if the spurious is not counted（because the frequency of the input signal is known）,how much dynamic range can be measured by AD6645.

the frequency of input is 3MHz,sample frequency is 90MHz.

after sample ,follow the process:

downsample to 10MHz;

do 65536 point fft;

question :

whether the input singal which amplitude is -100dBm(less than 1lsb) can be measured or not? why ?

i have done experiment on one ADC evaluation board(16 bit adc). all the signal from 10dBm to -110dBm can be measured.

Hey Chen,

I agree that this seems a bit like magic.

The key to understanding this is recognizing the statistical properties of both your signal and noise. Between subsequent data conversions, signal codes are correlated with previous ones and noise codes are uncorrelated.

For correlated signals, doubling the number of samples increases your RMS signal power by 6.02 dB.

For uncorrelated signals, doubling the number of samples increases your RMS noise power by 3.01 dB.

Given this, doubling the number of samples of a spectrum containing both signal and noise will net a signal-to-noise improvement of 6.02 dB - 3.01 dB = 3.01 dB each time.

To resolve signals smaller than an LSB, we need only to take more and more samples to leverage this behavior until the signal power becomes meaningfully higher than the noise power.