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Using Linear ADC and External Resistor RExt to Capture Thermistor Nonlinear Method

NTC / PTC thermistor is a sintered oxide body. Has a negative temperature coefficient, Compared with the metal resistance, resistance temperature coefficient, The sensitivity is about 10 times the metal thermal resistance. Simple structure, small resistivity, suitable for dynamic measurement, In the field of test and automatic control is widely used. But NTC / PTC thermistor thermoelectric serious non-linear. Compensating for its nonlinearity error or linearizing it is the primary problem to expand its measurement range and improve the measurement accuracy.
    When you need to convert the resistance value of the thermistor to a voltage value, Then the design of the device will be quite challenging. Thermistor is usually a high-impedance, non-linear resistance temperature components, The device simplifies one of the interface problems. However, the more challenging interface problem is, How to use the linear ADC and the external resistor RExt to capture the nonlinear behavior of the thermistor in digital form.

The term "thermistor" derives from the generalization of the description of "heat sensitive resistors". Thermistor includes two basic types, Respectively, positive temperature coefficient thermistor (PTC) and negative temperature coefficient thermistor (NTC). NTC Thermistors are ideal for high-precision temperature measurements. To determine the temperature around the thermistor, You can do this with the SteinHART-Hart formula: T = 1 / (A0 + A1 (lnRT) + A3 (lnRT3)). Where, T is Kelvin temperature; RT is the resistance of the thermistor at temperature T;While A0, A1, and A3 are constants provided by the thermistor manufacturer.

The resistance of the thermistor will change with the temperature changes, And this change is non-linear, The Steinhart-Hart equation shows this. During the temperature measurement, Need to drive a thermistor through the reference current, To create an equivalent voltage, The equivalent voltage has a non-linear response. You can use the reference table provided on the microcontroller,
An attempt is made to compensate for the non-linear response of the thermistor. Even if you can run such algorithms on the microcontroller firmware, But you still need a high-precision converter for temperature extremes in the data capture.

1, series and parallel resistance method
  The NTC thermistor has a nonlinear relationship with the temperature and must be linearized. The specific method is to first RT series of a suitable external resistor RExt, And then connected to the 1. 24V reference voltage URef, Then use the MAX6691 to measure the voltage on the RExt, The non-linearity of the NTC thermistor can be minimized in the selected temperature range.

To calculate RExt, follow these steps:
(1) Determine the temperature range to be measured (eg 0 ° C to + 70 ° C);
(2) Within this temperature range, the minimum value Rmin of the thermistor (corresponding to the highest temperature, for example + 70 ° C) is determined, The maximum value Rmax (corresponding to the lowest temperature, eg 0 ° C) and the intermediate value Rmid (corresponding to the intermediate temperature, here +35 ° C);

2, linear interpolation method:
Change the thermistor linear method, another method is, You can use "hardware linearization" techniques and a lower-precision ADC before digitizing. One technique is to connect a resistor RSER in series with the thermistor RTHERM and the reference voltage or power supply (see Figure 1). Set the PGA (Programmable Gain Amplifier) to 1V / V, However, in such a circuit, A 10-bit precision ADC can sense only a very limited temperature range (approximately ± 25 ° C).

Note that in Fig. 1 the high temperature region is not resolved. However, if the gain of the PGA is increased at these temperature values, The output signal of the PGA can be controlled within a certain range, In this range ADC can provide reliable conversion, Thus the temperature of the thermistor to identify.

The microcontroller's temperature-sensing algorithm reads 10-bit ADC digital values, And sends it to the PGA lag software program. The PGA lag procedure verifies the PGA gain setting, And the ADC digital values are compared with the voltage node values shown in Figure 1. If the ADC output exceeds the voltage node's value, The microcontroller sets the PGA gain to the next higher or lower gain setting. if necessary, The microcontroller will again acquire a new ADC value. The PGA gain and ADC values are then transferred to a microcontroller piece-wise linear interpolation program.
Getting data from a non-linear thermistor is sometimes seen as an "impossible task. "You can connect a series resistor, a microcontroller, A 10-bit ADC and a PGA with a reasonable use, In order to solve the non-linear thermistor after more than ± 25 ° C temperature measurement problems brought about after.

The thermistor is linearized with a logarithmic index circuit. The thermoelectric properties of the thermistor are exponential, A circuit configuration can be envisaged, Mainly to complete the logarithmic computing functions, The output of the circuit is linearly related to the temperature change. In line with this principle, The logarithmic exponential operation circuit is designed to realize the thermistor linearization output.

Thermistor Nonlinear Method
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