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Design >> Analog Design >> CMOS temperature sensors
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Message started by hchanda on Sep 25^th, 2007, 12:40pm

Title: CMOS temperature sensors
Post by hchanda on Sep 25^th, 2007, 12:40pm

Hi all,

I am referring to various papers on CMOS temperature sensors, in all the papers I see they show plot of error (in degree C) versus the temperature.
What is this error? I mean what is the definition?

I am looking into an existing architecture which uses a bandgap and first order sigma delta converter.

How does the op-amp offset in bandgap will contribute to temperature sensor inaccuracy? Is there any literature/ book that explains about it in detail.
I would like to get a clear understanding how the bandgap and sigma-delta will contribute to inaccuracy in temperature.

Also if there is any beginner's level book that explains the operation of sigma-delta ADC in detail. Cannot get a clear understanding how it will produce a different digital code for different analog inputs. And how to measure it's bandwidth?

Thanks,
hc

Title: Re: CMOS temperature sensors
Post by tosei on Sep 25^th, 2007, 4:01pm

Hi hchanda,

For any bandgap reference, the opamp offset will contribute to the temperature inaccuracy by means of its drift. If the opamp has offset, it will have an associated drift which will degrade the reference accuracy. In order to improve its accuracy (if required) the opamp ccan be chopped (therefore the offset drift is totally eliminated).

The sigma delta contribution to temperature inaccuracy I imagine might be mostly related to the quantization noise it will introduce. According to the BW and the oversampling ratio of your SD converter, the Q noise power in the band of interest (very low frequency) will dictate the "erroR" you will have when converting such temperature reading into a digital word.

In a few words, the SD converter quantizes the analog input signal by translating it into a "pulse density". The more input amplitude the more pulses within a certain time. Since the clock is fixed, the density is quantized.

Hope this helps
tosei

The SD converter can also be single or multi-bit. In either case, the "digital word" will be obtained only after (digitally) filtering and decimating the SD output by means of a decimator.

Title: Re: CMOS temperature sensors
Post by loose-electron on Sep 27^th, 2007, 12:58pm

due to process variance, an accurate thermal sensor generally uses two different currents in a single diode (time multiplexed) and then takes the difference between those and works with that.

this allows all the process variance, absolute accuracy issues, and offset issues to fall out of the equation. The thermal monitors for all the Pentium (and beyond) processors use this method to control the fan speeds - been there done that a while back.

poke around on the web for "thermal monitor and fan control" circuits, its out there.

jerry