Sampling and Nyquist/Antialiasing filters

[Ben Stuyts]

Hello,

On Tue, 05 Jan 1999, Duane R Balvage wrote:

> Is there anyone out there who has any basic guidelines on how many dB
> down "aliasing" frequencies have to be so they are not a problem?
>
> 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...

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.

Best regards,
Ben