I have been testing the NAU7802 non calibrated internal temperature sensor to see if it is usable as a replacement for e.g. the TSic306. A MCP9808 calibrated I2C temperature sensor helped me get some reference temperature readings. For testing I put the NAU7802 and the MCP9808 into the freezer of my fridge and after a couple of hours took them out and watched them warm up. From what I can tell the NAU7802 build-in temperature sensor is very linear (please see the image below). The only problem I encountered can be seen in the histogram below. There seems to be a systematic error in the NAU7802 ADC reading the temperature sensor of about +-0.3 degree Celsius. I'm guessing this has something to do with me recalibrating the analog frontend of the NAU7802 whenever input channels are changed (but I haven't investigated further). Despite these errors I think the build-in temperature sensor is still adequate for the birdhouse. It measures the temperature exactly in the right spot and helps save power compared to any extra hardware.