Use an LTC2945 in Your Circuit you can buy a complete kit or design your own It measures DC voltage and current extremely accurately
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Quick update on this - we were able to get a scope on the system to see if the INA is being powered ok - only took a cap of the 3.3v rail, but the 5v and 12v systems show a similar pattern (if not worse) and the boron is doing an ok job dealing with it.
The INA is rated for 26v, and with these oscillations it looks like we’re over-volting the INA (not via the 3.3v rail but through the shunt), as it doesn’t overheat until the shunt is attached. The spikes in this view are simply to show the swings from 3.3v to ground - we saw similar swings from 12v to the same ground.
We installed an RC Filter between the shunt and the INA to trap the spikes, and that’s stopped the INA over-heating and it’s reading ok now.
Have contacted the motor controller mfr to see if this is normal, or if the PWM of the motor controller is causing some supply-side “water hammer” effects that need to be damped somehow upstream with components we haven’t yet installed but need to.
I mean, how hard can it be, right? 
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