I'm looking into sampling a signal into a micro-controller, the ADC will be sampling at ~38.5 Khz with 8 bit depth.
The dynamic range is given by 6.021\*N + 1.763 dB, so in this case I'm getting ~49.92 dB which means I need to attenuate the signal about that much at the Nyquist freq. of 19.25 Khz.

After some experimenting with TI's filter lab I got the following implementation of a 4th order LPF:
[![enter image description here](https://i.stack.imgur.com/liqze.jpg)](https://i.stack.imgur.com/liqze.jpg)

I was wondering about the input impedance of the circuit so I quickly entered it into Multisim and got the following:

[![enter image description here](https://i.stack.imgur.com/LzkP4.jpg)](https://i.stack.imgur.com/LzkP4.jpg)

At 20 Hz the input Z is about 750 KOhm which is good enough for me, however as we move towards 1 KHz that figures goes down to about 16 KOhm. At 4 KHz (passband) the input Z is only 4 KOhm.

Questions are:

1. 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?
2. 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.

A passive filter also possible (but low input Z):

![schematic](https://i.stack.imgur.com/rNrMZ.png)

[simulate this circuit](/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2frNrMZ.png) – Schematic created using [CircuitLab](https://www.circuitlab.com/)