The simplest circuit to use is a shunt resistor and differential operational amp

Nice information for how to evaluate and select the right digital controller for your design

The clever “solar dice” design kit mentioned in the article (see https://goo.gl/YRPsFf). Anybody care to share application ideas?

The article referenced (see http://goo.gl/ZGlUQ4) has a very good discussion of microwave cavities, which are the equivalents of L-C resonant circuits

Power management is now inherently mixed signal. Need to know both digital and analog (see https://goo.gl/bn5Va4)

All designers of ultra-low power systems are concerned about battery life. How much time will elapse before the battery will need recharging?

In the Hall sensor described in post the battery (a coin cell) can last for 20 years (in theory, of course)