Anti aliasing filter + DC offset
How can I improve on that? Is there anything I could do without adding an additional op-amp buffer stage in front of the filter?


Increasing input impedance can be done by scaling the first filter components (R1,R2,C1,C2). Scale resistors UP while scaling capacitors DOWN maintains the same cut-off frequency. Still, source impedance adds to R1...if your signal source impedance is known, and stable, make R1 smaller.
Example: scale filter by ten: R1=27.4K, R2=37.4K, C1=1nF, C2=1.2nF
If your source resistance is 1K, then make R1= 26.4K.


  Since the ADC is single supply I'll need to bias the signal to VCC/2. How can I do that without killing the filter response? Looks like adding a cap at the output and a voltage divider will have a very dramatic effect on the low freq. response.


You can add a 2-resistor voltage divider to generate DC input. Since both op-amps have DC gain of 1, this voltage will appear at the filter output (plus-or-minus the small op-amp offset voltages). The 100k bias resistors shown will lower the source resistance (Rsource) only slightly. Incorporating filter scaling, and subtracting Rsource of 1K, and adding DC offset, your filter now looks like this:







simulate this circuit &ndash; Schematic created using CircuitLab
Use op-amps that have low bias currents, so that those large-value bias resistors plus series-connected filter resistors won't cause DC offset errors. Also, be aware that filter scaling increases thermal noise voltage. If your filter is used with very small input signals, scaling those input resistors up will impact the noise floor. For an 8-bit A-to-D, this shouldn't be a problem.