[time-nuts] Tbolt temperature sensor

Didier didier at cox.net
Thu Feb 5 13:15:22 UTC 2009


> -----Original Message-----
> From: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Bruce Griffiths
> Sent: Thursday, February 05, 2009 3:02 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Tbolt temperature sensor
> Mark Sims wrote:
> > Actually,  what is apparently going on with the newer 
> Thunderbolts is worse than that.  The reported temperature 
> seems to only end in 0.25C or 0.75C,  so it has an effective 
> 0.5C resolution.   The basic DS1620 resolution is 9 bits,  
> but the Tbolt firmware reduces that to 8 bits since the first 
> step in the high res temp algorithm is to mask off the lower 
> bit.  Whatever is going on  they are not getting the extra 
> resolution that they think they are,  and in fact they are 
> reducing the basic resolution of the chip.
> >
> > The firmware does seem do do some filtering on those values 
> since whenever the reading steps you can see some smoothing  
> going on.  A lot of times the  temperature value oscillatates 
> around the step point.  The filter apparently does not have 
> any hysteresis.
> >
> > The older Thunderbolts produced a nice smooth curve.  The 
> high res temperature reading (Bruce says is 12 bit/0.0625C) 
> coupled with the firmware filtering gave temperature curves 
> with microdegree scale resolution.  The newer ones clunk 
> around with effectively 0.5C resolution.
> >   
> Not quite right.
> According to the datasheet the resolution with the standard 9 
> bit output is actually 0.5C.
> The MSBit is actually a sign bit.
> Despite what the text may perhaps implies, 1/2 of a LSB is 
> actually subtracted before combining with the residual count 
> and other data (see attachment for formula).
> The resolution isnt in fact being truncated at all.
> An offset of 1/2 LSB is being subtracted from the coarse 
> reading, this has no effect on the resolution.
> Bruce

I have been casually following this thread because I use the DS1629 in
several of my projects and it also has a temperature sensor so I logically
assumed (that word again) that the 1620 and 1629 had a similar way to
measure temperature. The 1629 has 0.5 degree resolution but higher
resolution is available if you are willing to put up with additional work,
just like the 1620.

However, both devices uses completely different methods to measure
temperature: the 1620 uses a bandgap reference and an analog temperature
sensor while the 1629 uses a temperature sensistive oscillator and another
that is stable.

Since the 1629 is also a real time clock that can run from a lithium battery
for a long time, I suspect they had to use a lower power scheme for it than
the bandgap reference.


Didier KO4BB

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