Howdy I'm using a precision reference (Analog Devices REF191) an a classic three pin regulator constant current source configuration i.e. Vout is tied to the floating ground pin via a current setting resistor, and the load is connected to the floating ground pin. In this case, the regulator forces 2.048 volts across a 2.1K ohm resistor (Rset), giving me a nominal 1.01 mA constant current output. This circuit can be found on most three pin regulator data sheets and cook-books. The power supply is around 7-10 volts. The headroom of the regulator is 1.5 volts, and the Vout is 2.5 volts. In theory, I should have at least (7 - 1.5 - 2.048) 3.4 volts available to drive the constant current through the load--which at 1 mA should in theory be as much as 3.4K Ohms. What I am seeing is this: Regulation and constant current is VERY stable from a load resistance of zero to about 1.6K, at which point the current starts to drop radically. If I monitor the voltage across Rset, it remains stable until Rset = Rload, at which point regulation ceases. The tech folks at Analog Devices claim this can't happen, but I see it on every device tested. My problem is that the load is usually around 1.2K, but will go to 2K under high temperature conditions, and of course the excitation current to the sensor drops because of the instability in the current source. Any ideas, anyone? This one has a number of good folks completely buffaloed! Kelly William K. Borsum, P.E. -- OEM Dataloggers and Instrumentation Systems & San Diego, California, USA