[time-nuts] EFC tracking

Bob Camp lists at rtty.us
Mon Jun 28 17:18:32 UTC 2010


TI has a line of ADC's that drop a chopper stabilization loop around a fairly normal 24 bit "audio ADC" part. Their claimed benefit is that they take down the level of flicker noise and DC drift present in audio only parts. 

Since most of the "interesting" stuff on an EFC is long time span, flicker noise is indeed something to be concerned about.


On Jun 27, 2010, at 3:07 AM, Poul-Henning Kamp wrote:

> In message <HMEBKHFEACMNLMHIAFDNEEGPONAC.jmiles at pop.net>, "John Miles" writes:
>>> That is actually a very debatable proposition, a lot of them are
>>> tracking types that conveniently cover up any lack of linearity
>>> on the analog side of the fence.
>> Can you elaborate on that?  Linearity is linearity - wouldn't it ultimately
>> show up in the card's IMD specs?
> That would sort of assume there were relevant specs provided.
> The Audio ADC's are provided with audio relevant specs, which are
> not the specs you would care about in the suggested application.
> Broadly speaking, the important ability for a audio ADC is to be
> able to capture the top 16 significant bits of a continuous signal
> in an approx 24 bit space, sort of like a 16b*2^8b floating point
> format.
> Concerns like long term stability top out at "30 seconds", whereas
> woodoo-concerns like clock-jitter has gotten all the snakeoil
> recently.
> And then there are the features that really will totally get
> you in this application: autozero features, that are built with
> the assumption that any DC on the ADC should be compensated out.
> These ADC's are great for sound and they are almost perfect for Ham's
> SDR experiements.
> But they are not and will never be 7∏ digit 200 ksps instrumentation
> grade ADCs.
> You can get a brand new china-style DVM with USB port for approx
> 150USD on ebay, and as other said, you can get a HP3456A for as
> little as 50USD, but would need a GPIB-USB converter.
> By the time you have fixed a long term reference for your soundcard,
> which likely entails a buried zener chip, a totally new input
> section and a thermally controlled enclosure, you will have spent
> many times that, and still get a performance much worse than the
> HP3456A.
> So if the objective is to learn about analog/digital converters
> and their ailments: go for it.
> If the objective is to measure the EFC voltage over the long term,
> hit ebay, find instrument built for such purposes and move on.
> Poul-Henning
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe    
> Never attribute to malice what can adequately be explained by incompetence.
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