I have been sketching out a fairly high frequency switcher and noodling around with an idea for getting the output from a inductor DCR or high side resistive current sense setup out in a way that does not suffer too horribly with ground bounce, massive dV/dt and all the other nasties while retaining good CMRR and bandwidth, and think I just might have something.....
Basically the idea is to feed the differential voltage into a floating IF input on a double balanced mixer, with the RF input driven at some suitable RF frequency, then just sync demodulate the result once you are back on the control board away from the roar of the switching stuff. Obviously pick a RF drive frequency that is not the third harmonic of the switching rate (VHF drive is not out of the question so a few tens of MHz of bandwidth may well be reasonable, or square wave drive actually at the switching frequency as long as the transformers LF response is not too rotten, there will be a glitch at switching point but that is probably uninteresting anyway?).
Annoyingly the usual suspects (MCL) do not seem to have diode ring mixers that isolate the IF connections from the RF and LO ports, but there is no fundamental issue that I can see. Transformer isolation with response down to DC whats not to love?
The nice thing about this is that there is no need for active devices in the noisy domain, and the transformers only need a response down to whatever frequency the RF drive is, so they can be tiny little VHF things, it may even be possible to build both the required transformers as trifiliar windings on a single small pigs nose ferrite, in which case you are down to something like two four diodes and the transformer assembly in the power domain, all the rf drive and sync demod can be on the control board and is in any case trivial.
Thoughts?
I might need to order some diodes and small ferrites to have a play.
73 Dan.