The ISO721 and ISO722 are digital isolators with a logic input and output buffer separated by a silicon oxide (SiO2) insulation barrier. This barrier provides galvanic isolation of up to 4000 V. Used in conjunction 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, then differentiated by the capacitive 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 us, the input is assumed to be unpowered or not functional, and the failsafe circuit drives the output to a logic high state.
The symmetry of the dielectric and capacitor within the integrated circuitry provides for close capacitive matching and allows fast transient voltage changes between the input and output grounds without corrupting the output. The small capacitance and resulting time constant provide for very fast operation with signaling rates† from 0 Mbps (DC) to 100 Mbps for the ISO721/ISO722, and 0 Mbps to 150 Mbps with the ISO721M/ISO722M.
These devices require two supply voltages of 3.3 V, 5 V, or any combination. All inputs are 5 V tolerant when supplied from a 3.3 V supply and all outputs are 4 mA CMOS.
ISO722 device includes an active-low output enable that when driven to a high logic level, places the output in a high-impedance state and turns off internal bias circuitry to conserve power.
Both the ISO721 and ISO722 have TTL input thresholds and a noise-filter at the input that prevents transient pulses of up to 2 ns in duration from being passed to the output of the device.
The ISO721M and ISO722M have CMOS Vcc/2 input thresholds and do not have the input noise-filter and the additional propagation delay. These features of the ISO721M also provide for reduced jitter operation.
These devices also provide for glitch-free power up/down operation when powered up/down in typical applications. For more information, see Theory of Operation in the Application Information section.
The ISO721 and ISO721M are characterized for operation over the ambient temperature range of - 40°C to 125°C.
† The signaling rate of a line is the number of voltage transitions that are made per second expressed in the units bps (bits per second)