[time-nuts] Question for my new GPSDO

[Bruce Griffiths]

If the DAC update rate isn't excessive a 2nV-sec glitch is unlikely to produce a significant perturbation at the output of the lowpass filter following the DAC.

Bruce
> On 16 October 2019 at 21:01 Attila Kinali <attila at kinali.ch> wrote:
> 
> 
> On Wed, 16 Oct 2019 15:04:31 +1300 (NZDT)
> Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:
> 
> > Consistency of the glitch independent of DAC output is more important than 
> > its size. A constant amplitude glitch occurring at the dac update rate is 
> > more benign in its effect than a glitch whose amplitude varies with DAC 
> > output.
> 
> Yes. But it's actually not that bad. The glitch in the DAC is due to
> the CMOS switches in the DAC switching. Due to symmetry of the operation
> the glitch going from code A to code B will be the (almost) inverse of going
> from code B to code A. Which means they should (almost) cancel.
> 
> I can't find my notes from when I calculated this, but I remember that
> using an external CMOS switch had the same order of magnitude of uncertainty
> as using the DAC itself. And a few other effects would be also very close
> in magnitude. My conclusion was, considering that the back-of-envelope
> calculation of errors would result in a few bits more than necessary,
> the added complexity of using an external CMOS switch was probably not
> worth it. Of course, to be sure one would need to build both variants
> and measure them. Unfortunately, I don't have the means to do that.
> 
> Also, my guesstimate would be that using two DACs with metal-foil
> resistors for weighting would probably result in lower non-linearity.
> But the problem here is that the resistors add quite a bit of noise
> which in turn has to be accounted for.
> 
> 			Attila Kinali
> 
> -- 
> <JaberWorky>	The bad part of Zurich is where the degenerates
>                 throw DARK chocolate at you.
> 
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