Isolation Tech 101, Part 1: Digital Isolators | Symmetry Blog

Many considerations come into play when choosing the right isolator for your application. In a new blog series from Symmetry Electronics, we will be covering the different types of isolators available. For Part 1, we will be providing an overview of digital isolators, how they work, and when to use them.

Keep reading to learn everything you need to know about digital isolators.

Digital Isolators

In: Digital control or data signal
Out: Reproduced digital control or data signal

Digital isolators are a buffer for digital signals that provide  protection from high voltages and noise

• Can have single to multiple channels
• Available in different isolation ratings: 2.5kVrms & 5kVrms are most common
• Critical features are latency, skew, noise immunity and EMC considerations (emissions and susceptibility)

Why use a digital isolator?

Digital isolators are most commonly used when potential ground differences are present. Sensor inputs can operate at varying voltages, ranging from as low as 3 volts to 48 volts or higher, and a digital isolator helps provide for this type of application.

For example, if the microprocessor is operating at 3.3 volts and the inputs range from 24 volts to 48 volts, this could cause a significant potential difference in ground voltages, which can introduce damaging voltage levels to the devices present, skew sensor data, and introduce errors. Some form of isolation is needed to ensure accuracy. The sensor signal is usually conditioned by filters, protection circuits, an amplifier, and digitized by an ADC. This is the data signal that's needed by the PLC processor to function.

A digital isolator is used to eliminate any errors due to ground loops. And it's desirable for the digital isolator to have a low latency or propagation delay, low noise, and a high data rate. In effect, the less a digital isolator is visible to the input signal, the better.

How does a digital isolator work?

Digital isolators couple data across an isolation barrier. This is achieved by using a modulator to transmit high frequency carrier across the barrier to represent either a high or low digital state and no signal to represent the other state. The receiver demodulates the signal after advanced signal conditioning to produce an isolated output through a buffer stage.

Digital isolators use single-ended CMOS or TTL logic switching technology. The voltage range normally ranges from 3 volts to 5.5 volts for both supplies, VCC1 and VCC2, though some devices may support a larger supply voltage range. When designing the digital isolators, it is important to keep in mind that due to the single-ended design structure, digital isolators do not conform to any specific interface standard and are only intended for isolating single-ended digital signal lines.

Careful consideration of layouts should be used when using a digital isolator. A minimum of four layers is required to accomplish a low EMI PCB design.

Layer stacking should be in the following order from top to bottom:

1. high-speed signal layer
2. ground plane
3. power plane
4. low frequency signal layer

Routing the high-speed traces on the top layer avoids the use of vias and the introduction of air inductances and allows for clean interconnects between isolator and the transmitter and receiver circuits of the data link.

Placing a solid ground plane next to the high-speed signal layer establishes controlled impedance for transmission light interconnects and provides excellent low inductance path to the return current flow. Placing the power supply next to the ground plane creates an additional high frequency bypass capacitance. Routing the slower-speed control signals on the bottom layer allows for greater flexibility, as these signal lengths usually have margin to tolerate discontinuities such as vias.

If an additional supply voltage plane or signal layer is needed, add a second power or ground plane system to the stack to keep it symmetrical. This makes the second mechanically stable and prevents it from warping. Also, the power and ground plane at each power system can be placed closer together, thus increasing the high frequency bypass capacitance significantly.

What are ideal applications for Digital Isolators?

Industrial automation

• High noise immunity
• Reliability (high voltage lifetime)
• Signal integrity (failsafe data & low skew)
• UL, VDE/IEC certification

EV/HEV

• Low emissions
• Reliability (high voltage lifetime)
• Signal integrity (failsafe data & low skew)
• AEC-Q200 qualification

Power supply solutions

• Low latency, skew
• Protection features (overlap, UVLO etc.)
• Reliability (high voltage lifetime)
• UL, VDE/IEC certification

Inverters – solar & motor control

• Low latency, skew
• Reliability (high voltage lifetime)
• Signal integrity (failsafe data & low skew)
• UL, IEC/VDE certification

Where do I find the best Digital Isolators for my needs?

Silicon Labs has a single family to fit all needs, the Si86xx

• Si86xx: 1-6 channel, 1kv – 5kV ratings, low IDD
• Si86xxT: 10kV surge
• Offers the best noise immunity, latency, and emissions on the market 

Silicon Labs’ products provide the best performance and reliability in the industry. With Silicon Labs' isolators, new designs receive:

• Highest noise immunity
• Fastest data rates & lowest prop delays
• Failsafe data transmission
• Lowest EMI emissions
• Highest reliability

Looking to integrate Silicon Labs products with your design? Our Applications Engineers offer free design and technical help for your latest designs. Contact us today!