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:
I was wondering about the input impedance of the circuit so I quickly entered it into Multisim and got the following:
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:
- 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?
- 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):
simulate this circuit – Schematic created using CircuitLab