I know my question is a bit broad, but I want to ask: Using the single supply configuration, why does the first stage's non-inverting input and second stage's non-inverting input use different voltage dividers, the first one being: 200k ohm and 100k ohm, the second one being 20k ohm and 10k ohm.
edit: the input signal is shown as above: using a function generator to perform an AC sweep
It looks like a case of (roughly) wanting to balance the resistance seen by each input of the two op-amps in order to minimize errors due to the input bias current. However, they have not done this correctly.
If it weren't for the blocking capacitor C1, the inverting input of U1 would see the equivalent of 100 kohms parallel with 1 megohm, which the designer roughly equaled by setting up the non-inverting input to see a Thevenin equivalent source with 100 kohms parallel with 200 kohms.
Similarly, if not for blocking capacitor C2, the inverting input of U2 would see 10 kohms parallel with 100 kohms, and they roughly equaled that by attaching 10 kohms parallel with 20 kohms to the non-inverting input.
Matching the resistance seen by the two inputs ensures that bias current effects affect both inputs equally, so that no differential input voltage is generated that would be amplified at the op-amp output.
However, with the blocking capacitors present, these calculations are all changed completely, and the resistances set up for the non-inverting inputs are not well-matched at all to what's seen on the inverting inputs.
