Design review: Barrier circuits

[krook]

Evaluate how to integrate barrier circuits for better reliability in hazardous environments.

For intrinsic safety, every signal from every component which is exposed to the hazardous environment (i.e. directly exposed to gasses from the environment) will need a barrier circuit. Typically, this is done with two Zener diodes, rated appropriately for the temperature they are exposed to, and one or two limiting resistors. Every signal will need to be evaluated individually at the time of the actual design process. This may necessitate that the product is larger than currently anticipated, as these circuits generally take up a non-negligible amount of space.

These barrier circuits will also need to be carefully designed and tested to ensure that their function does not interfere with the proper function of the bus being protected. For instance, USB designs are not recommended because the protection circuits will likely add too much resistance and capacitance than the protocol allows. This will need to be individually evaluated on each sensor. Commonly, issues will come up if the sensor needs high-speed signaling, especially differential, if it has a low maximum bus capacitance, or if the signal is precision analog.