Active low pass filter with inverted output, currently tuned to frequency cutoff Fc = 1KHz. Formula is Fc = 1 / (2*pi*C*R2) where R2 is the feedback resistor above the op amp. Gain is currently set to 1 (or -1 if you prefer). Fomula is gain = R2/R1, meaning R1 can be adjusted to change the gain without affecting Fc.
