What's worked for me is to set the reference input (ground) of the op-amp (not device ground) to your "virtual ground". You'd do this for both the op-amps in your highpass filter. I'm not sure what your supply voltages/signal Vpp are, but for example, mine was centered at 2.5V on a 5V Vcc. Some more details might help...? Cheers, Ishaan ----- Original Message ----- From: "John Hansen" To: Sent: Sunday, June 22, 2003 10:43 Subject: [EE]: Dumb Analog question... Interfacing Active filters > Ok, I'm about to show why I'm a digital guy, not an analog guy.... > > I need to process an audio input before it runs into a modem chip. I've > designed an active high pass filter using a Microchip MPC6022 dual op amp > which I happened to have lying around. The filter works fine, but I need > to introduce a DC bias, since without it, the filter cuts off the negative > half of the incoming audio wave. I used a wheatstone bridge for doing > this (as explained in a Dallas Semiconductor ap note) and it works fine > too, adding about 2.5 volts DC to the incoming audio so the entire signal > is a positive voltage. The problem is that the first component in the > active filter is a capacitor, which, of course, removes the DC > bias. There must be some common way of handling this problem and I'm just > too inexperienced with analog circuits to know what it is. Can someone > point me in the right direction? > > Thanks, > > John Hansen > > -- > http://www.piclist.com hint: The list server can filter out subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics