APPLICATION NOTE 7217

Wireless Precision Temperature Detector Using the MAX11410 ADC

By: Tom Au-Yeung

Abstract:

The MAX11410 ADC is ideal for measuring a wide range of temperature from -200°C to +1250°C with a K-type thermocouple. It can be implemented as a wireless temperature detector when combined with an RF transmitter and receiver.


Introduction

This application note showcases a couple of electronic circuits that measure and detect a preset temperature threshold wirelessly using the MAX11410 Analog-to-Digital Converter (ADC), a Voltage-Controlled Oscillator (VCO), Logarithmic Detector, and Comparator.

Precision Thermocouple Temperature Measurements Using the MAX11410 ADC

The MAX11410 is a low-power, 10-channel, 24-bit delta-sigma (Δ-Σ) ADC with features and specifications optimized for precision sensor measurements. The device includes a low-noise, Programmable-Gain Amplifier (PGA) with very high-input impedance and available gains from 1x to 128x to optimize the overall dynamic range. This internal PGA is perfect for applications measuring a wide temperature range using thermocouples, which generate a very low thermoelectric voltage in the millivolt range. The programmable matched-current sources provide excitation for Resistance Temperature Detector (RTD) sensors, which can be used as the reference (cold) junction for the thermocouple measurement. An additional current source/sink aids in detecting broken sensor wires. The 10-channel input multiplexer provides the flexibility needed for complex, multisensor measurements.

The MAX11410 device incorporates two General-Purpose Input/Outputs (GPIO), which can be used to control external devices. The GPIO pin 16 is used to activate the MAX2623 VCO in this application note when the measured temperature inside an oven reaches the set threshold limit of +165°C. This is the safe temperature recommended by the Unted states Department of Agriculture (USDA) to cook chicken and poultry.

A thermocouple is made of a pair of dissimilar metallized electric cables connected to form two junctions. It can be used to measure a wide range of temperature from -200°C to +165°C such as the K-type device. It is therefore an ideal device to measure the temperature of favorite and delicious food such as roasted chicken. A thermocouple can be easily inserted into it for precision temperature measurement.

Temperature Measurement and Threshold Limit Detection

The circuit in Figure 1 depicts a temperature measurement and threshold-limit detection transmitter using the MAX11410 ADC and the MAX2623 VCO set at 915MHz. The frequency of 915MHz is chosen because it is in the middle of the license-free ISM 905MHz to 925MHz band, which is the radio spectrum reserved internationally for industrial, scientific, and medical applications.

Wireless temperature measurement and transmitterHigh-res Image ›
Figure 1. Wireless temperature measurement and transmitter.

Thermocouple temperature measurement is a two-step process. The first step is to establish the reference temperature, and the RTD feature of the MAX11410 is perfect for this purpose. The second step requires precision low-voltage measurement in millivolts generated by thermocouples.

A K-type thermocouple is connected to pins AIN1 and AIN2 of the MAX11410 through the 1kΩ overvoltage protection resistors, and the precision external reference voltage MAX6070 is connected to the REF2P and REF2N (Figure 1). The differential voltage across AIN1 and AIN2 produced by the thermocouple corresponding to the oven temperature is then amplified by the internal PGA with a gain of 32 and converted to the measured temperature as follows.

First, a reference or cold junction temperature is established by using the 2-wire RTD ratiometric measurement technique, which is accomplished by the circuit consisting of the RTD connected to pins 8 and 9, and the reference resistor RREF connected to REF1P and REF1N of the MAX11410.

The internal programmable matched current sources of the MAX11410 provide a known value of current up to 1.6mA to excite the RTD A in this ratiometric measurement technique. The same amount of current flows through the RREF to establish the reference voltage. This application note recommends 300µA current to provide 1.41V reference voltage. The error in the excitation current going through the RTD and RREF is canceled. The current flowing through the RTD and the RREF is the same. Hence, this RTD measurement accuracy depends entirely on the resistance accuracy of the RREF. Only an external resistor is used to implement the needed voltage reference in this ratiometric technique, saving cost and valuable PCB space.

A cold junction PT1000 RTD measures and generates an ambient temperature of +24.8°C based on the circuit in Figure 1, and the MAX11410 evaluation kit and software GUI (Graphical User Interface) (Figure 2).

Cold junction temperature measurement using the MAX11410 ADC and PT1000 RTDFigure 2. Cold junction temperature measurement using the MAX11410 ADC and PT1000 RTD.

The thermocouple temperature measurement uses +24.8°C as the reference temperature to measure the oven's internal temperature (Figure 3).

Oven temperature measurement using the MAX11410 ADC thermocouple featureFigure 3. Oven temperature measurement using the MAX11410 ADC thermocouple feature.

The MAX11410 GPIO (pin 16) activates the oscillator MAX2623 when the oven temperature exceeds the set limit of +165°C by setting the GPIO pin high to 3V. It transmits a 915MHz frequency signal with approximately -3dBm of power through the antenna at pin 7 when the MAX2623 is turned on (SHDN pin is high).

RF Signal Detector

The circuit in Figure 4 detects RF devices that emit RF frequencies in the 100MHz to 3000MHz range. The received power levels can be as low as -35dBm or 0.32μW. The log detector (MAX2015) senses RF signals at the INHI pin and produces an output voltage on pin 8 proportional to the power level of the received signal. This RF signal detector can be in the backyard to detect the MAX11410/MAX2623 temperature transmitter circuit in Figure 1 located by the oven.

This circuit sounds a buzzer alarm when it detects an RF signal in the range of 100MHz to 3000MHz with approximately -35dBm power levelFigure 4. This circuit sounds a buzzer alarm when it detects an RF signal in the range of 100MHz to 3000MHz with approximately -35dBm power level.

The MAX2015 generates a DC output of approximately 0.5V at low input power levels such as -35dBm at 915MHz. This voltage increases up to approximately 2.0V in proportion to the input RF power level. The reference voltage at the inverting terminal of the MAX9075 is preset to 0.6V by the resistors R1 and R2. The reference voltage is fine-tuned by adjusting the resistor R2 to avoid false alarms. The MAX2015 triggers the comparator MAX9075 to generate an output voltage of 3.0V on detecting an RF signal, which in turn activates a loud buzzer, sounding an alert on exceeding the desired temperature of +165°C.

Conclusion

The MAX11410 is optimized for precision temperature measurements using both RTDs and thermocouples by implementing the internally programmable matched excitation current sources, ratiometric features, low-noise PGA with very high-input impedance, and available gains from 1x to 128x,. Also, two internal GPIOs provide ease-of-control of external devices such as the MAX2623 VCO, which is used as an RF signal transmitter in this application note. The MAX11410 ADC and MAX2623 VCO combination, together with the MAX2015 logarithmic detector implemented as the receiver, creates a wireless temperature detector, which sounds an alarm upon reaching the set temperature limit inside an oven. Dinner is ready!