[time-nuts] Next upgrade
kb8tq at n1k.org
Sun Nov 26 11:53:55 EST 2017
Each time I’ve tried the method in the app note, there has been a tone in the output
spectrum at the sample rate of the ADC. I’ve never found a way to do the grounding
that eliminates it. The tone is large enough to show up as a spur on a “typical” OCXO
when it goes into the EFC port.
> On Nov 26, 2017, at 8:56 AM, Ole Petter Rønningen <opronningen at gmail.com> wrote:
> I guess everyone has seen this, but Linear has a nice appnote «A Standards Lab Grade 20-Bit DAC with 0.1ppm/°C Drift»
>> 26. nov. 2017 kl. 13:50 skrev Magnus Danielson <magnus at rubidium.dyndns.org>:
>>> On 11/26/2017 02:26 PM, Attila Kinali wrote:
>>> Though, if you have a decent 16bit DAC and want to get to 18bit,
>>> that's fairly simple using delta-sigma modulation... if you can live
>>> with a low pass fillter after the DAC. But the DNL will be the limiting
>>> factor here (unless you use some special techniques) and the (absolute) INL
>>> will not get better, for obvious reasons.
>> I needed 19 bit rather than 16 bit, so I implemented an interpolation scheme. A first degree sigma-delta would also be possible, but for low ratios what I did was more efficient.
>> A first degree sigma-delta is fairly simple thought.
>> The trick is that you want to push the noise high up so it becomes trivial to filter, then the filter will not be hard to design and won't be low enough to cause PLL instability and implementation troubles.
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