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ADS1118-Q1: FSR and internal impedance

Part Number: ADS1118-Q1
Other Parts Discussed in Thread: ADS1118

For an automotive prototype project, we use your component ADS1118-Q1 for read thermocouple(range of measure is -40°C to 900°C).

But we have problem/understanding.

We don’t understand why when we change FSR, converted values (ADC*LSB) are different.

Is there a link between the internal impedance and the different measurement impedances? Even if Rdiffa and Rdiffb are big(75k). I can't find why value are different. (Internal impedance is 710k to 22M)

See table below with thermocouple type K  (with temperature around 100°C=4mV), even if I subtract 1mV(25°C ambient). I must have 3mV. I don’t understand why I have 1.29mV for 0.512(best resolution normally).

FSR ADC Value LSB ADC converted mV
4.096 24 125u 2.98
2.048 44 62.5u 2.76
1.024 71 31.25u 2.23
0.512 82 15.625u 1.29