On 10/23/2017 03:52 PM, Charles Steinkuehler wrote:
The 2K resistor and the thermistor form a voltage divider. The
voltage divider feeds a 4.7K resistor and two capacitors which form an
RC filter to reduce noise. The BBB presents almost no load on it's
ADC input pins (CMOS inputs, so it's just leakage current), so there
is essentially zero voltage across the 4.7K resistor unless the
thermistor is changing value very rapidly (or there is noise on the line).
With no load on the THERM* pins and no effective voltage across the
4.7K resistor, the 4.7K doesn't enter into the temperature equations
and the ADC is directly reading the thermistor voltage (V_T). All the
thermistor current is flowing through the 2K pull-up (I_PU), and those
two values are used to calculate the thermistor resistance.
Ahh, beautiful explanation, thank you. I just couldn't get it out of my
head that the 4.7k resistor was tied to ground. D'oh!
John
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