AA filter before ADC inputs?

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10 Oct 2012
Posts: 21

I am wondering what anti-alias filters are in front of the ADC.

If the sampling rate is 100MSa/s, it seems you would need a <50Mhz AA in front of the ADC, but I can't find explanation of that. How about a block diagram?

Analog BW =100Mhz = repetitive sampling bandwidth

"instantaneous capture bandwidth" 20Mhz

will a 1Ghz signal cause aliasing?

I'd like a bit more explanation of this!

11 Oct 2012
Posts: 481

The front end to ADC has a two filters - one always on, and the other switchable.

The always on one sets the analog bandwidth to just above 100 MHz - we measure between 100 and 115 MHz. If this filter were not there, the BW would be set by the front end FET amp, which has a 200 MHz BW. All components after that have a higher bandwidth. So if you sampled 1 GHZ, I would not expect anything!

Secondly there is a switchable 20 MHz analog filter, but you have to switch it in your self. You use Settings/Acquisition Settings and then set the Prefilter A (or B) to 20 MHz or None. You have also got the option of turning on a digital moving average filter with a time constant of 40ns to 1.28 us.
After having set the filtering teh way you want it, you have to enable the filtering by clicking the Filter On or Off on the Cleverscope Control Panel.

So to complete the story, yes, if you have the filter turned Off, you will see Aliasing between 50 and 100 MHz.

I hope this helps.
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