I'm still learning about this, but it seems that an RC circuit performs the same function as an op amp integrator (http://www.electronics-tutorials.ws/opamp/opamp_6.html). In other words, they both can be used as integrators/low pass filters. If this is true, why would anyone bother with an op amp integrator? The only difference that I can see is that an RC circuit would be more sensitive to drawn current than an Op amp integrator would be. Am I missing something?