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# INA188: Rogowski coil calculator vs. board implementation

Part Number: INA188

I'm was looking at the TIDA-01063 reference design and wanted to replicate it on a much smaller scale as the frequency of the current we are trying to measure is around 150 times larger.  My questions have to do with the board layout and Rogowski Coil (RC) calculator plus INA188.  The RC calculator uses 2 radius values to calculate the number of coils required to attain the required differential output voltage, however, the actual board design has 4 different radius values for the thru-hole vias providing an offset between the forward helix coils vs. the return helix coils.  I assume the difference between the number of via radius values the calculator uses and the 4 via radius values on the actual board implementation were due to practical considerations making the board easier to fabricate.

My question here is the forward coil and return coil are offset from each other making the magnetic flux coupling into the forward coils slightly different than that of the returns coils.  Will this slight difference in magnetic flux coupling of each set of coils affect the CMRR since they are not precisely matched? And, by what percentage?

My other question, has to do with the break in the PCB which I'm guessing was provided for 2 reasons, but I'm not sure.  Was one of the reasons to allow you to slip the board over the current carrying conductor?  The cut in the board seems small for a modest sized conductor, but if the board is thin then you could probably open it up slightly at the break to accommodate a larger conductor.  My other thought for break was to avoid crosstalk at the end of coil where the propagating differential signal would see a short-circuit and could couple back into the coil where it starts.  Was this another reason for the break and why the coil count on the calculator doesn't exactly match the coil count of the final board implementation?  If your application doesn't need the break for inserting a wire into the center would keeping the board intact affect the performance?  In the board design included in the design kit there are 6 coils missing if I counted correctly to accommodate the break.  To prevent crosstalk do you need to consider scaling the size of break in the board for frequency?

Last question,  the front-end input on the INA188 for this Rogowski coil reference design has 2 passive differential integrators tied to ground bridged with another capacitor. What function does the bridging capacitor serve?

Thanks,

Mike