In an ideal world, the source, line, and load impedances
would be perfectly matched. Since this is almost never
achievable, the best one can do is get as close as possible.
And since most power sources are of a low-impedance
nature, matching networks or the equivalent are generally
necessary. Note that in a mis-matched transmission line,
there will be standing waves (current and voltage peaks)
at sub-multiples of the wavelength, always to the detriment
of maximum power transfer. In an environment where it is
difficult to control the line impedance (circuit board traces,
connectors, etc.) it is preferable to terminate both ends of
the line in a low impedance, to swamp the effects of the
variances in the line. It is preferable to lose some signal
amplitude, rather than to compromise the signal waveshape
(especially in digital signals, which are subject to the
effects of the transmission line causing signal distortion.

As to the question of whether it is preferable to match
the source or load ends, the load termination will prevent
reflections from that end only, and the source termination
will tend to reduce the effects of the reflections on the
source (harmonic generation, intermodulation, and
other nasty types of signal distortion). Either one will
reduce signal amplitude.

<als>

 

Perhaps you should consider getting rid of the FET switch and doing the switching in front of the op amp. See sim results below. As you know, the hardware may behave differently.
I show switching with an analog switch. You could do the switching digitally in front of the DAC, which would be preferable if your DAC is fast enough.

Attached Images

	led_driver_sch.gif (7.7 KB, 83 views)
	led_driver_waves.gif (9.1 KB, 83 views)