I measured my room temperature for a long time to evaluate two kinds of semiconductor tempeature sensors on my mother board. I assembled and soldered the two temperature sensors with 1mm gap on my mother board. I show a chart drawn by my app Slip21. I set offset value of Slip21 not to overlap the curves each other. The two sensors acutually should indicate the same temperature. I found noise of M3 is much bigger than M4. I looked into the noise by DSO Tektronics TDS310. The DSO averaging showed to suppress the noise well. I wrote it in my blog and you see detail in Japanese. I decided to amplify M3 by OP amplifier. I have LM358 and used it. I designed its circuit carefully because LM358 bias current is big and used a few resistors and a capacitor in my part box. I found MCU averaging process is useful for suppressing the semiconductor noise. I stopped fast scanning of A/D and adopted conventional scanning. I think it would be better. You can calculate the scanning time reading the MCU manual. I measured that the scanning time is 31.25kHz. It is far faster than varying temperature.MCU Board: Akizuki H8/3664F
I found the abnormal M4 output under 15°C, when I began measuring my room temperature by MCU averaging A/D inputs.
I stopped to work Slip21 and TDS298 and checked M4 output on my desk. It seemed that the sensor is different from my
order part model. When I opend the output from A/D input, the output value rised. I measured the value by DSO. I
doubted the load driving ability of the sensor. The data sheet describs to load 50uA. I calculated the output
impedance of source load. The equivalent resistor was 25kΩ! I thought that M4 indicated larger temperature than
M3 because the sink load impedance is rather big. It was lucky that I used LM358 OP amplifier. LM358 is dual type. I
decided the unused one as voltage follwer to lower output impedance. The below graph shows M3 temperature is the same
of M4 under 15°C, when the M4 offset is zero. It means that the senor model is different from my order one. The
lower temperature limit of the one on my board is 10°C.
I open my mother board circuit (3664.pdf) and hope that you will design another circuit. My circuit is not best because I used limited parts in my part box.