[time-nuts] AD5791 for EFC control ?

David davidwhess at gmail.com
Sat Apr 20 22:49:34 UTC 2013


On Sat, 20 Apr 2013 20:14:06 +0000, "Poul-Henning Kamp"
<phk at phk.freebsd.dk> wrote:

>
>I looked at the DAC1220 first, but in an EFC application it worries
>me to no end that it is a Sigma-Delta DAC.
>
>Even if I feed it a clock divided down from the OCXO you're EFC'ing,
>to avoid beatfrequency effects, I would still worry about harmonic
>effects and noise.
>
>Also, the noise-spec is pretty horrid, almost a microvolt/sqrt(Hz)
>all the way to one kHz.
>
>For comparison the AD5791 is purely static and its noise is below
>10 nv/sqrt(Hz) from 5Hz and up.

I wondered about that also but the low frequency noise is only a
couple times higher (RMS conversion factor of 6.6) than that of a
chopper or FET input precision operational amplifier which themselves
only have twice the noise of an AD5791.

The DAC1220 span and offset drift versus temperature on the other hand
are 20 to 50 times worse than that of the AD5791 unless you want to
spend the $30 difference in price for deglitching so you can use the
AD5791 autocalibration feature.



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