[time-nuts] Precision DACs (was: NCOCXO anyone?)
attila at kinali.ch
Sat Jul 23 14:36:28 EDT 2016
On Fri, 22 Jul 2016 12:15:25 -0500
David <davidwhess at gmail.com> 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
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
 "The 20-Bit DAC Is the Easiest Part of a 1-ppm-Accurate Precision
Voltage Source", by Maurice Egan, 2010
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