A question about ADC spec, thanks

[maxam]

Hi, All

I have a question about the spec of a ADC

"Sampling rate of 48Ms/s
SFDR>57dB @ 36MHz input frequency
3rd order intermoduation>57dBc (2 tone 35MHz and 37MHz)
"
Because the sampling frequency is 48MHz, so the bandwidth should be less then 24MHz, then why they test the SFDR outside the bandwidth? Thanks a lot!

 

[Henry.su]

I want to know it too

 

[maxwellequ]

I think you are confusing the Nyquist theorem: it says that the "BANDWIDTH of the input signal must be smaller than fs/2" and NOT that "the input signal frequency must be smaller than fs/2".

This means that you can sample input signals containing frequencies larger than fs/2, as long as its total bandwidth is smaller than fs/2.

See

https://www.answers.com/topic/nyquist-shannon-sampling-theorem

for more details

 

[jacobliu]

Hi, All

I have a question about the spec of a ADC

"Sampling rate of 48Ms/s
SFDR>57dB @ 36MHz input frequency
3rd order intermoduation>57dBc (2 tone 35MHz and 37MHz)
"
Because the sampling frequency is 48MHz, so the bandwidth should be less then 24MHz, then why they test the SFDR outside the bandwidth? Thanks a lot!

where did you find the spec?

in AD10265 (12-BIT 65MSPS ADC) spec:

SNR
@ 1.24 MHz 66 dB
@ 17 MHz 65 dB
@ 32 MHz 63 dB

have not meet your question,thx

 

[chency]

I think you are confusing the Nyquist theorem: it says that the "BANDWIDTH of the input signal must be smaller than fs/2" and NOT that "the input signal frequency must be smaller than fs/2".

This means that you can sample input signals containing frequencies larger than fs/2, as long as its total bandwidth is smaller than fs/2.


for more details

maxwellequ is right,it's a generalized application of the Nyquist theorem, adc's If(intermediate frequency) sampling has widely used in modem comunication circuits.

 

[Nicolas_Wang]

For DVB-C application(direct IF sampling), the signal at 36.15MHz is ampled at a freuqency in the order of 20MHz. If you want to know undersampling more detailed, you can survey ADI's application note.

 

[wonbef]

i think what maxwellequ say is very right!


regards,