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I'm designing a circuit to process an signal from an Rogowski coil. As the voltage is the differential of the measured current, we have to integrate the voltage to provide a representation of current.

My question is; with the RC integrator, is it as simple as ramming a capacitor on in the feedback of a inverting amplifier? As the RC integrator naturally acts as a high pass filter, do I just select the gain resistors I need and then select the capacitors value depending on what the -3dB cut-off I want? What considerations need to be made to provide the best output signal possible?

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  • \$\begingroup\$ You'll want stability, otherwise your readings will vary or oscillate -- all the more when you say provide the best output signal possible. Which means you're on your way of searching for more involved circuits than the classical RC opamp. \$\endgroup\$ – a concerned citizen May 29 '18 at 10:37
  • \$\begingroup\$ Ah, makes sense. When you talk about 'more involved circuits', are you able to point me in a direction at all? I'm unaware of any other integrator circuits (other than digital integration). Of course I'm going to add filters and gain selection, buffering ect, so the opamp integrator is not alone. \$\endgroup\$ – user160063 May 29 '18 at 10:43
  • \$\begingroup\$ @sidA30 TI have a pretty decent application note on op amps. It covers a few different integrator circuits so you could look through that. Section 3: Signal processing could have some bits to interest you: ti.com/ww/en/bobpease/assets/AN-31.pdf \$\endgroup\$ – MCG May 29 '18 at 11:45
  • \$\begingroup\$ An integrator (RC or otherwise) acts as a low-pass filter. \$\endgroup\$ – WhatRoughBeast May 29 '18 at 13:26
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An integrator acts as a low pass filter.

You will want to degrade the integration at lower frequencies than your fundamental because otherwise the integrator will eventually saturate from integrating even a slight offset voltage. Typically something like a resistor across the integrating capacitor, at a minimum, but you can use more sophisticated circuits to control the low-frequency roll-off.

Here is a TI app note describing the simple approach.

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