I have a circuit which takes the output of a one-shot and filter it. There are two stages. First the signal goes to two parallel capacitors between which there is a resistor, and one end of the resistor is connected to the negative input of the amplifier and the other to the output of the amplifier. The output of the amplifier through a resistance goes to a similar network with a second op-amp which output is the final output. The positive inputs of both op-amps are connected through a voltage divider to Vcc/2. To make it more clear I will number the components and I will describe one stage of the filter as the second is the same. C1 :p1,p2 - C2: p1,p3 - R1: p2, p3 - Op.amp. + : Vcc/2 - Op-amp - : p3 - Op.amp out : p2.
R2 = resistor between the two stages : p2, p1' (=p1 of the second stage) And now what is going on and I cannot understand. I have two almost identical prototypes of this circuit. In one of them the points p1', p2', p3' have a Vcc/2=2.5V voltage and in the other circuit for the same points there is a Dc component of Vcc/2 but there is a sinwave of
2V p-p added to the DC voltage. In the circuit with the aforementioned AC voltage, the voltage before the resistor which is placed between the two stages is only the DC component.