The design is becoming fairly mature, and as the results get better, more little things appear that are possible avenues for improvement.

The original Connor Winfield application note had an 18 bit DAC, but we went down to a 16 bit DAC instead. One reason for that was that the 18 bit DAC was interpolated, which was a technique I didn't fully trust.

But in the most recent results, I could see in some of the frequency graphs what appeared to me to be a stepwise behavior. The oscillator would dither between two frequencies, clearly wishing it were somewhere between. That led to a higher ADEV, of course.

So I decided to see if reverting back to the AD5680 from the AD5061 would make a difference.

Since the 5680 is interpolated, it requires a low-pass filter on the output so that the 10 kHz interpolation doesn't wind up frequency-modulating the output. This winds up being slightly complicated by the fact that we have the compression resistor network in place as well. The DAC datasheet limits the amount of capacitance on the output, so the RC filter can't have more than about a 0.01µF cap. But in order for the RC filter to work, the roll-off frequency has to be lower than 10 kHz (otherwise why bother?). To achieve a 1.5 kHz roll-off (recall that the roll-off frequency is 1/[2πRC]), that means we need to have a 10kΩ resistor. But the impedance of the compression network is more like 5kΩ - much too low, causing the DAC to basically be ignored. The solution was to multiply all of the resistors in the compression network by 10. The end result is 10kΩ in series with the DAC output, then a 0.01µF cap in parallel, then another 10kΩ in series, followed by two 100kΩ resistors in parallel to ground and Vref, and then finally the oscillator control voltage input. There's probably a better way to achieve this. It's conceivable that the second 10kΩ resistor could be replaced with 0Ω - that the resistor for the LPF can also serve as the resistance for the compression network.

The firmware needed a slight adjustment as well to reflect the slightly different data format of the new DAC. That's just a #define in the code. The plan going forward is for the DOT050V variant to retain the AD5061, since it is unlikely that it will benefit from the added resolution.