RF was only briefly mentioned as a mechanism for crossing the isolation barrier. Can you address the benefits of this technology against optical, capacitive or inductive means?
Asked by Jon K.
This is a complex question because the advantages of a technology are not linked just to the medium used to transmit the information. Some properties are affected by the how thick the insulation can be, for example Polymers like Polyimide or Epoxy resin can be made very thick and are very tolerant of transients, where SiO2 tends to have an upper limit due to stress in the coating.
The encoding scheme has a lot to do with the performance of an isolator, On Off Keying is very fast and robust, but is a power hog in all three media, but especially with optical systems, This is the RF scheme you asked about.
Pulse encoding is more challenging to implement since DC states must be encoded as well, but can be very low power. Optical systems can not be pulsed on and off quickly, so they are not good for pulse encoding. The inductive and optical fields are not as strongly attenuated by the thickness of the insulation that they have to cross as capacitive coupling. So it is actually the combination of these things that determines the properties of the isolator.
For the ADuM144x and ADuM124x parts ADI chose pulse encoding with a relatively thick barrier which keeps the low frequency power consumption down, but also give excellent surge performance. Another popular combination is capacitive coupling and SiO2 as an insulator, this gives good prop delay and high power consumption on both input and output since the transmitters can stay on for long time and the receivers draw quiescent current. The SiO2 barrier is not as robust to high voltage transients and is difficult to push any further.
Opto systems have thicker insulation and good surge properties, but are very power intensive, and limited in speed and inconsistent in prop delay. So the the medium is only part of the system and they must be looked along with the other considerations when constructing a system. The combination we chose is Inductive, single ended pulse encoding with slow refresh.
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