1kHz cutoff frequency for both passive high & low pass filters on the input blocks off any DC bias the input may have. The opamp provides unity gain for significantly higher/lower frequencies so only the target 1kHz signal actually gets amplified. This happens because the non-inverting configuration has a gain of Av=1+Z1/Z2 and each impedance will vary with frequency. At higher frequencies Z1->0 ohms so the gain Av->1+0/Z2=1 and at lower frequencies Z2->infinite ohms so Av->1+Z1/infinity=1. The opamp here has a +3V power supply so the filters all center around a +1.5V virtual ground because I was fiddling around with logic level signals at the time.
