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Update thermistor_21.h

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Removed 5V to 3.3V table conversion.
The conversion is leading to an erronous temperature reading, as for instance 1°C reading at room temperature.
The 5V table provided in _ADC-counts_ is valid as long as the ADC reference voltage and the INA826 supply voltage are identical.
The ADC is providing counts in relation to his reference voltage and the INA826 does the very same in the analog domain.
Validated with additional PT100 measurement for 100°C and room temperature.
Note, that the quantization error of the table in combination with rounding of the temperature reading might lead to a few degrees (seen up to 3°C) of error around the center between datapoints, e.g. 25°C, 35°C, etc.
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Till 2020-09-03 13:24:36 +02:00 • committed by GitHub
parent e0b413cf19
commit 710abdf03e
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1 changed files with 4 additions and 6 deletions
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Marlin/src/module/thermistor/thermistor_21.h
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@ -23,10 +23,10 @@
#define REVERSE_TEMP_SENSOR_RANGE_21 1
#undef OV_SCALE
#define OV_SCALE(N) (float((N) * 5) / 3.3f)
// Pt100 with INA826 amp with 3.3v excitation based on "Pt100 with INA826 amp on Ultimaker v2.0 electronics"
// Pt100 with INA826 amp with 3.3V excitation based on "Pt100 with INA826 amp on Ultimaker v2.0 electronics"
// As the uC ADC reference voltage is 3.3V as well as the INA826 supply/reference, the same table as for 5V excitation is valid.
// The ADC is referring his counts to the reference voltage as the IN826 opamp does scale the output voltage to the supply voltage in the analog domain.
// Note, that the quantization error of this table in conjunction with rounding of the temperature reading can result in a few degrees error most noticeable between datapoints, e.g. 25°C, 35°C, etc.
const temp_entry_t temptable_21[] PROGMEM = {
{ OV( 0), 0 },
{ OV(227), 1 },
@ -73,5 +73,3 @@ const temp_entry_t temptable_21[] PROGMEM = {
{ OV(614), 500 }
};
#undef OV_SCALE
#define OV_SCALE(N) (N)