How to roll of dc gain to unity via an opamp ac amplifier..

You don't need that Sallen-Key filter before your amplifier if you are already filtering with the input RC high-pass filter.

You can simply AC couple the input, bias the amp and gain it up.

^{simulate this circuit – Schematic created using CircuitLab}

Some key things to note from this circuit:

The input RC filter in your original circuit had a cutoff \$F_c\$ of 1 Hz but your text said 0.5Hz. 0.5Hz is actually better, so I kept that and changed the 0.1μF input capacitor to 0.22μF.

$$F_o(R1C1) = {{1}\over{2 \pi R1 C1}} = {{1}\over{2\pi(220nf)(1.6M)}} \approx 0.5Hz $$

Note that the RC combination of R2C2 with the opamp forms an active high-pass filter which should equal or be close to R1C1:

$$F_o(R2C2) = {{1}\over{2 \pi R2 C2}} = {{1}\over{2\pi(68μF)(5K)}} \approx 0.5Hz $$

Unfortunately, this means a fairly large C2 capacitor, because you need to balance that against R2 and the gain of the op-amp that you want (G=100).

You might be able to get around that by using two gain stages x10 of equal design. This way you can use larger R2 values and thus a smaller C2. For example, x10 would be R3=200k, R2=22k, C2=15μF, and you need two of them.

Also note that the LM324 cannot reach the top rail, so it will clip with such a high gain. You should use a better rail-to-rail output amp when working with 3.3V

EDIT: Oh, on further review, I think I see you want to follow the input high-pass filter @ \$F_c\$ = 0.5Hz with an active low-pass filter with \$F_c\$ = 5Hz, with the goal of making a bandpass filter out of the two.

If that's the case, then you will want to just AC couple the second stage again and bias it at Vbias. Furthermore, you'll want to change the Sallen-Key cutoff point to something higher than 5Hz if you want to pass 1-5Hz. Remember these are the 3dB down points, so you want some extra bandwidth. So if you want to pass 1-5Hz, then you should use a high pass at 0.5Hz or less and a low pass at 15Hz or more.

Change your 68K resistors on your Sallen-Key low pass filter to 22k and you will have a \$F_c\$ of 15Hz. This way you won't roll off and attenuate your 5Hz in-band signal too early.

^{simulate this circuit}

The answer is simple: For an ideal opamp your circuit provides a lowpass function with max gain of 40 dB and a cut-off (-3dB) at app. 100kHz. However, the real opamp (GBW=1MHz) allows a gain of 40 dB only up to app. 10kHz. Hence, it is the open loop gain of the opam that dominates and determines the frequency response of the whole circuit (3dB cut-off at app. 10kHz)