[time-nuts] EFC tracking

J.D. Bakker jdb at lartmaker.nl
Sat Jun 26 13:15:51 EDT 2010


At 01:43 +1200 27-06-2010, Steve Rooke wrote:
>  > I don't know if it qualifies as simple/cheap, but Analog Devices and others
>>  have single chip low-rate sigma/delta converters with good to excellent
>  > properties; these were meant for strain gauges but should be able to track
>  > slow-moving control voltages just fine. [...]
>
>Do you have any part numbers to hand that I could go and look up
>please?

(You didn't specify runtime or resolution; I am assuming that you 
plan to monitor an OCXO CV line over several days; this would imply 
that you're interested in sub-mV wiggles on a constantish offset of a 
few V. Do set me straight if I'm wrong)

I was looking at the Microchip MCP355x-series and Cirrus 
CS551x-series. Both are available in DIY-friendly SOIC-packages; if 
you can handle SOT-23-6 and TSSOPs your choices widen (ie the AD778x 
that Jim Lux mentioned). Both families are S/D converters with 
internal digital filtering, offering 20+ bit resolution (although not 
all bits are usable, depending on your config).

Most of these parts are available from Farnell and Digi-Key (and if 
it were me, I'd order from the latter).

At 10:20 -0500 26-06-2010, Didier Juges wrote:
>You may want to check the "Analog Devices MiniKit for ADuC702x-series".
>
>http://www.google.com/search?q=Analog+Devices+MiniKit+for+ADuC702x-series
>
>This kit includes a 24 bit ADC and integrated ARM processor in a 
>small PWB with all the tools and sample code to do what you want 
>with very little code to write (you can probably use the sample code 
>as-is).

Seconded, but note that according to the datasheet it only offers an 
internal 12-bit ADC (although the docs from AD seem to contradict 
eachother here). This may be Good Enough. The advantage of the 
ADuC7xxx series over most other microcontrollers with built-in ADCs 
is that the ADuC7xxx ADC has a differential input mode. For more 
cheap boards, have a look at Olimex (http://www.olimex.com/dev/).

At 06:21 -0700 26-06-2010, WB6BNQ wrote:
>I think using a voltage-to-frequency converter would solve that problem.  They
>are not too expensive and there are several flavors from Amalog 
>devices and some others.

Good idea, but might be hard to keep stable enough if my "sub-mV over 
days"-assumption is right.

JDB.
-- 
Years from now, if you are doing something quick and dirty,
you imagine that I am looking over your shoulder and say to
yourself, "Dijkstra would not like this," well that would be
immortality for me.          -- Edsger Dijkstra, 1930 - 2002




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