[time-nuts] EFC tracking

Bob Camp lists at rtty.us
Mon Jun 28 16:07:12 EDT 2010


Hi

The one I'm building up is a ADS1278

http://focus.ti.com/lit/ds/symlink/ads1278.pdf

There are others in the series with fewer inputs. As with a lot of converters, there isn't a whole lot of data on them.

Bob


On Jun 28, 2010, at 2:49 PM, Didier Juges wrote:

> Bob,
> 
> Do you have a part number to suggest a quick glance a the TI web site did not show up any parts in that category...
> 
> I found a Cirrus Logic ADC with chopper front end and good low noise specs, CS5530.
> 
> It's a 24 bit ADC but the ENB (noise free) drops to 15 at 1ksample/sec. Even at the lowest rate, it only has 19 bits noise free, so I assume digital filtering would be required to obtain 24 bits of resolution.
> 
> Didier
> 
> 
> ------------------------ Sent from my BlackBerry Wireless thingy while I do other things... 
> 
> -----Original Message-----
> From: Bob Camp <lists at rtty.us>
> Sender: time-nuts-bounces at febo.com
> Date: Mon, 28 Jun 2010 13:18:32 
> To: Discussion of precise time and frequency measurement<time-nuts at febo.com>
> Reply-To: Discussion of precise time and frequency measurement
> 	<time-nuts at febo.com>
> Subject: Re: [time-nuts] EFC tracking
> 
> Hi
> 
> 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.
> 
> Bob
> 
> 
> 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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> 
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