Which is the better for lowest spurs -the 0.5V biased ADL5365-05 vs. the 1.5V biased ADL5375-15?
I am driving the ADL5375 IQ mod with ADF9788 DAC, fixed LO freq =900M, with internal DAC DDS set to 150Mz. The data rate is 225M for Instantaneous BWidth (IBW) > 180MHz, hence I &Q baseand range 150+/-90MHz. The fixed 900M LO means the carrier and image leakage at the ADL5375 o/put can be removed by an analog filter centered at 1050MHz.
Currently I use the -05 becuse the DC biasing interface to the AD9788 DAC is easier than ADL5375-15. But I note from the data sheet for 2nd &3rd harmonics (Flo+2*Fbb) & (Flo+3*Fbb) , the the -15 is better than the -05.Eg if Fbb=70M, the signal is 970M with a spur at 1040M, inside the analog filter passband.
If lowest spurs is main criterion, which version is best? Are there two different bias types because of spur perfromance or is it because DACs have either 0.5 or 1.5V DC common-mode interface? If the -ADL537515 has better spurs nominally, how difficult is level shifting with a wide baseband from 60M to 240MHz?
Going forward, I am investigating the AD9122 DAC for IBW up to 500MHz, with the IQ mod in zero-IF mode (DAC DDS= 0Hz); RF out from 500M to 6G. The IQ baseband filter is a low -pass > 250MHz. Now, I will need carrier and image cancellation., and there is no filtering after the ADL5375. Which is best for optimum spurs?
-05 common mode version of ADL5375 is to interface our DAC(0.5V DC common mode voltage as default, 20mA Full scale current with 50 ohm termination) directly as like previous other modulators(ADL5370/1/2/3/4). And -15 version of ADL5375 would be from market requrement. 1.5V common mode voltage at modulator is one of popular common mode voltage at the time of development. So those are reason of two versions at DC common mode voltage for ADL5375-05 and -15 rather than spur performance isssue. From characterization data, -15 shows better 2nd,3rd harmonic performance overall frequencies bands including 900MHz than -05. But -05 has better 2nd harmonic performance at some frequencies. And plots below shows harmonics
performance vs Baseband input level(Fig13 for -05, Fig38 for -15) at 900MHz LO. We can see performance advatage of -05 harmonic distortion at some level. So it is hard to say one version has better harmonic performance than the other. Please refer "Driving the ADL5375-15 with a TXDAC" section at datasheet(page 26 of RevB) for the level shifting circuit with AD9122 DAC( which is same to AD9788).