I have an application for a 10-year completely sealed battery-powered Bluetooth temperature measurement system providing an accuracy of +/-0.5C over a range of 0C to 50C. The device cannot be calibrated during its working life, and I'd prefer not to calibrate in production. The simplest solution is something like a Si7054, which has a +/-0.4C acccuracy and a 0.01C drift per year = +/-0.5C. Great. $1.20 / 1K units.
However, can I do better/cheaper with fixed resistors, an NTC thermistor, and a ratiometric measurement using an onboard ADC? For instance:
NTC Thermistor:0.5% 100K, Beta 0.5%, $0.174/1K (TDK NTCG104ED104DT1X) Resistor 0.1% 100K, 25ppm/C = 0.1625% $0.066/1K (ARCOL APC0603B100KN)
Estimation of total accuracy ~1.1625%, cost $0.24
Now the bad part: The nrf52810 BLE IC has a +/-3% error on the 12-bit ADC after its internal calibration. The system error is > 4.2% once you account for propagation of errors. My understanding is that NTC sensors change ~4-5%/C so now we have an error of ~+/-1C. Darn.
Is there a simple way to calibrate this error out? The nRF52 can perform internal offset error calibration against ground. It seems using precision resistor dividers or fixed voltage references to provide a second reference point would allow me to figure out gain error and get enough accuracy. Am I missing anything here? Any other sources of error? Words of wisdom from the experienced?