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Can ADC provide constant samples?

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hi friends!, I have a doubt in ADC sampled.I have a 40MHz ADC(12-bit). I use to take 20 samples from a 2MHz sine source which is constant. My doubt is that in that 20 samples say 402,1,402,1566.....etc Can I expect the ADC to give the same sample values for each cycle?.Simply,will those 20 samples I take for a cycle be almost constant for all cycles?....Thanks
 

You mean something like:

Sample 1 = Sample 21
Sample 2 = Sample 22
Sample 3 = Sample 23
Sample 4 = Sample 24
...?

If your sample rate is exactly 40,000,000 Hz and your sine source is exactly 2,000,000 Hz, then those 20 samples will be almost constant for all cycles (only depending on noise then). So, theoretically, the answer would be: YES

In other words, (Sample rate / f_sine) has to be exactly 20 to get constant values for every cycle.

Though, in real life this almost won't be possible, as you can't set a frequency which exactly is 2000000.000000000000000...Hz.

The more the ratio diverges from 20, the more your samples will "wander".
 

Hi,

if your sinewave has no jitter and amplitude noise, the answer is yes.
 

if you create the clock driving your adc from the sine wave source (assuming it has a ttl reference o/p) using a PLL to multiply it, then you'll get a fairly good lock. There will of course be phase noise etc. So the somples will never be 'the same'.
 

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