William;

Please measure the current in the ground pin under different conditions.

I assume you have the shutdown pin pulled up?  With what value?

Chris Eddy

"William K. Borsum" wrote:

> 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
> <borsum@dascor.com> & <www.dascor.com>San Diego, California, USA