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A Basic Guide to RTD Measurements

Application Report

SBAA275–June 2018

A Basic Guide to RTD Measurements

Joseph Wu

ABSTRACT

RTDs, or resistance temperature detectors, are sensors used to measure temperature. These sensors are the among the most accurate temperature sensors available, covering large temperature ranges. However, getting accurate measurements with precision analog-to-digital converters (ADCs) requires attention to detail in design of measurement circuits and calculation of the measurement. This application note starts with an overview of the RTD, discussing their specifications, construction, and details in their use in temperature measurement. Different circuit topologies with precision ADCs are presented for different RTD configurations. Each circuit is shown with a basic design guide, showing calculations necessary to determine the ADC settings, limit measurement errors, and verify that the design fits in the operating range of the ADC.

Contents

1 RTD Overview 2 2 RTD Measurement Circuits 9 3 Summary 41 List of Figures

1 PT100 RTD Resistance From –200 C to 850 C 2 2 PT100 RTD Non-Linearity From –200 C to 850 C 3 3 Two-Wire, Three-Wire, and Four-Wire RTDs 4 4 Example of a Ratiometric RTD Measurement 4 5 Example of Lead Wire Resistance Cancellation 5 6 Swapping IDAC1 and IDAC2 to Chop the Measurement 6 7 Two-Wire RTD, Low-Side Reference Measurement Circuit 10 8 Two-Wire RTD, High-Side Reference Measurement Circuit 12 9 Three-Wire RTD, Low-Side Reference Measurement Circuit 14 10 Three-Wire RTD, Low-Side Reference Measurement Circuit With One IDAC Current Source 17 11 Three-Wire RTD, High-Side Reference Measurement Circuit 20 12 Four-Wire RTD, Low-Side Reference Measurement Circuit 23 13 Two Series Two-Wire RTD, Low-Side Reference Measurement Circuit 25 14 Two Series Four-Wire RTD, Low-Side Reference Measurement Circuit 27 15 Multiple Two-Wire RTDs Measurement Circuit 29 16 Multiple Three-Wire RTDs Measurement Circuit 31 17 Multiple Paralleled Four-Wire RTDs Measurement Circuit 33 18 Universal RTD Measurement Interface With Low-Side Reference Circuit 35 19 Universal RTD Measurement Interface With High-Side Reference Circuit 38 List of Tables

1 RTD Tolerance Class Information 3 Temperature (qC)

RTD Resistance -200 -100-*-***-*** 300-***-***-*** 700 800 900

0

100

200

300

400

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A Basic Guide to RTD Measurements

1 RTD Overview

RTDs are resistive elements that change resistance over temperature. Because the change in resistance is well characterized, they are used to make precision temperature measurements, with capability of making measurements with accuracies of well under 0.1 C. RTDs are typically constructed from a length of wire wrapped around a ceramic or glass core. RTDs may also be constructed from thick film resistors plated onto a substrate. The wire or resistance is typically platinum but may also be made from nickel or copper. The PT100 is a common RTD constructed from platinum with a resistance of 100 Ω at 0 C. RTD elements are also available with 0 C resistances of 200, 500, 1000, and 2000 Ω. 1.1 Callendar-Van Dusen Equation

The relationship between platinum RTD resistance and temperature is described by the Callendar-Van Dusen (CVD) equation. Equation 1 shows the resistance for temperatures below 0 C and Equation 2 shows the resistance for temperatures above 0 C for a PT100 RTD. For T < 0: RRTD(T) = R0 • {1 + (A • T) + (B • T2) + [(C • T3) • (T – 100)]} (1) For T > 0: RRTD(T) = R0 • [1 + (A • T) + (B • T2)] (2) The coefficients in the Callendar-Van Dusen equations are defined by the IEC-60751 standard. R0 is the resistance of the RTD at 0 C. For a PT100 RTD, R0 is 100 Ω. For IEC 60751 standard PT100 RTDs, the coefficients are:

• A = 3.9083 • 10-3

• B = –5.775 • 10-7

• C = –4.183 • 10-12

The change in resistance of a PT100 RTD from –200 C to 850 C is displayed in Figure 1. Figure 1. PT100 RTD Resistance From –200 C to 850 C While the change in RTD resistance is fairly linear over small temperature ranges, Figure 2 displays the resulting non-linearity if an end-point fit is made to the curve shown in Figure 1. Temperature (qC)

Nonlinearity Error -200 -100-*-***-*** 300-***-***-*** 700 800 900

0

5

10

15

20

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A Basic Guide to RTD Measurements

Figure 2. PT100 RTD Non-Linearity From –200 C to 850 C The results show a non-linearity greater than 16 Ω, making a linear approximation difficult over even small ranges. For temperatures greater than 0 C, temperatures can be determined by solving the quadratic from Equation 2. For temperatures lower than 0 C, the third order polynomial of Equation 1 may be difficult to calculate. Using simple microcontrollers, determining the temperature may be computationally difficult and using a look-up table to determine the temperature is common practice. Newer calibration standards allow for more calculation accuracy using higher order polynomials over segmented temperature ranges, but the Callendar-Van Dusen equation remains a commonly used conversion standard.

1.2 RTD Tolerance Standards

RTDs have good interchangeability. This means that there is little variation from sensor to sensor because of good accuracy tolerance. This allows for good measurement accuracy, even if RTD sensors are replaced from system to system.

There are two tolerance standards that define a grade or class for platinum RTD accuracy. The American standard is ASTM E1137 and is used mostly in North America. The European standard is known as the DIN or IEC standard. DIN IEC 60751 is used world wide. Both standards define the accuracy of the RTD starting with a base resistance of 100 Ω at a temperature of 0 C. Table 1 shows the specifications of different classes of RTDs. In both standards, the RTD has the tightest tolerance at 0 C. An absolute error is combined with a proportional error that has a temperature coefficient.

(1) 1/10 DIN is not included in the IEC 60751 specification but is an industry accepted tolerance for performance demanding applications. It is 1/10th of the DIN IEC Class B specification. Table 1. RTD Tolerance Class Information

TOLERANCE TOLERANCE VALUES

( C)

RESISTANCE AT 0 C

(Ω)

ERROR AT 100 C

( C)

ASTM Grade B (0.25 + 0.0042 • T ) 100 0.1 0.67 ASTM Grade A (0.13 + 0.0017 • T ) 100 0.05 0.3 IEC Class C (0.6 + 0.01 • T ) 100 0.24 1.6

IEC Class B (0.3 + 0.005 • T ) 100 0.12 0.8 IEC Class A (0.15 + 0.002 • T ) 100 0.06 0.35 IEC Class AA (0.1 + 0.0017 • T ) 100 0.04 0.27 1/10 DIN(1) (0.03 + 0.0005 • T ) 100 0.012 0.08 The specified temperature range of each RTD class tolerance becomes smaller with more accurate grades and classes. Additionally, the range varies with the RTD construction type. For more details about tolerance values and temperature ranges, consult the data sheets of the RTD manufacturer. ß

RRTD ADC

RREF

IDAC1

REFP REFN

Lead 1

Lead 2

Lead 2

Lead 1

Lead 3

Lead 1

Lead 4

Lead 2

Lead 3

2-Wire

RTD

3-Wire

RTD

4-Wire

RTD

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A Basic Guide to RTD Measurements

1.3 RTD Wiring Configurations

RTDs are made in three different wiring configurations described in this application note. Each wiring configuration requires a different excitation and circuit topology to reduce the measurement error. The three different wiring configurations are shown in Figure 3. Figure 3. Two-Wire, Three-Wire, and Four-Wire RTDs In the two-wire configuration, the RTD is connected through two wires connected to either end of the RTD. In this configuration, the lead wire resistances cannot be separated from the RTD resistance, adding an error that cannot be separated from the RTD measurement. Two-wire RTDs yield the least accurate RTD measurements and are used when accuracy is not critical or when lead lengths are short. Two-wire RTDs are the least expensive RTD configuration.

In the three-wire configuration, the RTD is connected to a single lead wire on one end and two lead wires on the opposite end. Using different circuit topologies and measurements, lead resistance effects can effectively be cancelled, reducing the error in three-wire RTD measurements. Compensation for lead wire resistance assumes that the lead resistances match. In the four-wire configuration, two lead wires are connected to either end of the RTD. In this configuration, the RTD resistance may be measured with a four-wire resistive measurement with superior accuracy. The RTD excitation is driven through one lead on either end, while the RTD resistance is measured with the other lead on either end. In this measurement, the RTD resistance is sensed without error contributed from the lead wire reacting with the sensor excitation. Four-wire RTDs yield the most accurate measurements, but are the most expensive RTD configuration. 1.4 Ratiometric Measurements

RTD measurements with an ADC are typically made with a ratiometric measurements. Figure 4 shows the basic topology of a ratiometric measurement. Shown are the ADC with a two-wire RTD and a reference resistor RREF. A single excitation current source (IDAC1) is used to excite the RTD as well as to establish a reference voltage across RREF for the ADC.

Figure 4. Example of a Ratiometric RTD Measurement ß

RRTD ADC

IDAC1

REFP REFN

IDAC2

RLEAD1

RLEAD2

RLEAD3

RREF

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A Basic Guide to RTD Measurements

With IDAC1, the ADC measures the voltage across the RTD using the voltage across RREF as the reference. This provides an output code that is proportional to the ratio of the RTD voltage and the reference voltage as shown in Equation 3. Ratiometric measurements will only produce positive output data, assuming zero offset error. For a fully-differential measurement, this is only the positive half of the full-scale range of the ADC, reducing the measurement resolution by one bit. The following equations assume a 24-bit bipolar ADC, with VREF as the full-scale range of the ADC. Output code = 223 • VRTD / VREF = 223 • IIDAC1 • RRTD / (IIDAC1 • RREF) (3) The currents cancel so that the equation reduces to Equation 4: Output code = 223 • RRTD / RREF (4)

In the end, the RTD resistance can be represented from the code as a function of the reference resistance.

RRTD = Output code • RREF / 223 (5)

The measurement depends on the resistive value of the RTD and the reference resistor RREF, but not on the IDAC1 current value. Therefore, the absolute accuracy and temperature drift of the excitation current does not matter. In a ratiometric measurement, as long as there is no current leakage from IDAC1 outside of this circuit, the measurement depends only on RRTD and RREF. ADC conversions do not need to be translated to voltage.

Assuming the ADC has a low gain error, RREF is often the largest source of error. The reference resistor must be a high accuracy precision resistor with low drift. Any error in the reference resistance becomes a gain error in the measurement.

1.4.1 Lead Resistance Cancellation

In Figure 5, the lead resistances of a three-wire RTD are shown and a second excitation current source is added, labeled IDAC2.

Figure 5. Example of Lead Wire Resistance Cancellation With a single excitation current source, RLEAD1 adds an error to the measurement. By adding IDAC2, the second excitation current source is used to cancel out the error in the lead wire resistance. When adding the lead resistances and the second current source, the equation becomes: Output code = 223 • [IIDAC1 • (RRTD + RLEAD1) – (IIDAC2 • RLEAD2)] / [(IIDAC1 + IIDAC2) • RREF] (6) If the lead resistances match and the excitation currents match, then RLEAD1 = RLEAD2 and IIDAC1 = IIDAC2. The lead wire resistances cancel out so that Equation 6 reduces to the result in Equation 7 maintaining a ratiometric measurement.

Output code = 223 • RRTD / (2 • RREF) = 222 • RRTD/ RREF (7) RRTD = Output code • RREF / 222 (8)

RLEAD3 is not part of the measurement, because it is not in the input measurement path or in the reference input path.

ß

RRTD ADC

IDAC2

REFP REFN

IDAC1

RLEAD1

RLEAD2

RLEAD3

RREF

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A Basic Guide to RTD Measurements

1.4.2 IDAC Current Chopping

As described in the previous section, the two current sources must be matched to cancel the lead resistances of the RTD wires. Any mismatch in the two current sources may be minimized by using the multiplexer (MUX) to swap or chop the two current sources between the two inputs. Taking two measurements in each configuration and averaging the results reduces the effects of mismatched current sources.

Using the configuration from Figure 5, Equation 6 results in the first measurement. Figure 6 swaps IDAC1 and IDAC2, and Equation 9 results in the second measurement. Figure 6. Swapping IDAC1 and IDAC2 to Chop the Measurement Output code = 223 • [IIDAC2 • (RLEAD1 + RRTD) − (IIDAC1 • RLEAD2)] / [(IIDAC1 + IIDAC2) • RREF] (9) To chop the RTD measurement, we average the first and second measurements. Take Equation 6, add it to Equation 9 and then divide by two to average the result. This is shown in the following: Averaged output code = 223 • {[IIDAC1 • (RLEAD1 + RRTD) − (IIDAC2 • RLEAD2)] + [IIDAC2 • (RLEAD1 + RRTD) – (IIDAC1 • RLEAD2)]} / {2 •

[(IIDAC1 + IIDAC2) • RREF]} (10)

Then combine (IIDAC1 + IIDAC2) terms:

Averaged output code = 223 • [(IIDAC1 + IIDAC2) • (RLEAD1 + RRTD) – (IIDAC1 + IIDAC2) • RLEAD2)] / [2 • (IIDAC1 + IIDAC2) • RREF] (11) Then cancel the IIDAC1 + IIDAC2 terms and set RLEAD1 = RLEAD2 = RLEAD to get the following equations: Averaged output code = 223 • [(RLEAD + RRTD) − RLEAD] / (2 • RREF)] (12) After this, the RLEAD terms are cancelled as well. Averaged output code = 223 • RRTD / (2 • RREF) = 222 • RRTD / RREF (13) Going through the results to Equation 13, it is not important that IIDAC1 and IIDAC2 are not equal, it is only important that IIDAC1 and IIDAC2 are the same values after they are swapped. If they are the same, then the

(IIDAC1 + IIDAC2) terms cancel out.

There may still be errors in the system. Here, RLEAD1 and RLEAD2 are assumed to be the same. If they are different, this becomes an error. Also, if there are leakage currents in the measurement (from TVS or other protection diodes for example), then the leakage contributes to the error. www.ti.com RTD Overview

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A Basic Guide to RTD Measurements

1.5 Design Considerations

Designing an RTD measurement system requires balancing several different design goals and circuit considerations. After selecting components and excitation magnitude, the designer must verify that the design fits in the operating range of the ADC which includes reference voltage magnitude, the input range of the PGA, and the compliance voltage of any excitation current sources. This section is a basic guide to setting the parameters of operation to design an RTD measurement system with precision ADCs. The basic ratiometric measurement shown in Figure 4 will be the starting point for an RTD measurement circuit.

Later sections describe different circuit topologies used for measurement of different RTD wiring configurations. By extension, calculations found here can be applied to different topologies shown later. 1.5.1 Identify the RTD Range of Operation

Start by determining the expected temperature measurement range required for the system, because this will set the range of RTD resistance measurement. As an example, start with a PT100 RTD. The resistance of a PT100 RTD over temperature was shown in Figure 1. If the required system temperature measurement range is –200 C to 850 C, this requires the full measurement range of a PT100 RTD. With this temperature range, the RTD would have an equivalent resistance range of 20 Ω to 400 Ω. Use this resistance range to start the design of the measurement system. Determining the temperature range and then the RTD resistance range helps set the excitation current, gain, and the reference resistance in the design.

1.5.2 Set the Excitation Current Sources and Consider RTD Self Heating Many precision ADCs used for RTD measurement will have programmable excitation current sources

(IDACs) in several magnitudes. A precision ADC device may have a matched pair of IDACs used for excitation. These IDACs can be set to currents of 10, 50, 100, 250, 500, 750, 1000, 1500, and 2000 µA. Excitation currents are used to drive both the RTD, the reference resistance and biasing resistors for some designs.

For the best noise performance, maximize the excitation current used for the RTD and reference resistance excitation. However, most excitation currents should be kept lower than 1 mA because of self heating. Because there is current running through the RTD, the RTD itself will dissipate power through heat. This self heating will cause an error in the measurement. The change in temperature (ΔT) is determined by the power dissipation of the RTD divided by the self-heating coefficient E, in mW/ C. This change in temperature becomes a temperature measurement error and is shown in Equation 14. ΔT = (IIDAC)2 • RRTD / E (14)

The typical range of RTD self-heating coefficients is 2.5 mW/ C for small, thin-film elements and 65 mW/ C for larger, wire-wound elements. With 1-mA excitation at the maximum RTD resistance value and a larger self-heating coefficient, the power dissipation in the RTD is less than 0.4 mW and will keep the measurement errors due to self-heating to less than 0.01 C. Self-heating coefficients will vary with RTD construction and the measurement medium (in air or in water, for example). Consult the RTD manufacturer data sheet for sensor characteristics. Referring back to Figure 4, this topology uses a single IDAC current source. Other topologies may use matched sources to for lead current calculation.

1.5.3 Set Reference Voltage and PGA Gain

After selecting the IDAC current, use the maximum reference resistor possible, but consider several factors in the setting the reference. The reference voltage must be within the minimum and maximum reference voltages for operation. Many ADCs will have a minimum value of the reference of 0.5 V. Some devices will have a reference maximum of AVDD – AVSS, while others may have a lower maximum of AVDD – AVSS – 1 V. Consult the ADC data sheet for more specifications on the external reference input range.

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A good selection for the reference voltage is using a value close to the midpoint of AVDD – AVSS. Often, this reference voltage is used to set up the common-mode voltage for the input measurement. PGA amplification may be limited by its input range and output swing. By setting the input common-mode voltage to the midpoint in the supplies, the PGA will have the maximum range possible. Many precision ADCs have a PGA that can amplify small input signals. These PGAs often will have gains from 1 V/V to 128 V/V in factors of 2.

Also, select a reference resistance that maximizes the usable input range of the ADC. As an example, it helps to show this with several values. Start with a 2-wire RTD ratiometric measurement with a PT100 where the maximum resistance is 400 Ω. This is the setup shown for a basic ratiometric measurement in Figure 4.

If the IDAC current is selected to be 1 mA, then the reference resistor could be chosen to be 1620 Ω. The measurement of the 400 Ω could be set to a PGA gain of 4. This would make the input voltage 1.6 V, while the reference voltage is set to 1.62 V. This would maximize the input voltage range of the ADC to 98.8% of the positive full-scale range. A reference resistor of 1600 Ω could have been chosen to maximize the ADC, however a small gain error or resistance error may push a 400 Ω measurement out of the range of operation. For this example, the next largest 1% resistor value above 1600 was selected. Another benefit of setting the reference voltage to 1.62 V is that it sets the RTD measurement near the midpoint of the supply voltage. A reference of 1.62 V sets the input voltage for the ADC negative input. The input voltage is highest at the maximum RTD resistance is 0.4 V using an IDAC current of 1 mA and RTD resistance of 400 Ω. This sets the input voltage to 2.02 V for the ADC positive input. Selecting a marginally larger resistance only reduces the resolution of the measurement. If the reference resistor is selected to be 2400 Ω then the reference voltage becomes 2.4 V. With an input to the ADC of 1.6 V (from 0.4 V after PGA gain of 4) compared to a reference voltage of 2.4 V, the ADC uses only 67% of the positive full-scale range.

1.5.4 Verify the Design Fits the Device Range of Operation After determining the RTD range of operation, selecting the IDAC currents, the reference resistance, and the PGA gain, verify that the design still is within the range of operation of the device. The PGA will have an input range dependent on the input common-mode voltage and the PGA gain. This may be different for each ADC. Determine the minimum and maximum input voltage and the common- mode voltage for each input voltage operation. By setting the input common-mode voltage to near mid- supply, the input voltage should be within the PGA range of operation. However, it is important to verify this through the equations given in the data sheet of the selected ADC. Consult the ADC data sheet for descriptions of the PGA and limitations in its input range. Additionally, calculate the voltage at the output of the IDAC current sources. As the output voltage rises near the supply, the IDAC current will lose compliance as the output impedance of the current source is reduced. Calculate the voltage based on the IDAC currents driving the RTD resistance, reference resistance and bias resistance if necessary. If this voltage gets too close to the positive supply, the current may be reduced. Note that this compliance voltage will be different from device to device, and may vary by output current magnitude. Again, it is important to verify the compliance voltage based on the IDAC current source specifications in the data sheet of the selected ADC. 1.5.5 Design Iteration

If the design does not fall within the range of operation for the PGA, or is outside the compliance voltage of the IDAC, then another iteration the design may be necessary. It may be necessary to reduce or increase the reference resistance, or change any biasing resistors to set the PGA input range or set the IDAC output to with the compliance voltage.

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A Basic Guide to RTD Measurements

2 RTD Measurement Circuits

NOTE: Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality. The following sections describe circuit topologies for the three RTD wiring configurations. Each section provides the basic topology, with benefits and drawbacks for the circuit. Different topologies have different connections for analog inputs, reference inputs, and IDAC outputs. A basic theory of operation is provided with notes to guide the reader through important considerations in the design. However, a design procedure similar to the Design Considerations section can be followed to determine system values and parameters. Later sections describe measurements with different combinations of RTDs, allowing for more versatile temperature measurement systems. The circuits use a single ADC with a multiplexer to measure multiple elements and route excitation current to the sensor. Conversion results are shown with a generic 24-bit bipolar ADC, using the positive full-scale range of the device. Conversions with 16-bit ADCs are similar in calculation. Results are shown as functions of the reference resistance. Conversion to temperature depends on the linearity and error of the individual RTD model, and is not discussed in this applications note. PGA ADC

AIN0

Mux

AIN1

AIN2

REFP0

REFN0

RREF

RRTD

AVSS

IDAC1

IDAC2

Lead 1

Lead 2

AVDD

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A Basic Guide to RTD Measurements

2.1 Two-Wire RTD Measurement With Low-Side Reference The most basic RTD measurement uses a two-wire RTD for temperature measurement. Shown below is a schematic and design for a two-wire RTD measurement with an ADC. A ratiometric measurement is created with the RTD as the input and a precision resistor as the reference input. 2.1.1 Schematic

Figure 7. Two-Wire RTD, Low-Side Reference Measurement Circuit 2.1.2 Pros and Cons

Pros:

• Simplest implementation of RTD temperature measurement

• Uses only two analog input pins for measurement and one IDAC current for sensor and reference resistor excitation

• Good for local measurements, where the lead length and resistance are small

• Ratiometric measurement, IDAC noise and drift are cancelled Cons:

• Least accurate measurement for RTDs

• No lead wire compensation; lead resistance affects measurement accuracy 2.1.3 Design Notes

An IDAC current source drives both the RTD and the reference resistor, RREF. Because the same current drives both elements, the ADC measurement is a ratiometric measurement. Calculation for the RTD resistance does not require a conversion to a voltage, but does require a precision reference resistor with high accuracy and low drift.

The measurement circuit requires:

• Single dedicated IDAC output pin

• AINP and AINN inputs

• External reference input

• Precision reference resistor

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First, identify the range of operation for the RTD. For example, a PT100 RTD has a range of 20 Ω to 400 Ω if the temperature measurement range is from −200 C to 850 C. The reference resistor must be larger than the maximum RTD value. The reference resistance and PGA gain determines the positive full- scale range of the measurement.

Then, choose the reference resistor and IDAC current value. Ideally, choosing the largest IDAC current provides the best performance by increasing the sensor signal above any noise in the system. However, there are several other considerations in determining the values. First, higher current may lead to self- heating of the RTD, which adds error to the measurement. Second, the reference resistance acts as a level shift for the sensor measurement. This level shift is used to raise the DC bias of the analog input signal so that the voltage is within the input range of the PGA. Generally, the analog input signal is set near mid-supply for best operation.

To verify that the design is within the ADC range of operation, Calculate the voltages for AIN1 and AIN2 and the maximum differential input voltage. Verify that VAIN1 and VAIN2 are within the input range of the PGA given the gain setting and supply voltage. Use the maximum RTD resistance based on the desired temperature measurement.

VAIN1 = IIDAC1 • (RRTD + RREF) (15)

VAIN2 = IIDAC1 • RREF (16)

Additionally, the output voltage of the IDAC source calculated from VAIN1 must be low enough from AVDD to be within the compliance voltage of the IDAC current source. When the IDAC output voltage rises too close to AVDD, the IDAC loses compliance and the excitation current is reduced. The reference resistor, RREF must be a precision resistor with high accuracy and low drift. Any error in the RREF reflects the same error in the RTD measurement. The REFP0 and REFN0 pins are shown connecting to the RREF resistor as a Kelvin connection to get the best measurement of the reference voltage. This eliminates any series resistance as an error from the reference resistance measurement. The lead wire resistance is an error term in the two-wire RTD measurement. The previous calculations neglect the lead resistances, but can be added to the RRTD term. 2.1.4 Measurement Conversion

Output Code = 223 • Gain • VRTD / VREF = 223 • Gain • (IIDAC1 • RRTD) / (IIDAC1 • RREF) = 223 • Gain • RRTD / RREF (17) RRTD = RREF • Output Code / (223 • Gain) (18)

2.1.5 Generic Register Settings

• Select multiplexer settings for AINP and AINN to measure leads 1 and 2 of the RTD

• Enable the PGA, set gain to desired value

• Select data rate and digital filter settings

• Select reference input to measure RREF for ratiometric measurement

• Enable the internal reference (the IDAC requires an enabled internal reference)

• Set IDAC magnitude and select IDAC1 output pin to drive lead 1 of the RTD PGA ADC

AIN0

AIN1

AIN2

REFP0

REFN0

RREF

RRTD

AVSS

IDAC1

IDAC2

RBIAS

AVDD

Lead 1

Lead 2

Mux

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2.2 Two-

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