Capacitive isolation moves on-chip
By Richard A Quinnell, Contributing Editor -- EDN, February 16, 2006
Sporting an isolation rating of 560V operating and 4000V transient, the ISO721 and ISO721M interface devices from Texas Instruments (picture) use on-chip capacitors to protect signal lines from becoming pathways to disaster. The result is isolation that is three times faster than optical and 35% faster than inductive, with speeds as high as 150 Mbps. The design also uses less power than optical isolation, is virtually immune to the magnetic fields that plague inductive isolation, and has an estimated MTBF of 25 years.
The ISO721 includes built-in noise filtering to eliminate transients shorter than 2 nsec with a 100-Mbps throughput. The ISO721M eliminates the filter to boost throughput to data rates as great as 150 Mbps and reduce latency from 17 to 10 nsec. Both devices have high-voltage transient protection to 25 kV/µsec.
Capacitive coupling normally imposes a highpass-filter response on the signal, but the ISO721 family eliminates that restriction. An input-signal-conditioner stage converts the incoming signal to differential signaling, coupling through on-chip capacitors to a receiver stage. In parallel, a pulse-width-modulation generator senses the low-frequency component of the incoming signal and encodes it before connecting to the receiver stage. The receiver stage recombines both the high- and the low-frequency signal components, replicating the input signal with a 4-µsec delay on the low-frequency component.
The ISO721 multichip modules have two stages of isolation, which take place on separate die; the only connections between them are the bond wires from the first stage to the integrated capacitors that serve as the second stage's input. The capacitive coupling, along with a split lead frame and two-die construction, ensures that no dangerous signals can couple from the input to the output. Devices in the ISO721 family cost $1.65 (1000) and are available now.
Texas Instruments, www.ti.com.
