7 Optocoupler Current Input vs Digital Isolator CMOS Voltage Input

All optocoupler inputs are current-driven and require > 2 mA of steady bias current for the device to operate. Many optocouplers may need > 10 mA of input current for them to meet minimum application performance requirements. This makes them less suitable to be directly driven by any TTL or CMOS outputs and hence they may need a buffer to be able to drive the optocoupler.

Optocouplers are also not suitable to be used with low-voltage digital circuits (< 3.3 V) as the optocoupler performance can drastically change with a small change in input voltage. Digital isolators like ISO6741 offer high impedance CMOS inputs that are voltage driven. The CMOS inputs consume a maximum of ±10 µA of steady current and hence can be directly driven by any TLL or COMS outputs without requiring any external buffer. This makes them compatible to be directly interfaced with most other digital devices like MCU, ADC, and so on.

Digital isolators can also work with a wide range of power supply and logic voltage levels and also support 1.8-V low-voltage operation. Some variation in input supply voltage or logic voltage levels also does not affect the output logic voltage levels. The input capacitance of digital isolators (about 1.3 pF for ISO6741) is also significantly low compared to an optocoupler (about 60 pF for a typical high-speed optocoupler), thereby making digital isolators switch much faster and easier compared to optocouplers.