Information Merchants canned

How does a low pass filter demodulate a pulse width modulated signal? - design of pulse oximeters webster rapidshare

I am stuck at home, away from the library of science: (

I need a good explanation or a Web site explaining how to take the pulse width modulated signal (original signal varies in amplitude and frequency) - and demodulated with a low pass.

It was proposed to use an operational amplifier in my filter design. If you have an idea why someone wants to put operational amplifiers in a filter, I wonder, too!

Thanks