Adam Bryant writes: [in respect to filtering an ADC input]

>Forgive my limited electronics background, but what is the purpose of the
>RC component in this diagram?  I understand the purpose of the diodes,
>but why would you use the RC component?  I would guess as a method to
>reduce spikes and "slow down" the rate of change of the incoming signal?


You guessed right.

Wagner just gave more electronics info on RC filters than I ever knew, but
from the DSP side of the fence...

Given an adc system that samples at a constant rate, any frequencies above
half the sampling frequency (called the 'Nyquist' frequency) will cause
interference ("aliasing artifacts"). This can be demonstrated by drawing a
high frequency sine wave on a piece of paper and then "sampling" is at a
small nember of evenly spaced points (more than half a wavelength apart) -
the "voltage" at these points could be anything within the amplitude of the
waveform, and bears little relation to either the average level of the
signal or the wavform of the signal.

So the rule of thumb is to block any frequencies above the nyquist from
entering the ADC. The RC filter hasn't got a sharp frequency rolloff so it
should probably be tuned well below the nyquist, or an active filter could
be used. Another technique called "oversampling" involves sampling at a
(integer ratio) higher rate than needed (but still performing the analog
filtering for the original sample rate), and then filtering the signal in
the digital domain and decimating down to the required rate - I'm doing a
bit of both.

Regards

Ross.