How to Measure Multiple Temperatures Using a Single MAX11410 and Resistance Temperature Detectors

Introduction

A resistance temperature detector (RTD) is a precision temperature measurement device containing a resistor that changes the resistance value proportionally to environmental temperature changes. RTDs typically consist of a fine wire wrapped around a glass or ceramic core, often sheathed in a protective material. RTDs accurately measure temperatures from -200°C to +850°C in laboratory and industrial applications despite their fragility as they are very accurate, repeatable, reliable, and stable. Multiple temperature measurements are ideal and implemented in the same hardware in many process control and industrial applications.

Precision Temperature Measurements using the MAX11410

The MAX11410 is a low-power, 10-channel, 24-bit S-? 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. The programmable matched current sources provide excitation for RTD sensors. An additional current sink and current source aid in detecting broken sensor wires. The 10-channel input multiplexer provides the flexibility needed for complex, multi-sensor measurements. GPIOs provide ease of control of external switches or other circuitry. Additionally, the MAX11410 has internal 50Hz and 60Hz filters to provide improved common-mode rejection. It also has self and system calibration options to reduce further software effort.

2-Wire RTD

A voltage is generated across the 2-wire RTD by the MAX11410 internally programmable matched current sources from 10µA to 1.6mA through the AIN0/REF0P pin to accurately measure the temperature based on the RTD resistance. This current first goes through the RTD cable parasitic resistance R_C. It then passes through the 2-wire RTD and the bottom cable parasitic resistance R_C. Finally, the current travels through the R_REF 4K? resistor to establish the reference voltage at the REF1P pin. Pins AIN1 and AIN2 are used to measure the voltage drop across the RTD, V_RTD. The RTD value is calculated as follows:

V_AIN1 – V_AIN2 = V_RC + V_RTD + V_RC = 2 × V_RC + V_RTD = (2 × R_C + RTD) × I
V_REF = R_REF times × I

The MAX11410 ADC compares the analog input voltage to the reference voltage and produces the output code, which is a digital binary number and is obtained as:

Code(V_AIN1 – V_AIN2) = Code(2 × V_RC + V_RTD) = (2 × V_RC + V_RTD) × 2^N/V_REF = (2 × R_C + RTD) × I × 2^N/(R_REF × I) = (2 × R_C + RTD) × 2^N/R_REF

So,

Code(V_AIN1 – V_AIN2) = (2 × R_C + RTD) × 2^N / R_REF and Code(ADC Fullscale) = 2^N
Code(V_AIN1 – V_AIN2) = (2 × R_C+ RTD) × Code(ADC Fullscale)/R_REF

Therefore,

2 × R_C + RTD = Code(V_AIN1 – V_AIN2) × R_REF/Code (ADC Fullscale)

The calculated RTD is independent of the excitation current from the device in this ratiometric measurement. Any error in the current is canceled and the measured voltage across the RTD (with parasitic resistance) depends solely on the precision of the reference resistor, R_REF.

The voltage drop across the RTD is measured differentially between the AIN1 and AIN2 pins in this 2-Wire RTD topology. This voltage includes the voltage drop across the two cable parasitic resistances, R_C, which is the voltage error caused by the RTD cable. Therefore, the 2-wire RTD should only be used when the RTD cable length is short.

Figure 1 shows the 2-wire RTD temperature measurement using the MAX11410.

Figure 1. Measuring the 2-Wire RTD using the MAX11410.

3-Wire RTD

A voltage is generated across the 3-wire RTD by the MAX11410 internally programmable matched current sources from 10µA to 1.6mA through the AIN0/REF0P pin to accurately measure the temperature based on the RTD resistance. This current first goes through the top RTD cable parasitic resistance, R_C, and then passes through the 3-wire RTD. Finally, the current travels through the R_REF 4K? resistor to establish the reference voltage at the REF1P pin.

A second matched current source from 10µA to 1.6mA is sent through the AIN3 pin in this 3-wire RTD measurement. This second current travels through the second cable parasitic resistance, R_C. Finally, the current travels through the R_REF 4K? resistor to establish the reference voltage at the REF1P pin as the first excitation current. This reference voltage is equal to twice the current multiplied by the R_REF 4KO resistor. The measured RTD resistance precision is also dependent entirely on the accuracy of the R_REF. The voltage drop across the RTD is measured differentially between the AIN1 and AIN3 pins as:

V_AIN1 – V_AIN3 = (V_RC + V_RTD) - V_RC = V_RTD = RTD × I
V_REF = R_REF × 2 × I (twice as much current flowing through R_REF)
Code(V_AIN1 – V_AIN3) = Code(V_RTD) = (V_RTD) × 2^N/(2 × V_REF) = (RTD) × I × 2^N/(2 × R_REF × I) = (RTD) × 2^N/(2 × R_REF)

So,

Code(V_AIN1 – V_AIN2) = (RTD) × 2^N/(2 × R_REF)

Also,

Code(ADC Fullscale) = 2^N
Code(V_AIN1 – V_AIN2) = RTD × Code(ADC Fullscale)/(2 × R_REF)

Therefore,

RTD = Code(V_AIN1 – V_AIN2) × 2 × R_REF/Code (ADC Fullscale)

The RTD is independent of the excitation current from the MAX11410 device in this ratiometric measurement. Any error in the current is canceled and the measured voltage across the RTD depends solely on the precision of the reference resistor, R_REF. Additionally, the voltage drop across the top parasitic resistor is subtracted from the voltage drop across the bottom parasitic resistor if the parasitic resistances (R_C) in the cables are the same. Therefore, the temperature measurement in this 3-Wire RTD topology is more precise than the 2-Wire RTD.

Figure 2 shows the 3-wire RTD temperature measurement using the MAX11410.

Figure 2. Measuring the 3-Wire RTD using the MAX11410.

4-Wire RTD

A voltage is generated across the 4-wire RTD by the MAX11410 internally programmable matched current sources from 10µA to 1.6mA through the AIN0/REF0P pin to accurately measure the temperature based on the RTD resistance. This current first goes through the top RTD cable parasitic resistance, R_C. It then passes through the 4-wire RTD and the bottom RTD cable parasitic resistance, R_C. Finally, the current travels through the R_REF 4K? resistor to establish the reference voltage at the REF1P pin.

The measured RTD resistance precision is also dependent entirely on the accuracy of the R_REF. The voltage drop across the RTD is measured differentially between AIN1 and AIN2 like the 3-Wire RTD topology. The voltage error seen in 2-wire and 3-wire RTDs does not exist in this 4-wire RTD topology as no current flows through the parasitic resistances of the cable connected at analog inputs AIN1 and AIN2. So, the 4-wire RTD is the most accurate method of measurement.

Figure 3 shows the 4-wire RTD temperature measurement using the MAX11410.

Figure 3. Measuring the 4-Wire RTD using the MAX11410.

Measuring Additional RTDs

There are 10 analog channels available in the MAX11410. Only three sets of RTDs can be implemented for the 2-Wire RTD topology because three channels are used for each RTD. Only two sets of RTDs can be used for both 3-Wire and 4-Wire because four channels are required for each RTD. An external multiplexer or a switch like the MAX4735 or equivalent can be used to implement the additional RTDs, if needed.

The number of RTDs can be doubled with an additional MAX4735 switch. Set the switch to the NC1 and NC2 pins to select RTD1 (Figure 4). Set the switch to NO1 and NO2 to select RTD2.

Figure 4. Measuring additional RTDs using the MAX11410 and a switch.

Conclusion

The MAX11410, with the implementation of internally programmable matched excitation current sources and the ratiometric feature, is optimized for precision temperature measurements using three different types of RTDs: 2-Wire, 3-Wire, and 4-Wire. The error due to the parasitic resistance in the long cables in the 3-Wire (assuming the top and bottom parasitic resistances, R_C, in the cables are the same) and 4-Wire RTDs are eliminated. Additionally, the measured temperature accuracy depends solely on the precision of the reference resistor, R_REF. So, no external voltage reference is required, which saves board space and system cost.