ADC input frequency and input bandwidth?

[terry_kchai]

Hi, guys

I'm confused about input frequency and input bandwidth of ADC, what's the difference?

I found that some commercial ADC with 80Msps can handle the input up to 180MHz and can allow the input bandwidth of 500MHz, why is that?What's the relation between them? I know that when input is higher than 1/2 of Fs, aliasing happens. But also found that in RF/IF sampling application, the required input frequency for the ADC can up to be 500MHz or higher, but the sampling frequency is only 125Msps.How can it operate well?

I hope some one can remove my doubts, thank you in advance!

 

[biff44]

The answer to one of your questons is that you can use a lower clock frequency than your input signal IF you are trying to undersample. Your output will be aliased, but if you know ahead of time the frequencies involved, you can handle it mathematically. But you would need an ADC with an analog bandwidth much larger than the expected (lower) clock frequency.

 

[terry_kchai]

Thank you for your reply, does the S/H determine the input analog bandwidth?