I am using ADS5681 in a system in which it acts as an Arbitrary Waveform Generator (AWG) with a sampling clock frequency of 312.5 MHz.
When characterizing it with sine signals, I have found that the desired sinusoid seems to be modulated by low frequency random noise and various tones. There is nod additive noise but multiplicative. I came to this conclusion when programming the DAC with a constant digital signal, thus making the potential clock jitter irrelevant. The interfering signals where also present.
Then, I configured register BiasLPG_A (CONFIG6) to 0, thus programming a 95 kHz low pass filter in the DAC current source (instead of default value of 472 kHz). Then, the most offending spurs at +-610 kHz at -56 dBc vanished. Other reduced their amplitude.
However, some other remain, and most offending one is at +-4.8 MHz. I have made the following findings about it:
- Spur level is proportional to carrier frequency (see figure below)
- When changing the Full Scale current (by programming CONFIG7 register), the spur maintains relative level with desired signal
- When reducing the sinusoid amplitude, the spur maintains relative level with desired signal
- When changing the sampling frequency from actual 312.5 MHz to 250 MHz, the spur level reduces in an approximate ratio of 20*log10(Fs1/Fs2)
- The spur is visible when configuring a DC signal, but level is low (as corresponds to low frequency tone)
- The spur level changes level at DC depending on configured level: -80, -77 and -74 dBm when configuring maximum positive, zero and maximum negative value in the DAC.
- When disconnecting the DAC active output driver and measuring with a transformer, the interference relative level does not change.
- When bypassing RBIASJ, there are no observable changes.
There are other spurs at lower frequencies and with lower amplitudes.
In the board there are no switch mode power supply at this frequency. I have tested to change clock frequency of other digital elements in the board, without effect.
I have observed this behaviour in all the boards I have tested.
Do you have any clue on what may be happening?
Regards
Luis Miguel