Why do I need to put a low pass filter in front of an analog to digital converter (ADC?)
A low pass filter is needed for most analog to digital converters (ADCs.)   Without the filter, frequencies above half the sampling frequency of the ADC will &quot;wrap around&quot; into the sampled data.  This is called &quot;aliasing.&quot;
For example, assume you have an ADC sampling at 20000 samples per second.   If you sample a 9000 Hz sine wave and record the data, then sample a 11000 Hz sine wave and record the data, then both sets of data will &quot;look the same&quot; - they will seem to have the same frequency content.
This picture from the Wikipedia aliasing page show how it works:

Both the red and the blue sine waves produce the same sampled result (the black points.)
This effect is usually unwanted.  You usually want to capture all of the signal up to half the sampling rate without aliases.
You get rid of aliased signals with a low pass filter.
The low pass filter also removes high frequency noise.
Some ADCs have built in filters, some don't.  Even those that do will usually require a moderate low pass filter to reduce signals above some specified multiple of the sampling rate.

Sometimes you can make use of aliasing.
Say you have an ADC with a sampling rate of 2MHz.  Normally you would be limited to sampling frequencies below 1MHz.
If you use a band pass filter to pass signals only from 1MHz to 1.5MHz, then you could capture signals between 1MHz and 1.5MHz with your 2MHz ADC.
The frequencies will be mirrored, but that can be accounted for in whatever analysis of the data you do - or mathematically mirrored again so that everything is &quot;right side up.&quot;
That kind of thing requires an ADC without a built in filter and some knowledge of the internals of the ADC.  Besides the sampling rate, you have to be sure that the analog parts of the ADC will pass the signals that you are interested in.