Op amp circuit DC offset removal

[crypton]

Heya all!

Im sorry if this has been discussed somewhere, but i haven't been able to precisely keyword my problem (If anyone knows a good search term, please, do tell).

So i'm doing some more in-depth op-amp stuff than I've used to.

This is my specific op-amp circuit:
not sure how to embed attachment pictures...
See attachment picture

The circuit basically is like a variable PWM, depending on the input signal. I need PWM from both periods of the input signal around the set threshold (so I've created positive and negative threshold simultaneously).

I get around 800mV offset (green and blue lines) at the final comparator 'switches', which I want to be at 0V when at rest. Any ideas what should I do ?

I've messed around adding some negative feedbacks (makes the transitions slow though :S ) and trying differential op-amp modes, but maybe I'm doing it wrong...
OR maybe there's some better circuits to achieve this behaviour (but this is what I came up myself though).

(there can be more issues, when i want to build it up in real life - decoupling, actual resistor sizes etc, but these would be left for another topic I guess.)

[orolo]

Look for "DC output voltage swing" in the datasheet. For a 5V supply, the output of the AD8039 goes from 0.9 to 4.1V, not reaching ground. For a 2.5V supply as in your case (note that minimum supply is 3V according to datasheet) it's in that zone. If you want an op-amp whose output reaches the negative rail, look for a single supply op amp. An LM358 would work in your circuit (as long as the frequency doesn't go much higher than a few kHz), save again for the 2.5V supply again.

[Ian.M]

That's what OPAMPs do when you run them open loop.   In real life, most OPAMP outputs will probably slam to one of the rails even if you directly tie both inputs together and bias them to half supply. 

The AD8038/9 is a  special case - its optimised for applications like buffering analog video with a closed loop gain of x1 or x2, and is only specced for operation at loop gains upto x10. The open loop gain is 70dB, which is a bit over 3.1V/mV (compare with uA741 at 200V/mV) so the output may not rail.  However the typical input offset voltage of +/-0.5mV means you'll probably get anything up to about +/-1.5V offset at the output and worst case input offset of +/-3mV could give you +/-9.5V offset at the output.

Also Vin goes below ground and your AD8038 chips are powered between +v and ground so you are exceeding their absolute maximum ratings and will probably destroy them. If you want a comparator with a ground referenced output, use one with an output ground pin, not an OPAMP!