The problem is that the gain and offset of the 2 DACs changes with time
and temperature so that the required corrections also change.
Ideally an autocalibration technique would be used to dynamically track
such changes.
Since changes in the coarse DAC are only required infrequently and the
mismatch only affects the region around coarse DAC transitions which are
relatively infrequent (or should be) most designers choose to live with
the increased loop settling time at such transitions.
With sufficient overlap between the coarse and fine DACs only small fine
DAC changes should be required to compensate for mismatch between the
coarse and fine DACs after a change in the coarse DAC output.
The coarse + fine DAC approach is used in some GPSDOs and in particle
accelerator control systems.
Bruce
[email protected] wrote:
Hi,
just a clarification, I did write 4066 it is a 4046 that I replaced. Take
a look at the MCP 4822 dual 12 bit D/A In the data sheet they have an
example using one for coarse, the other for fine steps, I realize that the
transition is not perfect but maybe code can compensate for the transition.
Bert Kehren
In a message dated 6/29/2010 5:10:39 A.M. Eastern Daylight Time,
[email protected] writes:
Moin,
On Sat, 26 Jun 2010 21:14:02 EDT
[email protected] wrote:
What you want is basically a Shera Board. That design has been around
for
quite some time and has served me very well.
Yes. The Shera Board and similar designs serve as an example for me.
I have a total of six running
including two controlling Rubidium. There are in my opinion a couple of
problems: not every 4066 works on the design the 18 bit D/A is very hard
to
find and now expensive and the single step of the D/A is intended for a
1.7
E-13 frequency step.
Yes. My goal is to update the venerable 4066 with something more
modern and have components that are easy to get trough farnell, digikey,
mouser, and all the other distributors. Yes, 16bit D/A seems to
be the maximum that is currently available. It crossed my mind
to build a 24bit R-2R D/A using discrete components, but this might
have actually a worse performance than a off the shelf 16bit D/A.
(temperature drifft, resistor values missmatch, EMI, etc)
Attila Kinali
--
If you want to walk fast, walk alone.
If you want to walk far, walk together.
-- African proverb
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