2 Overview

The ISO721 and ISO721M digital isolators have a logic input and output buffer separated by a silicon oxide (SiO₂) insulation barrier. Used with isolated power supplies, these devices prevent noise currents on a data bus or other circuits from entering the local ground and interfering with or damaging sensitive circuitry.

A binary input signal is conditioned, translated to a balanced signal, and then differentiated by the SiO₂ isolation barrier. Across the isolation barrier, a differential comparator receives the logic transition information, then sets or resets a flip-flop and the output circuit accordingly. A periodic update pulse is sent across the barrier to ensure the proper dc level of the output. If this dc-refresh pulse is not received for more than 4 μs, the input is assumed to be unpowered or not functional, and the fail-safe circuit drives the output to a logic-high state.