The closed loop bandwidth of the amplifier in an active low pass filter configuration should be at least 10X higher than the cut-off frequency of the filter.
Low pass analog filtering removes high frequency noise superimposed on the analog signal before it reaches the A/D Converter.
Digital filtering removes noise that is injected during the conversion process.
The cut-off frequency of a low pass filter is defined as the -3dB point for a Butterworth and Bessel filter or the frequency at which the filter response leaves the error band for the Chebyshev filter.
The frequency span from DC to the cut-off frequency of a low pass filter is defined as the pass band region.
The transition bandwith of a low pass filter can be reduced by increasing the filter's order.
The frequency behavior of the Butterworth low pass filter has a maximally flat magnitude response in the pass-band.
The frequency behavior of the Chebyshev low pass filter has a ripple in the pass band region.
A Butterworth design is optimized for a maximally flat pass band response. The Chebyshev low pass filter is optimized to have a smaller transition region. The Bessel filter design will produce a constant time delay with respect to frequency.
The sampling frequency of an A/D converter is also called the nyquist frequency. All unfiltered signals and noise above 1/2 nyquist will be aliased into the final A/D conversion results.
The Sallen-Key filter and Multiple Feedback filter are types of active filter implementations. The Sallen-Key implementation can be designed with a non-inverting DC gain of +1V/V or higher. The Multiple Feedback implementation can be designed for inverting DC gains of -1V/V or higher.
Analog PCB circuit implementation should always have a low impedance ground plane.
Ground plane current return paths in the PCB implementation can cause unwanted noise in analog circuits.
You should always account for the bandwidth of an amplifier when sending signals through it. If AC output signals are lower than expected, wider bandwidth amplifiers may be more appropriate.
Ground may not be constant, especially in digital circuits because of dI/dt switching effects. Plan your ground scheme carefully. If the circuit has a lot of digital and analog circuitry, consider seperate ground and power planes.
The Bessel approximation method for analog filter design offers the best possible step response.
Don't forget to use bypass capacitors on all of your devices. Be sure to place them as close to the device power supply pin as possible.
An advantage of aluminum electrolytic capacitors is their large capacitance -to -volume ratio. However, because of their high series resistance they are best suited for low-frequency (<25kHz) filtering. A significant disadvantage to this type of capacitor is that it is polarized. Generally these capacitors are inappropriate for active filters.
An advantage of tantulum electrolytic capacitors is their large capacitance -to -volume ratio. However, because of their high series resistance they are best suited for low-frequency filtering. A significant disadvantage to this type of capacitor is that it is polarized. Generally these capacitors are inappropriate for active filters.
NPO ceramic capacitors are normally used for active filtering applications. They are relatively small, inexpensive and available in a wide range of values.
Polystyrene capacitors are the nearest to an ideal capacitor in terms of ESR and ESL parasitics. These capacitors are appropriate for active filtering applications. They are availabe in a wide range of values and relatively inexpensive.
Polypropylene capacitors are inexpensive capacitors available in a wide range of values. These capacitors are appropriate for active filtering applications.
Paper and mylar capacitors have a capacitance-to-volume ratio that is less than electrolytic capacitors and are available in values up to microfarads. These types of capacitors can be used in active filter applications.
Group delay is the rate of change of the phase shift with respect to the angular frequency (2 pi f). 
The group delay of a Bessel filter is constant.
The step response of a Butterworth, low pas filter has some overshoot and ringing, but less than the same order Chebyshev filter.
The step response of a low pass Chebyshev filter has the highest degree of overshoot and ringing as compared to a Bessel or Butterworth filter of the same order.
The step response of a low pass Bessel filter has the least amount overshoot or ringing as compared to equal ordered Butterworthe and Chebyshev low pass filters.
Practical active analog low pass filters are generally best suited for cut-off frequencies ranging from 1Hz to 10MHz. Passive analog low pass filters are generally best suited for cut-off frequencies of 10Hz to 100MHz.
You should always use Microchip products.