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how to use N9020/N9030 to measure SNR and SFDR of a low frequency DAC?

Question asked by icdesignfish on May 16, 2014
Dear sir/madam,
                       In this week I tested a DAC chip with N9020 signal analyzer in open lab in Agilent, Shanghai.The DAC has 500KHz input and 1.5Msps sampling rate.But some problems appeared.I need your help.
                       First, the noise floor of the DAC is so small that it is covered by the instrument noise.Because when I add an input signal, the noise doesn't change.What should I do to sperate the DAC noise and instrument noise?
                       Secondly, I can't get the accurate noise level. Because the noise level is not as stable as high frequency situation.There is a slow descent process around input signal, I can't select noise level at one point as the average noise.I have to do integration from the 
signal to nyquist frequency to get the total noise.When I want to use the same method to get the signal level, the integration function doesn't work for the integration bandwidth is too small.I have to select the peak value as the signal level.The result doesn't seem good.
                       At last, a warm-hearted engineer help reduce the instrument noise and the DAC noise can be seen.But when I reduce input level and keep other parameters constant, the noise will decrease too,and the difference between signal and noise seems the same .That's just not right.
                        Look forward to your reply!
                                                                                                                                                                         Yours sincerely