Hi, I need to construct a filter to pass 5Hz and above. I'm also using this circuit to prevent any DC signals from passing (I have a DC offset that I need to get rid of). I'm using the equation f = 1 / 2*pi*RC (in which I found after derivation). As of right now, I have two designs: 1) Series RC - works fine in SPICE. 2) Differentiator (OpAmp config) - can't get working on SPICE. I have no idea why it's not working. I've tried connecting pins 1 and 5 of the opamp to GND, as well. This is what I have below: http://img138.imageshack.us/img138/3764/14914103um3.png [Broken] Code (Text): * Schematics Netlist * V_V2 $N_0001 0 5v V_V3 0 $N_0002 5v R_R1 $N_0004 $N_0003 79.5k C_C1 $N_0005 $N_0004 .1u R_R2 $N_0003 $N_0006 79.5k X_U2 0 $N_0003 $N_0001 $N_0002 $N_0006 uA741 C_C2_BYPASS $N_0001 $N_0002 .01u V_V_INPUT $N_0005 0 DC 0 AC 1 +SIN 2.4 50m 100 0 0 0 R_R3 0 $N_0006 10k I am using PSpice Schematics, Evaluation Version 9.1 - Web Update 1 for my simulations.