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# AD9652: Confusion with Input Frequency Range settings

Category: Datasheet/Specs

In AD9652, there given a register 0x22A which is useful for "Input frequency settings" (in which it says till 150MHz it is 1st Nyquist, 150-300MHz as 2nd Nyquist & more than 300MHz belongs to 3rd Nyquist). How does that work with respect to the input clock we give. How does this Nyquist values given related from our way of visualizing frequency in Nyquist's through FFT which we check according to our sampling frequency (CLOCK INPUT). Can you please clarify me with my confusion regarding this!

I hope I will listen from you very soon

Thanks & Regards
Saravana S R K

• Hello Saravana,

To give you understanding on visualizing frequency in Nyquist zone, consider the illustration below:

where in:

fs = sampling frequency

From the datasheet  ADC AC specification, the clock input = 1240MHz divided by 4 provided by AD9652 clock divider which will give you 310MHz (1240/4)  sampling frequency..

Thus as illustrated in Table9:

0-155MHz - 1st Nyquist zone

155-310MHz - 2nd Nyquist zone

310MHz above - 3rd Nyquist Zone

Hope this help.

Regards,

jmoreno5

• Thank you for the reply,

What if my input clock = 100MHz then, my 1st Nyquist will be <50MHz, 2nd will be in between 50-100MHz and >100MHz will be my 3rd Nyquist. My question is does these settings on datasheet according to "310MHz" as its "Fs" will be the same or will be according to my 100MHz as Fs.
Because if I give an input clock = 500MHz, does this setting need to be set to the register "0x22A" should be according to 310MHz as Fs or 500MHz as Fs.

• Hello,

The sampling frequency (sample rate) is taken after the input clock rate conversion. From your example above of input clock rate = 100MHz using clock divider 1 will give you 100MSPS sample rate. The Nyquist zone always reference on your sampling rate (Fs) (here after conversion is 100MSPS) so is your register "0x22A". Thus your 1st Nyquist <50MHz, 2nd 50-100MHz and 3rd>100MHz as you mention.

From your example no.2 given input clock of 500MHz, please take note that from table4 in the datasheet:

The maximum conversion rate (sample rate) = 310MSPS, this is taken after the input clock rate conversion of 1240MHz with the divider of 4.

Thus, with input clock=500MHz , you need to take first the conversion rate, note that the minimum conversion rate is 80MSPS and max of 310MSPS so you need to use clock divider of 2 which will give you 250MSPS (500/2) so your register 0x22A will follow according to your 250MSPS sampling rate, not your input clock.

Hope this clears out.

Regards,

jmoreno5

• Thank you again for the clarification

Thanks & Regards
Saravana SRK