I'm using an ADC (adc081c021) with a pretty noisy input:
There is also a 5-second video here: http://tinypic.com/player.php?v=2ue47cm&s=5
The objective is to detect the two peaks. How could I smooth this input in order to have better ADC readings? Is there a better option than a LC filter? Any recommended values?
Since you have an example of the signal on your scope, the best thing to do is capture the data and transfer it to a PC. Then use a tool like Matlab or Octave to simulate the effect of different filters.
You are looking for a filter, just defined in terms of poles (and maybe zeros) that minimizes the noise, without disturbing the desired features of the signal.
When you have a filter definition, then worry about how to build it.
If a single-pole filter is adequate, a simple RC circuit solves your problem.
For a two-pole filter, the Sallen-Key op-amp circuit is known for having relatively good tolerance for changes in the component values. An LC combination is also possible.
For higher-order filters (which I doubt you need), a cascade of Sallen-Key filters is preferable to a ladder of LC stages, because the op-amp provides buffering that prevents component value shifts in one stage from affecting the characteristics of other stages.
Edit In reply to your comment, I'm not a DSP guy, but here's how I'd work out the equivalent continuous time filter:
Your filter function in discrete time is
\$y_n = a x_n + (1-a) y_{n-1}\$
Given an impulse input, the time constant is the time it takes to decay to \$e^{-1}\$ of the value of \$y_0\$.
This is given by
\$(1-0.025)^n = e^{-1}\$
Solving this, n is about 39 samples, or 156 us.
So you want to choose R low enough that the input impedance of the ADC doesn't affect the filter performance much, then choose C to give RC = 156 us.
A quick and simple option to investigate is to average the ADC values over a given number of measurements, resulting in a simple low pass filter. Best option would be a ring buffer of a certain size in which you push the most recent value at the end and average across all values in it. This method does come with a maximum delay of the ring buffer length times sample frequency.
What I would do would be to only use the ADCs input if the value was higher than x. You can do this by using a comparator to check if the value is above x and then reading the ADC if it is.
