Sampling and Nyquist/Antialiasing filters

[Ben Stuyts]

On Tue,  5 Jan 99, I wrote:

> > I'm retrofitting my Prophet-2000 sampler with 2 switched-cap
> > 6-pole filters in series (12 poles, 72 dB/oct) for the antialiasing
>
> I'm not 100% sure about this, so correct me if I'm wrong but...

And wrong I was...

> You have to be very careful with switched cap filters when using them for
> anti-aliasing purposes. As they themselved are sampling systems, they will
> need an anti-alias filter at the input, or else you will get aliasing in the
> filter. So if they switch at, say, 40 kHz, you will definitely need to make
> sure nothing above 20 kHz gets in! Which kind of makes the switched-cap
> excercise futile.

Duane pointed out to me that they actually sample at 50 times their cut-off  
frequence, so it is less of a problem. However, I've had problems with this in  
the past, but it was soooo long ago, that I forgot the details. I had a  
switched cap filter at 10 Hz for a low frequency amp, and I did have trouble  
with interference up to a few 100 Hz.

Best regards,
Ben