Selecting the right digital isolator for your design

Most digital isolators are designed with specific system requirements and applications in mind, so it’s worth considering whether the device you select meets your systems’ requirements. When selecting a digital isolator, the following tips may help narrow down your choices.

1. Understanding your isolation specification requirements
Before selecting your digital isolator, some parameters in the datasheet related to the isolation design are worth considering.

  • Isolation withstand voltage
    General regulatory requirements often dictate this specification, which represents the voltage the isolator can handle with breakdown for at least 60s.
  • Working voltage
    Some factors such as package size, pollution degree and material group may affect the working voltage of a component.
  • Surge isolation rating
    If your design requires reinforced isolation, you may consider this parameter and ensure this isolator can withstand <10-kV surge pulse.
  • Creepage/ Clearance
    This parameter will be dictated by the isolator’s package and lead frame.
  • Common mode transient immunity (CMTI)
    If your design is in a noisy environment, higher CMTI rating will be important for your digital isolator. To learn more CMTI, please refer to this post:
  • Power consumption
    Is overall system power consumption a critical specification in your design? You may consider the per-channel current consumption specification of each isolator device if so.
  • Data rate
    What data rate does your communication interface require?

2. Choose the right package
Packages may be very different in terms of isolation, because the size and characteristics of the package directly affect the high voltage capability of the device. Larger packages with larger creepage distances and clearances will allow higher isolation voltage specifications. Small packaging can save board space and cost. In addition, you may consider how many isolation channels your communication interface requires, because the number of channels also impacts the type of package.

3. Determining the number of required channels and configuration
Before selecting your isolator, you need to determine how many channels you need to isolate your signals and which direction each signal will go. Those will determine the number of required channels and configuration. Also considering the preferred default output state (or fail-safe state) will help to determine the predefined state of output pine (high or low) when input channel of a digital isolator is unpowered or the pine are left floating. Options may be available for both default-high or default-low outputs.