[time-nuts] "Audio" DAC for GPSDO?

[Chris Albertson]

I think these kinds of DACs are meant to be clocked out at some fixed
sample rate, like 44.1KHz and your software has to stuff a FIFO so
there is some milliseconds of delay in the queue.     Before you use
these write some pseudo-code and see if you can make it work.

One idea is to never write to the FIFO and change the "default" value
that gets written when the FIFO is empty, that way you never mess with
filling a FIFO thousands of times per second.

My Arduino's PWM outputs are working well.  The GPSDO has been running
now for months.  It's proof that you can do this with no external
active components.  I'm sure there must be some version of a PIC that
has analog outputs that can directly drive a XO's EFC pin.    If you
need even one external IC, you'd be best off getting a different uP.



On Mon, Jun 2, 2014 at 2:02 PM, Bob Stewart <bob at evoria.net> wrote:
> Now that my TIC is working with Bert's board, I'm considering taking the next step of designing a GPSDO from scratch.  There are several projects I'd like to do with a dsPIC33, so that was a natural choice.  But I now understand that it has an "audio" DAC and is not recommended for process control.  Could someone explain to me how such an audio DAC differs from a non-audio DAC and why it's not suitable for this application?  Is this just a disclaimer from microchip to avoid liability or is there some practical reason to go with a traditional DAC?
>
> On reading through the various datasheets, it appears to me that the concern might be that the input data to the DAC might be interrupted, thus causing it to go to some programmable "safe" output voltage.  My initial thought was just to control the value of the safe voltage and not bother to feed the DAC, though I haven't really explored the idea.
>
> Bob - AE6RV
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-- 

Chris Albertson
Redondo Beach, California