You could always do what I suggested earlier "The solution is simple. Drop the 50 ohms by, say, a factor of 10. Then add a potential divider to drop the DAC output by a factor of 10 to the opamp input. Then you only lose 0.33V at maximum brightness."
Keith.
Or you can use this circuit with a volage divider in the input of the transistor, I have used the variable resistor in the input to simulate the dac
You can change R2 to a lower value depending on your led and how much you want the maximum current to be.
Alex
The circuit should be able to switch at 100kHz easily, even with the LM358. Are you sure you aren't being limited by the DAC/software? What happens when you try to switch faster?
Keith.
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