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AD7760 bad THD/Harmonics?

Question asked by trioflex on Sep 13, 2012
Latest reply on Sep 17, 2012 by trioflex

Hi

 

we just got AD7760 working - now we are not sure where the problem is or if there is any, we are using 1KHz sinus generator, that as of the author should have 0.3ppm THD and all harmonics down at least 120dB, but this is what we see when we capture the signal with AD7760 (default filter, 625k ODR). Signal is feed to +input, -input is grounded.

130db.JPG

Noise floor seems to be below 130dB what is not that bad, but the harmonics are way high? We will try other signals tomorrow, but maybe someone can already comment on the FFT plot, could it really be that the good signal generator isnt that good, or ah, yes jitter. we are clocking from FPGA pin at the moment not from our clock generator. So probably the clock jitter adds some harmonics to the spectrum.

 

Still any comments welcome!

 

/Antti

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