Low-Cost Calibration Techniques for Smart Temperature Sensors

Smart temperature sensors generally need to be trimmed to obtain measurement errors below +/- 2 degrees C. The associated temperature calibration procedure is time consuming and therefore costly. This paper presents two, much faster, voltage calibration techniques. Both make use of the fact that a voltage proportional to absolute temperature (PTAT) can be accurately generated on chip. By measuring this voltage, the sensor's actual temperature can be determined, whereupon the sensor can be trimmed to correct for its dominant source of error: spread in the on-chip voltage reference. The first calibration technique consists of measuring the (small) PTAT voltage directly, while the second, more robust alternative does so indirectly, by using an external reference voltage and the on-chip ADC. Experimental results from a prototype fabricated in 0.7 mu m CMOS technology show that after calibration and trimming, these two techniques result in measurement errors (+/- 3 sigma) of +/- 0.15 degrees C and +/- 0.25 degrees C, respectively, in a range from -55 degrees C to 125 degrees C.