The ISO721, ISO721M, ISO722, and ISO722M are digital isolators with a logic input and
output buffer separated by a silicon dioxide (SiO2) insulation barrier. This
barrier provides galvanic isolation of up to 4000 VPK per VDE. 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
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 µs, the input is assumed to be
unpowered or not being actively driven, and the failsafe circuit drives the output to a logic-high
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 fast operation with signaling rates from 0 Mbps (DC) to 100 Mbps for the ISO721 and the ISO722
devices, and 0 Mbps to 150 Mbps with the ISO721M and the ISO722M devices.
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.
The ISO722 and ISO722M devices include 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 devices have TTL input thresholds and a noise filter at the
input that prevent transient pulses of up to 2 ns in duration from being passed to the output of
The ISO721M and ISO722M devices have CMOS VCC / 2 input
thresholds, but do not have the noise-filter and the additional propagation delay. These features
of the ISO721M device also provide for reduced-jitter operation.
The ISO721, ISO721M, ISO722, and ISO722M devices are characterized for operation over the
ambient temperature range of –40°C to 125°C.
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