ADC question about high analog BW > conversion rate

[Mantle279]

Hi edaboard,

I am looking at RF ADCs; the trouble is, I am a digital type with an appreciation for analog.

I am looking at the datasheet for an Intersil KAD5512P-25 ADC. https://www.intersil.com/content/dam/Intersil/documents/kad5/kad5512p.pdf

They provide several specifications on SFDR and SNR as a function of analog input frequency. However the device itself has a peak conversion rate of 250MSPS.

The question I have is: why specify an analog input frequency that goes up to 1GHz is the conversion rate is only 250MSPS? This is not a block converter. I think it has to do with the RF front-end and how well the filtering is before the A/D. Is that the right idea? So you could have a fairly wide simple LPF with a roll-off of 20dB/decade, but the ADC will filter it further per Nyquist. Or you could have a fairly complex filter at the front end with a steeper roll-off. The benefit is that this improves your SNR and SFDR, but the cost is increased complexity at the front-end.

Is that the right idea?

Thanks

 

[crutschow]

It may have a high frequency analog input for use in sub-sampling RF processing techniques. For that the analog BW must be higher than the Nyquist limit would imply.