Hi there, we are considering ADS8863 as replace for AD7982 in a cost down project. When comparing AC performance specification I go back to the definitions of SNR, THD,SINAD and SFDR(Spurious free dynamic range).
This referecne article (https://www.ni.com/content/ni/locales/zh-cn/support/documentation/supplemental/06/understanding-frequency-performance-specifications.html#section-1740667349) gives simple and clear explains on these definitions.
In my understanding, all these specs can be uniformly described as power (or volatge) of Signal divided by X , that is (Sig/X); and when represented in decibels, it will be Sig(db) - X(db). Here the letter X refers to:
(1) rms of noise , for SNR
(2) RSS of harmoniscs (9th order for ADS8863) , for THD. (positive dB value)
(3) RSS of rms noise + harmonics, for SINAD. so SINAD value is always less(worse) than SNR and THD.
(4) max. (worst) spur (harmonically or nonharmonically related spur ) , for SFDR.
My questions are :
(1)If in a same test setup (as in the ref article, pictures snapshot below), the amplitudes of harmonics and spurs shoud always be higher than the noise floor level (if not , you wont see any "spikes"), so we could intuitively get a Larger SNR dB value than SFDR(or THD) from the definitions. I also marked this idea on the bottom-right figure . However, the actual datasheet data is completely opposite to this thought (Datasheet : SNR 96.5dB < SFDR 112dB) . Something may be wrong with my thinking, hope some expert could advice. This is confusing for me....
(2) When choosing or comparing a ADC, how should we use the parameter SFDR ?
Thanks a lot !