rsdio writes: In conjunction with the recent discussion of how difficult it is to control current draw (in a USB Device), here is an example of a circuit that sinks (draws) a precise current, rather than creating a specific voltage.
Precision Current-Transmitter/Source-Sink Calibrator Has Wide Compliance Range
Based on a precision op amp (MAX4236), this loop-powered, high-compliance, precision current sink (or source) can be modified for use as a 4–20mA sink transmitter.
A similar version of this article appeared in the March 24, 2009 issue of ED magazine.