> -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Bruce Griffiths > Sent: Saturday, September 13, 2008 10:48 PM > To: Discussion of precise time and frequency measurement > Subject: Re: [time-nuts] Thunderbolt Monitor (Didier Juges) > > > 7. Provide the ability to read several temperature devices. > > Something as simple as the DALLAS One-wire devices as > > probes would work. > > The capability of using higher accuracy, higher resolution > temperature sensors such as RTDs and thermistors may also be useful.
Regarding temperature sensing, Using easily obtained NTC thermistors and the built-in 12 bit ADC of a Silabs processor, I was able to obtain stable temperature readings with about 0.02 degree C resolution at 25 degree without any digital filtering or analog processing (other than decoupling capacitors), by directly feeding the voltage from a voltage divider composed of the thermistor and a precision 10k resistor to the ADC, and using the processor's built-in voltage reference to drive the divider. A small amount of dithering (or simply by averaging several samples and using noise as the dither) will easily give 0.01 degree C resolution if you like to see all the digit values used. I did not bother to calculate the actual precision, but a rough check shows the main absolute error to be the thermistor itself, all other errors can be made negligible compared to it by using a precision resistor and clean layout. The basic unadjusted accuracy is a fraction of a degree, sufficient for most applications. This with a thermistor that costs about $1.00 at Digikey and is as big as a 1/8 Watt resistor... The software does the voltage/temperature conversion using math functions (log particularly), so make sure your compiler supports it. Alternately, you could use lookup tables, but that becomes impractical for anything greater than 8 bits of resolution. Due to the non-linearity of the circuit and that of the NTC thermistor itself, resolution of that particular circuit degrades away from 25 degree C to about 0.08 degree C at -35 and at +85 degree C. Different choices of thermistor/resistor values can optimize the resolution at the temperature of interest, but make sure you take the dissipation in the thermistor itself into consideration, as it will create a non-linear offset. Any type of analog linearization (in front of the ADC) over a broader range of temperatures will cost a lot of money because you will need an exponentially greater number of precision components. If greater resolution is needed, a 16 bit ADC would be the best solution in terms of cost and hassle. Didier KO4BB _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
