[time-nuts] Precision DACs

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Jul 27 17:47:21 EDT 2016


On Wednesday, July 27, 2016 08:55:20 PM Robert LaJeunesse wrote:
> The concept of track & hold was well understood in 1966 and documented in
> the classic Philbrick Researches amplifier application book, now available
> at
> http://www.analog.com/library/analogdialogue/archives/philbrick/computing_a
> mplifiers.html
> 
> Bob LaJeunesse
> 
> > Sent: Wednesday, July 27, 2016 at 2:15 PM
> > From: "Scott Stobbe" <scott.j.stobbe at gmail.com>
> > To: "Discussion of precise time and frequency measurement"
> > <time-nuts at febo.com> Subject: Re: [time-nuts] Precision DACs
> > 
> > The first reference at hand I checked was the ADI Data Converter Handbook,
> > 1986. Pg 60 Discusses track & hold, with a reference to the HDD-1206 as
> > including track/hold on die.
> > 
> > On Tue, Jul 26, 2016 at 2:46 PM, Richard (Rick) Karlquist <
> > 
> > richard at karlquist.com> wrote:
> > > On 7/25/2016 10:42 PM, Scott Stobbe wrote:
> > >> dramatically different due to glitching on code transition. That being
> > >> said, they are kept separate not to confuse sources of error.
> > > 
> > > FWIW:
> > > 
> > > The 5071A has a "home brew" DDS that was designed by the late
> > > (and great) Robin Giffard.  He used what he called a "blanking"
> > > circuit that disconnected the DAC during the time period when it
> > > is at risk for glitching on code transitions.  He described it
> > > in terms of some new innovation he had invented.  It seemed to
> > > me an "obvious" (that loaded word) idea that surely must have
> > > been used before.  In any event, he was clearly doing something
> > > right (as usual).
> > > 
> > > Rick
> 
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For the particular application a track and hold circuit is not necessary a 
much simpler circuit that merely disconnects the DAC from the filter input and 
connects the filter input to ground will suffice.
This would result if done properly in a set of constant amplitude glitches at 
the sample rate which can easily be rejected by the reconstruction  filter.
The DAC output in effect would be modulated by a square wave with a duty cycle 
and phase with respect to DAC transitions selected to remove DAC glitches from 
the filter input. The only effect of this modulation on the filtered output 
signal will be a small reduction in amplitude to that achievable with a sample 
and hold style deglitcher.

Bruce



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