[Groth]

I'd thought about adding gain at the difference amp, but then I wouldn't be able to use the resistor networks I can get 'cheaply'. Having a first stage output that is of known accuracy without any tweaking would cut down on the number of pots needed to get an accurate output.

Mere mortals such as I are limited to hand matching out resistors - dare I ask how 'cheaply' you get laser-trimmed resistors arrays?

There is much to said for the ease of error checking with the first stage unity gain. But with the pain I went through trying to stamp out the tiniest input bias current errors and input voltage offset errors before they got amplified, I'm ready to try some early gain. I had more consistent luck with tweaking the bits-to-amps equations than I did tweaking the pots.

I can't think of anything to be gained by preserving high frequency components unless, a much higher speed A/D is going to be used.
I'm just not seeing any practical benefits to higher bandwidth, and I'm seeing a lot of difficulties to overcome.

I'm not planning to keep the high frequencies for long, and I don't want to challenge Nyquist to a wrestling match.

As I picture it, off each shunt we get a superposition of load current, inductor current ripple, and assorted high frequency spiking, ringing, etc. A one Mhz low-pass should peel off most of high frequency junk while leaving the triangle waves of the inductor ripple relatively intact.

At the summing stage, the combined ripple from the three channels should have a third the amplitude and thrice the frequency. And that's where I want to kill it - when it is smallest in comparison to the load current signal, and when it's higher frequency makes it easier to filter.

I'm sorta tryin' to replicate how the current get to the CPU - ceramics by the inductors bypass the high stuff, then the electrolytics filter the combined current. And I have this recurrent dream/nightmare of three phases of 33% duty cycle square waves, which are sometimes low-passed and then added, sometime added and then low-passed.

Sorry, I'll leave the risk of damaging a motherboard to others.

No sense of adventure, what a pity.