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RF430-TMPSNS-EVM : Question about circuit

hyosung kim
Intellectual 550 points

Part Number: RF430-TMPSNS-EVM

There is one part of the circuit that I don't understand:
I know SVSS is about 100mV compared to Ground.
The voltage to ADC2 will not change,
I thought the voltage entering ADC1 could be changed by the thermistor.
However, the circuit alone does not indicate whether the voltage entering ADC1 changes as the resistance changes.
Can you tell me what the principle is?

  • 0
    TI__Guru*** 128235 points
    Hello Hyosung,

    When using a thermistor, the resistance changes based on the temperature. To measure this, when in Thermistor measurement mode, the ADC outputs an identical current through both the reference resistor, and the thermistor.

    Then the device gets voltage measurements for both the thermistor, and the reference resistor.

    From here, math comes into play. Specifically, Ohm's Law.

    The voltage across the reference resistor is a known amount, as is the reference resistor value. These are used to calculate the current flowing through the reference resistor (which is the reason we need the reference resistor at all).

    With the current known, which is identical to the current flowing through the thermistor, then we now know the voltage across the thermistor, and the current through the thermistor. This allows us to calculate the Resistance of the thermistor.

    Then a look-up table based on the thermistors data sheet, or a formula provided by the thermistors manufacturer can be used to derive the temperature based on the resistance of the thermistor.

    Hopefully this explains the operation for the ADC in thermistor mode.

    Note that this method only applies for thermistors, and for other sensors, you would just read the voltage out of the sensor as there wouldn't be a reference resistor or a need to measure the exact current through the sensor.