Thermistor combined with high-resolution ΣΔA/D converter to measure temperature

Temperature is one of the most commonly measured variables in industrial, consumer, and computer applications, and thermistors are one of the primary means of monitoring this physical condition. But the thermistor output must be linearized in the digital or analog range to obtain accurate measurements. The excitation source and compensation must also be selected for the self-heating effects of the thermistor itself. Errors caused by overheating cause the device resistance to change, allowing errors to enter the measurement system.

In most applications where temperature is measured, the measured value must be converted from analog to digital form. The use of high-precision Σ△ converters can greatly reduce the large number of signal conditioning components required for conversion. This is a high-precision, low-cost system implementation solution.

Thermistor for temperature measurement

Thermistors are circuit components made of semiconductor materials. They have high negative temperature coefficient (NTC) or high positive temperature coefficient (PTC) characteristics. An NTC thermistor is equivalent to a resistor, with a temperature coefficient range of -3% ~ -5%/℃. For a thermistor, the absolute value output changes by one ten thousandth over its operating temperature range. High sensitivity allows thermistor circuits to detect instantaneous changes in temperature that are not possible with thermocouples or RTDs. NTC thermistors are best for precision temperature measurements, while PTC thermistors are better suited for switching applications.

In many applications, thermistors exhibit high stability, precision, small size, flexibility, and low cost. They also have fast response times and are among the most sensitive temperature sensors. Engineers, scientists, and technicians use thermistors in communications, instrumentation, automotive, medical, aviation, aerospace, and consumer applications.

The thermistor temperature curve can be approximately expressed by the Steinhart-Hart equation:

1/T=A+B(lnR)+C(lnR)3

Where T: temperature (k)

R: Thermistor resistance (Ω)

A, B, C: Curve fitting constants

A, B, and C can be found by selecting three data points of the data curve and solving three simultaneous equations. When the selected data point spacing does not exceed the thermistor temperature range calibration midpoint of 100°C, the equation approximates a ±0.02°C curve fit.

High resolution data collection

In data acquisition applications, a high-resolution A/D converter is required to digitize the signal generated by the thermistor contained in the measurement circuit. Analog Devices' AD7711 (see Figure 1) is a signal conditioning A/D converter that is particularly suitable for temperature measurement applications using thermistors.

This integrated solution provides an on-chip power supply and voltage reference that can be used to excite the thermistor with a constant current source or a ratiometric voltage. The AD7711 also contains a programmable gain amplifier, providing a gain of 1 to 128, enabling the front-end signal range to be 20mV to 2.5V (unipolar) and ±20mV to ±2.5V (bipolar). AD7711 uses various methods in the ΣΔ modulator to achieve the gain required for digitization, avoiding the noise and offset problems associated with discrete signal conditioning circuits.

The analog input is differential, with common-mode rejection greater than 90dB (up to several kHz). This input eliminates large DC voltages from measuring. The reference input is also in differential form, enabling ratiometric work in the front end.

On-chip ΣΔ processing provides high-energy filtering. The main benefit of on-chip filtering is that it excludes dominant frequency components and sensor excitation frequencies from the input signal, which must be removed in the application.

In addition to attenuation, this filter provides 120dB attenuation notch filtering. The notch filter can be set at 50Hz or 60Hz to eliminate line frequency components from the system. On-chip digital filtering not only provides out-of-band signal attenuation but also reduces anti-aliasing requirements (as a result of the high oversampling rate used in ΣΔ processing).

The offset and gain errors caused by the A/D converter can be eliminated by adding a known voltage to the input and comparing it with the desired code output, and calculating the offset and gain correction factors based on the A/D conversion difference. On-chip correction not only eliminates offset and gain errors caused by the A/D converter, but also eliminates errors that occur in the front-end circuit. The surface mount package (SOIC) used by the AD7711 is particularly suitable for integrated designs with limited board area.

In portable instruments or handheld healthcare products, power is an additional expense. The AD7705 A/D converter column is suitable for these applications. It consumes only 1mW and has the same signal processing front end, sigma delta modulator and digital filter as the AD7711 (it has no on-chip current source or base).

Current-excited sensing resistor application circuit

The circuit shown in Figure 2 uses the AD7711 to digitize the output voltage generated by the thermistor, using an on-chip 200μA current source.

The thermistor used in this circuit is a 0.3k1A1 from Betatherm Ireland Ltd., with a calibrated resistance of 300Ω at 25°C. The same current source that energizes the thermistor in this circuit also produces the reference voltage. Therefore, excitation current changes do not affect performance. The most common wiring configuration in applications is the 4-wire punch/sense configuration, which also reduces the impact of lead resistance on system performance.

The lead resistance of the drive lines only changes the common-mode voltage without degrading circuit performance. The lead resistance of the sense wire is unimportant because there is no current flowing in the AD7711 analog high input impedance connection.

Many applications use only two wires to the thermistor because the wire overtemperature resistance is significantly lower than the thermistor element value. But the reference design resistor must have a low temperature coefficient to prevent over-temperature errors in the reference voltage. The operating temperature range of the circuit is -30℃～100℃. The low-end limit is the current source output compliance related to the increase in thermistor impedance with decreasing temperature, determined by the voltage range in the circuit concerned.

The AD7711 accepts thermistor output voltages ranging from 7mV at 100°C to 0.75V at -35°C. The converter programmable gain amplifier maximizes system SNR. Using a 2V reference and a gain range of 1 to 128, the AD7711 can accept unipolar signals between 0mV and 15mV and 0V and 2V. For example, if the operating range is 25°C to 100°C, the maximum output voltage generated by the thermistor is 60mV. Allows the converter to achieve the full range with a gain of 32. At a gain of 32, with a 10Hz programmed update rate, this converter provides 17 bits of effective peak-to-peak resolution.

The circuit in Figure 3 is a complete ratio measurement system designed with a ΣΔ converter without using an on-chip current source or voltage reference. In this application, the constant current source is generated by the voltage reference and op amp. The same constant current source used to excite the thermistor also generates the reference for the A/D converter. Because the ratio works, drift errors in the measurement system are eliminated. This converter does not have an on-chip voltage reference or current source, so the thermistor excitation and AD7705 reference must be generated externally. The thermistor in this application is excited by a constant current source generated by an op amp and an external reference. The constant current source generates the reference for the AD7705. This implementation is fully ratiometric, and excitation current changes do not degrade system performance. When the gain is 1, the AD7705 can provide 16-bit peak-to-peak resolution at a buffer mode base temperature range of -55°C to 70°C and a 50Hz output rate.

Voltage excited thermistor application circuit

In the circuit shown in Figure 4, the voltage source that excites the thermistor comes from the AD7711. The thermistor is 10k3A (Betatherm Company), and the calibrated impedance is 10kΩ at 25°C. The two resistors (RS and R1) in the circuit are connected in series with the thermistor, which can also be implemented with RS=0Ω. Resistor RS limits thermistor power dissipation, and resistor R1 linearizes the thermistor output. When RS=0Ω, the output voltage across R1 varies from 33mV at -50℃ to 2.329V at 100℃. When operating in a gain of 1 unipolar operating mode, the AD7711 is suitable for an input signal range of 0V to 2.5V. If the application requires the full dynamic range of the A/D converter, the system correction feature on the converter can be used to eliminate the low-end 33mV. Therefore, when the converter is at 33mV at the input, the converter digital output will be 0. Similarly, at the high end, the system full-scale calibration can be used, and the 2.329V input voltage output is all 1.

In this circuit, the AD7711 programmable gain amplifier is used to amplify the signal generated by R1 to maximize the system SNR.

Thermistor output linearization

There are two methods for linearizing the thermistor output in general applications. In the digital range, a device characteristic lookup table can be used. To generate this table, the Steinhart-Hatt equation was used to implement a 3rd order linearization formula, providing an accuracy of ±0.02°C. In the analog range, additional series or parallel resistors are used to achieve linearization such that the voltage at three equally spaced points on the linear temperature scale or the resistance of a fixed resistance value thermistor has zero error.

Assuming RS = 0Ω, the circuit of Figure 4 linearizes the thermistor output with R1 in series with the thermistor. Force the voltage applied to R1 to have zero error at three points on the linear temperature scale in the desired temperature range. Apply the temperature range to determine the maximum error. In this circuit, the temperature range is -50℃~100℃. The errors at 50°C, 25°C and 100°C are zero, and the other temperature errors are distributed by the S-shaped curve.

In this way, the thermistor's nonlinear negative temperature characteristic is transformed into a linear relationship with a peak error of 12%. As the temperature range shrinks, the error becomes significantly smaller at 0.01°C in the 10°C range, 0.05°C in the 30°C range, and 2.0°C in the 60°C range.

where RTMID, RTLO and RTHI are the mid-range, low-end range and high-end range thermistor impedances respectively.

Eliminate self-heating effects

Thermistor self-heating effects can degrade overall system performance. These effects are more pronounced in still air. If the thermistor is placed in flowing air, liquid or solid, the natural error is quite small. In order to keep the maximum natural error within 0.1°C, the current in the circuit needs to be limited to keep the power consumption in the thermistor RT to a maximum of 0.1mW. The circuit shown in Figure 2 uses a 200μA constant current to excite the thermistor. The thermistor consumes 12μA constant current source at 25℃, and the thermistor consumes 3μW at 25℃. The thermistor's power consumption is less than 0.1mW in the working range of -55℃~70℃.

In Figure 4, the maximum power consumption point (when the self-heating effect is the worst) is when the RT value is equal to R1. The series resistor RS in Figure 4 will limit the current in the circuit and keep the thermistor power dissipation at an acceptable level, so the self-heating effect of the thermistor can be ignored. The 6kΩ resistance is sufficient to limit the current in the circuit and ensure that the maximum power dissipation in the thermistor is less than 0.1mW.

Conclusion

Thermistors are particularly suitable for temperature measurement applications with limited temperature ranges. Its price is lower than both RTD and thermocouple. Particularly useful for measuring temperature at remote locations because the lead resistance is not significant compared to the high resistance of the thermistor. Such devices have significant resistance changes for small temperature changes. Enables thermistors to be used for high-resolution measurements.

Thermistors are highly interchangeable and small in size. Combining the thermistor with a high-resolution A/D converter can form a high-precision, high-resolution temperature measurement system.

The disadvantage of the thermistor is that it requires linearization and the thermistor is used within a limited temperature range.