I will indeed have to seriously consider designing my own xfmr, but I will leave that for a 2nd iteration. At the moment it is not a possibility as the current sensing is part of a larger system and I have a time constraint for a working prototype.
In terms of the bandwidth I need, the FIS155 is the the only off-the-shelf part I found that met my spec. The price to pay is the turns ratio.
In any case, while the circuit could certainly be improved, it doesn't seem all that bad, does it?
Thanks again for your insight JS.
Cheers.
Designing a transformer shouldn't chew much time, as it's working with very little voltage they are easy to work with, build and test. Also, once you have a proper transformer, the next step is much easier, building an amplifier for a proper transformer could be much easier than working with a wrong transformer to start with.
As they work in current mode they are usually much more linear with less effort than voltage transformers, where core saturation can happen and makes things awful.
I've used quite a few measurement transformers, usually 5A output, for the desired input, 100A, 500A, whatever, as 5A-10A is usually an easy current to deal with and many instruments support that, as any half decent DMM or using a shunt to probe around it. 5A are nominal, but the specs usually hold tight up to 120% and they can stand over that for a short period. When transients are huge and you need the lower range is common to short the transformer during start up, and then start measuring the steady state.
1:1000 is also common, but you need to wind 1000 turns on the sec, kind of a pain compared to 100 turns, as the primary is usually 1 turn.
The circuit for this is quite straight forward, µCurrent could do a great job if one of the ranges is fine for your needs, if not adjust that design, you don't really need to care for the burden voltage so you can have a much higher one and let your transformer make it small enough, that eases up the opamp choise, as voltage errors become much smaller, still the topology remains, shunt, non inverting amplifier with gain to get the desired output, dual rails makes it easy. Then there are the frequency considerations, to get up to a few kHz isn't hard at all, filter over the needed bandwidth so you don't have unnecessary noise.
If you want to make it more complex there are several things you can add, for a current mode transformer you could compensate for the winding resistance and get a much wider frequency range and improved linearity.
JS