current sensor for small currents?
Update: Subsequent to posting this answer, I switched to a Melexis MLX91206 linear Hall effect current sensor, with the current to be measured being passed through a coil with the sensor at its core. This permitted measuring currents down to 100 mA, with isolation. See this answer for more details.



One of my projects required high side current sensing of up to 500 mA full-scale at a voltage of 24 volts unregulated. We could not find any integrated device with isolated current sensing like the Allegro ACS parts for that current range.

A non-isolated solution using Analog Devices AD8217 Current Shunt Monitor ($2.44 single unit at Digikey) was chosen, based on this article which provided useful insight into the several options we were considering. 


Disadvantage: Unidirectional current sensing, not suitable for AC
Advantages: Minimum part count, and device contains internal LDO so unregulated Vcc was fine.


For sensing bidirectional current flow we did consider using an AD8210 Bidirectional Shunt Monitor (nearly $5 each!), but eventually just went with current sensing before the coupling capacitor stage. This does introduce some error, but it was approximately linear error within our range of interest, hence eliminated in software.

An useful background reference was Linear's Current Sense Circuit application note.

Also, if someone does identify, or introduce, a hall effect isolated current sensor like Allegro's range, but for low currents, we would happily change over to it.