[time-nuts] DAC performance [WAS: Papers on timing for lunar laser ranging]

Bob kb8tq kb8tq at n1k.org
Sun Jul 16 16:51:21 EDT 2017


One gotcha with any ADC or DAC is going to be the reference. There, you are in the same 
“get what you pay for” dilemma. Stable and noisy, can do. Quiet and not very stable, can do. 
Both stable and quiet, not so easy if you want it cheap. 

Noise can also be the sigma delta ADC’s weak point. Even at slow rates, some of them need
a lot of averages to quiet down. 

Does this or that design need this or that level of stability or noise? That depends a lot on the 
approach used. In a GPSDO, cutting down on the EFC range is a great way to “cost reduce” the 
rest of the circuit in terms of noise and stability.  Well made modern OCXO’s don’t drift a lot ….


> On Jul 16, 2017, at 2:24 PM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> Bert wrote:
>> We limited to affordable  and solderable.
>> The LTC1655 was the clear winner because
>> of linearity and temperature
>>      *     *     *
>> Five years later I know no better alternative
> Well, to name just the first one that comes to mind, how about the LTC1650?  Like the 1655, it is available in SO and DIP packages.  Its differential nonlinearity is >2x better than the 1655, it settles 5x faster (4uS vs. 20uS to 1 LSB) and is nearly 100x (40dB) quieter (30nV/sqrtHz vs. 280).  The 1650 has substantially lower glitch energy, as well (1.8nV-S for the 1650, 12nV-S for the 1655).
> Not all of the better specs are required for steering an oscillator at GPSDO rates, but others are a distinct advantage.
> I have not surveyed the field to see what other "SO or easier to solder" DACs are available with better performance than the 1655, but I'm sure there are others.
> This is to take nothing away from the 1655 -- I have accounted for thousands of them in my own designs, and think very highly of it.
> Best regards,
> Charles
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