I have worked with integrator circuit using op-Amp. Should simillar circuit be used to integrate di/dt from rogowski coils? What about the offset which will continously get integrated and will be cumullative?

Hallo Prahlad,

if your signal current to be measured is Isig(t), then Rogowsky coil gives a signal voltage of Urog(t) = M x dIsig(t) / dt. If you would apply this voltage to the input of following integrator



then, assuming having an ideal operational amplifier without showing any input bias current and input offset voltage, at output the signal Uout(t) can be found, according to:



The minus results from the fact, that integrator is built using an inverting circuitry.

Unfortunately, every operational ampflifier will show unideal behaviour in terms of input current and offset voltage. But as your signal current Isig(t) is an AC signal you could put a big capacitor in series to 'R'. This would prevent DC currents caused by offset voltages from permanently flowing into capacitor 'C' of integrator. If this additional blocking capacitor is choosen big enough, its effect on signal fabrication (integration process) would be negligible.
The unavoidable input bias current can be kept under control by connecting a high ohmic resistor in parallel to 'C'. If you choose a FET operational amplifer like TL051A for instance, this resistor can be choosen very high, means its influence on signal fabrication (intergration process) can also be kept negligible.

Of course, a naked integrator like the one shown above, would not be enough for your application. You would also need a buffer amplifer directly at input, which provides some gain eventually, and drives the integrator low ohmically. At input of buffer a protection circuit should be placed, which prevents damage from fast glitches.
Behind the integrator you would need an additional inverter to produce a positive signal.

Some words about 'inherently linear' performance of a Rogowski coil:
Linearity is provided by avoiding a core made of material showing 'µr' > 1. But what, if such material would be near the Rogowsky coil at close distance?? As no Rogowski coil can ever be bend perfectly symmetrically, magnetic stray fields resulting from these magnetized iron containing parts and hitting the Rogowski coil will not totally vanish. The result is an introduce of some unlinearity, caused by the unlinear 'µr' of these iron containing parts. So, take care that no iron containing parts come close to Rogowski coil or to conductor carrying the current to be measured!
Another cause of unlinearity is skin effect: If the current to be measured contains higher frequencies, then skin effect forces according high frequency currents more and more to flow on surface of conductor. In combination with unsymmetrically wound Rogowski coil this can result in different mutual coupling factors 'M' for lower and higher frequencies.

Kai