DAC controlled LDO as current source
Once again, the 4-resistor difference amp is your friend. The circuit they show you has the output of the LT1991 referenced to ground, but it can also be referenced to Vout, like this:







simulate this circuit &ndash; Schematic created using CircuitLab

OA2 acts to keep the voltage difference between OUT and REF equal to the voltage difference between IN+ and IN-. This means that you can now control the voltage across R1 &mdash; and therefore the output current &mdash; by means of a ground-referenced voltage Vset coming from your DAC.

This circuit shows a gain of 1, which means that Vset must be limited to 0 - 1V, but you could actually use a gain of less than 1, which would allow you to use a wider range of DAC voltages. Just keep R3 = R5 and R4 = R6.

You can get amplifiers with matched resistors built in, such as the LT6363 family. In particular, you might want to use the LT6363-0.5.