[time-nuts] Temperature sensors and quartz crystals (was: HP5061B Versus HP5071 Cesium Line Frequencies)

Bob kb8tq kb8tq at n1k.org
Mon Jun 5 07:41:46 EDT 2017


> On Jun 5, 2017, at 7:30 AM, Attila Kinali <attila at kinali.ch> wrote:
> On Mon, 5 Jun 2017 01:18:59 +0100
> Adrian Godwin <artgodwin at gmail.com> wrote:
>> Where do digital sensors (e.g. ds1820 and some more recent parts from TI)
>> fit into this ?
> AFAIK, these are all band-gap temperature sensors. But unlike a discrete
> sensor, you have the problem that they only contain a low resolution
> ADC on die (somewhere between 8 and 14 bit). If your goal is to measure
> temperature and report it with an accuracy of about 1°C, then these are
> the easiest to use sensors you can buy. Sensor noise doesn't really matter
> with them, as it is dominated by the low ADC resolution. I don't have any
> long term stability data on those, but given their use-case I do not think
> that they are very stable.

Based on using them in a lot of designs, they are indeed quite stable. They are not
going to rival a thermistor or an RTD, but compared to their resolution they are stable. 
Put another way, if they read out at the (say) 0.5 C level, you can come back a year later 
and the temperature repeats at < the 0.5 C level. 

None of this is simple or straightforward. All temperature sensors have a sensitivity 
to strain. They all exhibit some level of hysteresis.  That can make aging measurements 
a bit challenging. 


> Although long term stability might not be an
> issue at all, again due to low ADC resolution.
> If you need better precision, accuracy, or stability, then choosing one
> of the modern delta-sigma ADCs that directly support thermistors
> (e.g. like AD7124) is not much more difficult, though a bit more expensive
> (around 10USD instead of 5USD like for an TMP107). Additionally you need
> to calbirate the system, which means you need a reference temperature sensor
> and a setup with which you can produce different temperatures. Though for
> an oven kind of temperature control, one can live without calibration.
> 				Attila Kinali
> -- 
> You know, the very powerful and the very stupid have one thing in common.
> They don't alters their views to fit the facts, they alter the facts to
> fit the views, which can be uncomfortable if you happen to be one of the
> facts that needs altering.  -- The Doctor
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