edges. Range examples include:

• 100kHz to >10MHz: High frequency digital isolators
• 100Hz to 150kHz: Linear analog isolators

So opto-isolators tailor bandwidth optimization for either linearity or speed based on analog versus digital focus.

Accuracy and Linearity

Analog opto-isolators concentrating on linear transfer require precision correlation between input and output signals, rated by:

• Total Harmonic Distortion (THD)
• Nonlinearity

Lower distortion and nonlinearity provides higher signal accuracy for analog sensor measurements and other precision applications.

Temperature Stability

Opto-isolators exhibit some variation in characteristics across operating temperature ranges. Key traits regarding thermal change include:

• Output Current Temperature Coefficient
• Propagation Delay Temperature Coefficient

Temperature coefficients approaching zero deliver consistent operation regardless of ambient heat levels — an important requirement in harsh environments.

Switching Speed

For handling rapid digital pulses, fast edge speeds rated in voltage per nanosecond, such as 1V/ns and higher, enable sharp digital input/output response without distortion.

Output Current Rating

The sustained forward current that the phototransistor can provide differs widely, with common levels falling between 15mA to 300mA. Higher current directly increases load driving capacity.

Power Dissipation Rating

This indicates maximum power the device can handle before overheating, with common ratings of 100mW to 1W. This ties closely to thermal design.

Isolation Rating

Also called insulation resistance, this signifies resistance between input and output sections, rated from megaohms up to teraohms. Higher values enhance noise immunity.

Safety Standards

For high voltage industrial applications, many opto-isolators carry safety agency certifications, such as:

• UL1577 for isolation equipment
• IEC 60747–5–5 for optoelectronics
• CSA Component Acceptance Notice #5A marking

Certification ensures suitability for human safety in heavy equipment and high power environments.

Opto-Isolator Selection Flowchart

With the background on key specifications and customization options covered, the following flowchart provides generalized guidance through critical branching decisions that narrow down toward the optimal opto-isolator choice for your unique signal isolation needs:

This flowchart poses key questions on:

• Signal type: Analog or digital?
• Voltage: Low, medium or high level signals?
• Speed: Slow, moderate or fast edge requirements?
• Accuracy: General purpose or precision linearity needs?
• Power and loads: Light or heavy drive requirements?
• Interface: Compatibility needs with downstream circuits?
• Environment: General commercial grade or rugged?
• Budget: Low cost or premium grade?

Traversing the tree by answering these questions related to critical parameters guides intelligent selection tailoring device characteristics to your specific application criteria.

The following sections explore five common opto-isolator product families aligned to typical design scenarios, mapping specifications back to the flowchart traits.

Digital Signal Opto-Isolators

For standard logic trigger and pulse transfer in digital circuits, families like the HCPL-07xx and HCPL-3700 provide robust low cost isolation.

Typical applications: Digital signal transfer, buffering, pulse trains, noise immunity