[time-nuts] Precision DACs (was: NCOCXO anyone?)

Scott Stobbe scott.j.stobbe at gmail.com
Sun Jul 24 23:48:05 EDT 2016

I doubt the AD5791 does much better than 16 bits operating at 1 Msps, when
you include glitch energy, noise, and distortion.

On Saturday, 23 July 2016, Attila Kinali <attila at kinali.ch> wrote:

> On Fri, 22 Jul 2016 12:15:25 -0500
> David <davidwhess at gmail.com <javascript:;>> wrote:
> > If you expect analog specifications in line with the claimed digital
> > resolution of ADCs and DACs, you will be disappointed.  20 bits is
> > about where they top out no matter how many bits are available; the
> > best you can hope for is that they are monotonic but how meaningful is
> > that when it is buried in noise?
> Depends on your application. If the circuitry that follows the DAC has
> some integrative/low-pass characteristics, then bits burried in noise
> might be not that bad. E.g. when controlling an VCXO, any noise beyond
> 10-50kHz will be filtered out by the crystal and its high Q.
> Similarly, low frequency noise might be eaten up by the surrounding
> control-loop. This is especially beneficial when dealing with a
> circuit that has high 1/f noise. The drawback is, that high loop frequency
> (something around 10-100Hz is the minimum required to filter 1/f noise)
> is not that easy to achieve. It requires carefull design and makes things
> generally a lot more expensive.
> > The LTC2400 is considered suitable for 6 digit designs before software
> > calibration is used which the application note and datasheet mention.
> > In this case, it is its repeatable INL which can be corrected for and
> > its low gain and offset drift which matter.
> Yes, but the LTC2400, as all delta-sigma converters, has the big problem
> that it only reaches the full performance at a very low sampling rate.
> In case of the LTC2400 it's a whooping 7sps. Ie, that would limit the
> DAC build with an LTC2400 in its feedback path to at most 3sps, probably
> even lower.
> On the other hand, a modern DACs like the AD5791 reaches full 20bit at
> 1Msps
> (resp 1us settling time to 0.02% @10V step, or 1us to 1LSB @500 code step).
> But using the AD5791 in a design isn't easy either. The dual voltage
> reference
> that is required to reach full spec is kind of inconvenient. And as phk
> already
> wrote, these DACs deliver you the reference accuracy and noise very
> precisely.
> A nice write-up on issues in this area can be found at[1]
>                         Attila Kinali
> [1] "The 20-Bit DAC Is the Easiest Part of a 1-ppm-Accurate Precision
> Voltage Source", by Maurice Egan, 2010
> http://www.analog.com/library/analogdialogue/archives/44-04/ad5791.html
> --
> Malek's Law:
>         Any simple idea will be worded in the most complicated way.
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