--- It looks like the problem is having to take the inversion in order to do the current summation and get the true current to voltage conversion in the DAC's input opamp, and then having to do the second inversion to get you right-side-up again so you can do the successive approximation.
Since you say accuracy isn't all that important, I'd cheat and sum the DAC's input currents in a smallish resistor and then use a single opamp as a non-inverting amp with as much gain as you need to make the SAR work right. Like this:
DAC3>---[R4]----+--------|+\\ | | >----+--> Out to SAR comparator DAC2>---[R3]----+ +--|-/ | | | | DAC1>---[R2]----+ +---[R6]---+ | | DAC0>---[R1]----+ [R7] | | [R5] | | | GND>------------+-----+
The smaller R5 is, the smaller the error will be in your current-to-voltage conversion, but the greater the gain you'll need to get the output to the SAR right. Unless you scale the reference voltage input to the SAR's comparator. In any case, the gain of the opamp, above, will be:
R6 + R7 Av = --------- R7
-- John Fields Professional Circuit Designer