An op-amp integrator produces an output proportional to the time integral of the input. In the classic inverting form, the signal passes through resistor Rin into the inverting node; a capacitor C from output to that node provides feedback, and the non-inverting input is grounded. Ideally, Vout/Vin = −1/(Rin C s), so vout(t) = −(1/(Rin C))∫vin(t)dt + V0. Because low-frequency gain is very high, DC offsets and bias currents can drive the output into saturation. Practical designs add a resistor Rf in parallel with C to limit DC gain and prevent drift, and choose op-amps with adequate bandwidth. You can modify the DC offset of the AC input to see this saturation in action.
