[time-nuts] yet another GPSDO design, or so

Attila Kinali attila at kinali.ch
Tue Jun 29 17:50:59 UTC 2010

On Tue, 29 Jun 2010 12:54:52 -0400
"Bob Camp" <lists at rtty.us> wrote:

> Some of the TI (Burr Brown) 16 bit parts are 1/4 lsb DNL on > 98% of the
> transitions. Most of the time you have a "coarse" DAC that's at 18 bits.
> Some of the errors are predictable and you can take them out with a simple
> training process. You won't easily get 24 bits, but 20 is very achievable. 
> 20 bits is a nice round million to one factor. A 1 ppm EFC becomes a 1 ppb
> step if everything is perfectly linear. Even with a 4:1 slope ratio it's
> still quite small. With a more restricted EFC of 2.5x10^-8 you are at
> 1x10^-13 with the 4:1 slope ratio.  

What currently puzzles me with such high bit D/A solutions is, how
to stabilize the output voltage. Given a 20 bit D/A working over
e.g. an 5V range (0-5V), 1LSB becomes 4.7uV. With 24 bit it's 
already at 300nV. This is getting awfully near to the noise voltages
that a low noise circuit has. Not to talk that designing a power
supply with a noise of less than 1uV is very daunting task.

			Attila Kinali
If you want to walk fast, walk alone.
If you want to walk far, walk together.
		-- African proverb

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