AN70698_001-70698_0H

www.cypress.com Document No. 001-70698 Rev. *H 1

AN70698

PSoC 3 and PSoC 5LP Temperature Measurement with an RTD

Author: Praveen Sekar and Todd Dust

Associated Project: Yes Associated Part Family: All PSoC 3 and PSoC 5LP Parts

Software Version: PSoC Creator 2.2 SP1 and higher

Related Application Notes: AN75511, AN66477, AN60590

AN70698 explains the theory of temperature measurement using an RTD, and then shows how to do so with a single PSoC 3 or PSoC 5LP without the need for external ADCs or amplifiers. To make it easy to calculate temperature from ADC readings, PSoC Creator provides an RTD Component. Four example projects are included to demonstrate operation with both low and high levels of accuracy and resolution.

Contents

Introduction ....................................................................... 2 RTD Theory of Operation ............................................... 2 RTD Resistance Measurement Methods ........................... 3

Two-Wire Measurement ............................................... 3 Three-Wire Measurement ............................................. 3 Four-Wire Measurement ............................................... 4

Reference Resistor Method ............................................... 4 Reference Resistor Selection ....................................... 5 Offset Error Cancellation .............................................. 5 Gain Error Cancellation ................................................ 5

RTD Resistance-to-Temperature Conversion ................ 5 Positive Temperatures.................................................. 5 Negative Temperatures ................................................ 6 Choosing the Right Polynomial Order .......................... 6 RTD Component .......................................................... 6

RTD Temperature Measurement with PSoC ..................... 7 Hardware Used CY8CKIT-025 EBK .......................... 7

Project Description ............................................................ 8 Firmware Flow .............................................................. 9 Testing the Project ..................................................... 10

Interfacing Multiple RTDs ................................................ 10 Broken RTD Reconfiguration .......................................... 10 Performance Measures ................................................... 10

Temperature Resolution ............................................. 10 Temperature Accuracy ............................................... 11 List of all Errors .......................................................... 14 Test Results ............................................................... 14

Summary ......................................................................... 15 About the Author ............................................................. 15

Appendix A ...................................................................... 16 Appendix B ...................................................................... 18

Broken RTD reconfiguration ....................................... 18 Worldwide Sales and Design Support ............................. 26



Introduction

Temperature is one of the most frequently measured environmental variables. Temperature measurement is typically done using one of four sensors: resistance

temperature detector (RTD), thermocouple, thermistor, and diode. Table 1 compares these four sensor types.

Table 1. Comparison of Temperature Sensors

Parameter RTD Thermocouple Thermistor Diode

Temperature range 200 to +850 250 to +2350 100 to +300 50 to +150

Sensitivity at 25 C 0.387 /C 40 V/C (K-type) 416 /C 250 V /C

Accuracy High Medium to High Medium Low

Linearity Good Fair Poor Good

Typical cost (US $) $3 $80 $3 $15 $0.2 $10



Equation 1 and Equation 2 are also referred to as CallendarVan Dusen equations, and A, B, and C coefficients are also known as CallendarVan Dusen coefficients; the term is named after the scientists who discovered and refined the equations.

The values of A, B, and C for PT100 RTD are specified in IEC 60751 for standard industry-grade platinum and are:

412

27

13

10*183.4

10*775.5

10*9083.3

CC

CB

CA

and the resistance at 0 C,

1000R (PT100 RTD)

Plotting the resistance versus the temperature yields a nearly linear curve, which is shown in Figure 1. However, it is not a perfectly straight line. Figure 1 also shows the straight line approximation imposed on the RTD curve.

Figure 1. RTD Resistance versus Temperature

RTD temperature measurement involves two steps:

1. Measure the RTD resistance accurately

2. Convert the measured resistance accurately to temperature.

The following sections describe the two tasks.

RTD Resistance Measurement Methods

The techniques commonly used to measure resistance, and their effectiveness for RTDs, are described in the following section.

Two-Wire Measurement

In a two-wire measurement, a current is passed through the RTD and the voltage across the RTD is measured, as shown in Figure 2.

Figure 2. Two-Wire RTD Temperature Measurement

Rw1

IsRRTD

VRTD

Rw2

In Figure 1 the Linear Approximation is made by assuming

the RTD changes by 0.385/C. A 1-ohm error in the resistance measurement leads to a temperature error of about 2.6 C(1/.385). Therefore, while measuring RRTD, the RTD resistance has to be determined with an accuracy of



Figure 3. Three-Wire RTD Temperature Measurement

IsRRTD

VRTD

Vwire

Rw1

Rw2

Rw3

1

2

Caveat

Although this method is better than the previous method, the three-wire method gives accurate results only if Rw1 = Rw2.

To avoid all error associated with the wire resistances, use the four-wire measurement described next.

Four-Wire Measurement

The four-wire measurement, as shown in Figure 4, greatly reduces any error caused by wire resistances.

Figure 4. Four-Wire RTD Temperature Measurement

Rw1

ISRRTD

VRTD

Rw2

Rw3

Rw4

In this method, a known constant current is passed through the RTD. The voltage across it is measured using a separate sensing path. The separate sensing path ensures that the voltage drop across the wire resistances, Rw1 and Rw4, does not affect the RTD voltage measured.

There will be little voltage drop across resistances Rw2 and Rw3, which are in the ADC measurement path, because there is negligible current flow into the high-input impedance terminals of the ADC.

The RTD resistance in this method is given by Equation 7.

RTD

RTDRTD

I

VR Equation 7

To find the RTD resistance, the current source and the ADC measuring the voltage must be accurate. Specifically, the current source and the ADC should be free from offset, gain and non-linearity errors. Even a small error in voltage measurement can result in a large temperature error at higher temperatures.

To overcome the gain/offset error caused by the ADC and the current DAC (IDAC), add a reference resistor to your design.

Reference Resistor Method

The reference resistor makes the measurement error independent of both the current source accuracy (gain error,), and the ADC accuracy (gain error). The measurement error depends primarily on a reference resistance.

The schematic of this method is shown in Figure 5. A constant current is passed through a known accurate reference resistance in series with the RTD. The voltage, Vref, across the reference resistor and the voltage, VRTD, across the RTD are measured. The RTD resistance is given by Equation 8:

ref

ref

RTDRTD R

V

VR * Equation 8

Figure 5. RTD Temperature Measurement

IDAC

ADC

RRTD

Rref, 100 ohm0.1%

0

1

Rw1

Rw2

Rw3

Rw4

PSoC

The current path is shown by the bold blue line, and voltage measurement paths are shown in red. The current does not flow through wire resistances Rw3 and Rw4, because of the high-input impedance of the ADC. The rest of the application note discusses this method and the associated project based on it. The following sections discuss how to choose a reference resistor, and how the offset error and the gain error of the ADC and IDAC become null.



Reference Resistor Selection

Choose a small reference resistor that doesnt load the IDAC. This project uses a reference resistance of 100 ohms. The IDAC has a compliance voltage of VDDA-1V. This means that the voltage at the IDAC cannot exceed VDDA-1V. If the maximum RTD resistance is 400 ohms, the reference resistor is 100 ohms, the internal PSoC routing is ~400 ohms and a 1-mA current is passed through the three in series, then a voltage of 900 mV will be produced (1 mA * 400 ohms + 100 ohms + 400 ohms). This is well below the compliance voltage.

In addition, choose a reference resistor that uses the same ADC range as the RTD. This will reduce the time it takes to measure the RTD, because the ADC will not need to be reconfigured. Again, if the maximum RTD resistance is 400 ohms, and 1 mA is passed through it, the RTD will produce a voltage of 400 mV. The DelSig ADC has an input range of 512 mV. Keep the voltage across the reference resistor in this 512-mV range. If it is outside this range, you will have to reconfigure the ADC every sample; reconfiguration takes time.

Offset Error Cancellation

In this application note, offset cancellation is done by correlated double sampling (CDS). In CDS, the offset is measured and then, in firmware, it is subtracted from the other voltage measurements. Using this method reduces the ADC sample rate by 50%, because offset is measured every other sample.

You can use CDS to measure offset in various ways. See AN66444 PSoC

3 and PSoC 5LP Correlated Double

Sampling for details.

For this application note, offset is measured by passing zero current through the IDAC and measuring the voltage directly across channel 0 or 1.

With CDS, the equation for resistance measurement becomes:

RTDref

ref

RTD RVV

VV*

0

0

Equation 9

Where, VRTD is the voltage measured across the RTD

Vref is the voltage measured across the 100- resistor

V0 is the offset voltage measured when the IDAC current is set to zero.

The offset current of the IDAC does not cause any error because it would be nulled by the difference in Equation 9.

VRTD = (I+I0) * RRTD + ADC offset

V0 = I0 * RRTD + ADC offset

Subtracting V0 From VRTD removes ADC offset. The difference nulls the I0 term, which is caused by the IDAC offset. If CDS is done regularly, then offset drift is also canceled out.

Gain Error Cancellation

Assume that the ADC has a gain error of k and the DAC has a gain error of k. These errors reflect as multiplicative factors in the voltage measurements, VRTD and Vref. Because Equation 9 includes a ratio, the multiplicative errors k and k cancel out.

RTD = ref

ref

RTD RVVkk

VVkk*

)(**

)(**

0

'

0

'

Now the error depends primarily on the accuracy of the reference resistor, Rref.

This method also removes any errors associated with gain drift, because the ratio metric measurement is being taken every time.

Using the reference resistor method, you can determine the RTD resistance accurately. The next step is to convert the RTD resistance to temperature. This conversion is discussed in the following section.

RTD Resistance-to-Temperature Conversion

The straightforward method to obtain temperature from resistance is to use the CallendarVan Dusen equations. But Equations 1 and 2 show resistance in terms of temperature; you need to know temperature in terms of resistance. The solution is given below.

Positive Temperatures

Solving Equation 1 for T,

B

R

RBAA

T

T

2

140

2

Equation 10

The other solution of the quadratic equation is eliminated using the known point (T = 0; R =100).

Using Equation 10 involves a square root, which requires a math library and about 5800 8051 CPU cycles for computation. This is a lot of time to spend doing a single calculation.

Instead of Equation 10, use a polynomial to calculate temperature. Compute the polynomial by first constructing a resistance versus temperature table and then using curve-fitting techniques on the table. Excel is a good tool to use for curve fitting.



Polynomial computation, which does not require a math library, executes faster. The temperature accuracy increases as the order of the polynomial increases. Using a fifth-order polynomial

[1] can reduce the conversion error

to



For example, if you are using PT100 RTD, your range is -100 C to 400 C, and you require 0.05 C temperature calculation accuracy, select PT100 RTD, enter the maximum and minimum temperatures in their respective text boxes, and select 0.05C temperature accuracy. The

component automatically chooses the appropriate polynomial for you.

Figure 8 shows that a third-order polynomial meets that requirement.

Figure 8. Customizer for 100 C to 400 C Range

In your code, use the RTD_GetTemperature(int32 res) API to calculate temperature. The API takes resistance (in milliohms) and computes the temperature using the polynomial coefficients and returns the temperature (in 1/100

th of C).

The temperature error depends on many factors in addition to resistance-to-temperature conversion error. The RTD customizer shows only the error due to resistance-to-temperature conversion. To accommodate other errors, ensure that the resistance-to-temperature conversion error is less than one-tenth of the total error budget. The other errors are discussed in Accuracy of the Measurement section.

RTD Temperature Measurement with PSoC

This application note has four example projects (RTD_HighEnd, RTD_MidEnd, RTD_LowEnd, and Broken RTD reconfiguration) to showcase RTD temperature measurement using PSoC. The first three projects display the RTD temperature on an LCD. These projects have the same signal chain. This section describes the RTD_MidEnd project in detail. Differences between the projects are explained in detail in the Performance Measures section. The fourth project, Broken RTD Reconfiguration, is explained in Appendix B.

Hardware Used CY8CKIT-025 EBK

The PSoC Precision Analog Temperature Sensor EBK (CY8CKIT-025) is used in the example project. The kit provides four sensorsthermocouple, thermistor, RTD, and diodefor measuring temperature. In addition, interface slots let you plug in your own thermocouple, thermistor, RTD, and diode. You can connect the EBK to the CY8CKIT-030 PSoC 3 Development Kit (DVK), or to the CY8CKIT-050 PSoC 5LP DVK. Figure 9 shows the kit and Figure 10 shows the RTD portion. For more details on the kit, go to www.cypress.com/go/Cy8CKIT-025.

Figure 9. PSoC Precision Analog Temperature Sensor EBK



Figure 10. RTD Section of the EBK

Project Description

The RTD_MidEnd PSoC Creator project associated with this application note displays the RTD temperature directly on an LCD. The schematic of the project, which uses the CY8CKIT-025 PSoC Precision Analog Temperature Sensor EBK, is shown in Figure 11. This figure is similar to Figure 5 except that the reference resistance is in a separate path, and is not in series with the RTD.

The CY8CKIT-025 does it this way to share the reference resistance for diode temperature measurement. Because the reference resistance is not in series with the RTD, the current through the reference resistance can differ from the current through the RTD. If the currents through the reference resistance and RTD are not equal, you can get a temperature error. Manual analog routing is used to avoid a significant error. See Appendix A for a detailed explanation.

If you are designing your own board for RTD measurement, use the reference resistor in series with the RTD, as shown in Figure 5. You can avoid the 0.7 C error at 850 C (see Appendix A). You require only one pin from IDAC, and the current mux shown is not required (see Figure 11). Use an IDAC direct connection (P0_6, P0_7, P3_0, P3_1) to connect the series combination of RTD and reference resistor to the IDAC. The PSoC Creator schematic for that project is shown in Figure 12.

Figure 11. PSoC Creator Top Design Schematic



Figure 12. PSoC Creator Top Design Schematic for Reference Resistance in Series with RTD

Firmware Flow

The following chart shows the firmware flow for the low- and mid-end RTD temperature measurement projects associated with this AN. The projects differ only in the ADC resolution and the filter attenuation factor used.

Read voltage across reference

resistor at zero current for

Correlated Double Sampling (CDS)

Connect IDAC current mux to

Reference resistor and measure

voltage V1.

Connect IDAC current mux channel

to RTD and measure voltage V2.

Calculate temperature using the

RTD_GetTemperature(uint32 res)

API of the RTD component

Start IDAC and set to source 1-mA

current

END

CDS V1 and filter (IIR Filter)

CDS V2 and filter

Measure RTD resistance

from V2 and V1RRTD = (V2/V1)*Rref

Read voltage across RTD sensor

at zero current for CDSStart



If the reference resistance is in series with the RTD, the firmware flow remains as shown, except that the IDAC output is not multiplexed.

Testing the Project

1. Plug CY8CKIT-025 into PORT E of the CY8CKIT-030 or CY8CKIT-050.

2. For the CY8CKIT-030, choose the CY8C3866AXI-040. For the CY8CKIT-050, choose the CY8C5868AXI-LP035

3. Build the attached project and program the PSoC device.

4. The LCD displays the RTD temperature.

Interfacing Multiple RTDs

The easiest way to interface to multiple RTDs is to wire them in series with one another. However, make sure you dont violate the compliance voltage of the IDAC, as discussed earlier.

Alternatively, you can easily interface as many as four RTDs with PSoC 3 / PSoC 5LP using the four IDACs in the device.

Broken RTD Reconfiguration

If one of the RTD wires is broken, you can reconfigure the four-wire RTD to a three-wire RTD and continue to show the temperature without a considerable loss of accuracy. Appendix B describes the broken RTD reconfiguration in detail. A project (Broken RTD reconfiguration) is also associated with this application note.

Performance Measures

The RTD-based temperature-sensing market can be categorized into three performance segments: high end, mid end, and low end (see Table 3).

Table 3. RTD Performance Ranges

Market segment Resolution* (C) Accuracy* (C)

High 0.01 0.1%

Mid 0.1 0.2 0.5%

Low >0.1C >0.5C

* Resolution is generally specified only for temperatures > 0C. Accuracy doesnt include the sensor accuracy and is usually specified with a fixed offset, such as 0.1% or 1C, whichever is greater.

In this section, we will see how to use PSoC 3 and PSoC 5LP to address all three segments. Unit test results appear at the end.

Temperature Resolution

The temperature resolution depends on three factors:

The temperature range to be measured The IDAC current The ADC resolution High End

Lets calculate the resolution required for measuring -200 C to 850 C with a 0.01 C resolution. First, we need to translate the temperature resolution to voltage resolution. From Equations 1 and 2, you can calculate the resistance change per unit change in temperature .

A change from 850C to 851C causes a 0.292- change in resistance.

A change from -200C to -201C causes a 0.432- change in resistance.

Take the smaller of the two: 0.292 for the calculations. Use an IDAC current of 1 mA; the current used in the associated project, for the calculations.

The voltage resolution required for 1 C = 0.292 * 1 mA = 292 V.

If you can resolve 2.92 V, you will achieve a 0.01 C resolution.

This means the IDAC (operating at 1-mA current) and the ADC together should have a noise of less than 2.92 V.

Next, calculate the full-scale voltage range of the RTD:

Resistance at -200 C = 18.52 (using Equation 2)

Resistance at 850 C = 390.481 (using Equation 1)

Voltage output at -200 C = 18.52 * .001 = 18.52 mV

Voltage output at 850 C = 390.481 * .001 = 390.481 mV

The ADC has an input range of 512 mV; this range fits well with the output voltages of the RTD.

Using a 20-bit ADC with an input range of 512 mV, each bit represents 1.024 V/2^20 = 0.9765 V.

According to the DelSig ADC datasheet, the ADC has an RMS noise of ~1 count in the 512-mV range. This indicates the noise is below the required 2.92 V. However, when the voltage is close to a temperature transition, the noise causes flicker in the output.

To eliminate flickering bits, the attached project adds a small software-based IIR filter to the code. See AN2099, Single-Pole IIR filter for a detailed description of a software-based IIR filter.



Mid End

For mid-end applications, the resolution requirement is 0.1 C. Using the same logic we used before, we need to resolve only 29.2 V to achieve a resolution of 0.1 C. For the mid-end project, the same ADC configuration is used as the one in the high-end project. The only difference is that the filter attenuation is lower in the mid-end project. Low End

For low-end applications, a resolution of 1 C is often sufficient. Based on our earlier calculations, we need to resolve only 292 V. Using a 12-bit ADC in the 512-mV range gives a voltage resolution of 250 V. The 250 V is below the requirement of 292 V. Using a 12-bit ADC allows us to move into lower-cost PSoC devices. The project RTD_LowEnd associated with this application note demonstrates how to use a 12-bit ADC to measure an RTD.

Temperature Accuracy

You can calculate the temperature accuracy by summing all possible individual errors, which fit into one of two categories:

1. Error due to the measurement system

2. Error due to RTD

Error Due to the Measurement System

The error due to the measurement system is due to the circuit shown in Figure 5. Consider Equation 8, which is used to obtain RTD resistance.

ref

ref

RTDRTD R

V

VR *

VRTD is the voltage measured across the RTD

Vref is the voltage across the 100- resistance

Rref is the reference resistance

As discussed, the only major source of error using this method is the accuracy of the reference resistance. The offset error is nulled by the difference, and the gain error is nulled by the ratio. The other source of measurement error is non-linearity in the ADC.

Error Due to ADC Integral Non-Lineari ty

The integral non-linearity (INL) of an ADC at any point is the difference between the ideal ADC count and the actual ADC count at that point after gain and offset corrections have been completed. The datasheet specifies the maximum INL of all points across process, voltage, and

temperature (PVT). PSoC 3 ADC has an INL of 32 LSb; 32 LSb corresponds to 64 V for 20-bit resolution and 1.024-V range. The ADC INL has the same analog value, 64uV, in 0.512 V range. Lets calculate the error due to INL at 850 C for IDAC current of 1 mA.

mVRIV RTDRTD 481.390481.390*1* (The RTD resistance at 850 C = 390.481, using Equation 1)

mVRIV refref 100100*1*

Using equation 8

100*100

064.0481.390100*

100

481.390errorResistance

064.0

Substituting the worst-case INL at the numerator, we get 0.064- resistance error corresponds to a temperature error of 0.16 C(.064/.385). The INL is at its worst when it applies only to the numerator.

Note that we have taken the worst-case INL across PVT and substituted worst-case positive INL at the numerator. In practical application, the error due to INL is much lower. For example, for an INL of +8 LSb at the numerator and 0 LSb at the denominator, the temperature error at 850 C is 0.054 C.

Error Due to Reference Resistance Tolerance

In Equation 8, we substituted 100 for the value of the reference resistor, Rref. But the actual value of Rref will change because of its tolerance and temperature coefficient. Therefore, the value Vref, which is measured across the reference resistance, will be erroneous. Assume that the tolerance of Rref is 0.1 percent and the temperature coefficient is 10 ppm/C.

RTDRTD RIV *

refref RIV *

Substituting the value of Rref with the tolerance and temperature coefficient yields Equation 11.

)25(*00001.0001.01(100* sysTempIVref Equation 11

The measured value of the RTD resistance, Rmeas, is:

))25(*00001.0001.01(

sysTemp

RR RTDmeas

Equation 12

where the 0.1 percent resistance tolerance contributes to the additive factor 0.001, and the temperature coefficient



(10 ppm/C) contributes to the additive factor 0.00001*(sysTemp-25). Note that the temperature coefficient is usually specified with reference to 25 C. Therefore, the effect of the temperature coefficient is zero at 25 C, but it increases as the temperature deviates from 25 C.

The measured temperature can be calculated from the measured resistance using Equations 1 and 2.

Figure 13 shows the temperature error due to the reference resistor error at different RTD temperatures and ambient temperatures. Ambient temperature (see legend) is the temperature of the reference resistance on the printed circuit board (PCB) while RTD temperature (x axis) is the actual temperature to which the RTD is exposed.

Figure 13. Temperature Error Due to Reference Resistor Accuracy and Temperature Tolerance

The orange line shows that for a 0.1 percent reference resistor, the error is



Figure 15. Temperature Error due to RTD Interchangeability Error

At 25 C, the worst-case temperature error = 0.3 + 25 * 0.005 = 0.43 C

At 800 C, the worst-case temperature error = 0.3 + 800 * 0.005 = 4.3 C.

If this error is not acceptable, the RTD has to be calibrated. For most users seeking a high-performance RTD (0.1% or 1C), calibration will be required.

Ideally, the resistance at 0 C, R0, is expected to be 100 for a PT100 RTD. But, practically, it will have a tolerance that causes the interchangeability error. Going back to Equations 1 and 2, note that a deviation in the value of R0 causes a scale error in the measured temperature. Therefore, performing a scale correction corrects the RTD interchangeability error.

For RTD calibration, perform the following steps.

1. Make sure that all other sources of error are nulled (Offset should be nulled by correlated double sampling, and the gain error is automatically nulled by the ratio).

2. Adjust the RTD to a known temperature, T1, for this method T1 should be close to 0 C. Validate and measure that temperature, T1, with an accurate thermometer.

3. Measure the RTD resistance, Rmeas, at T1.

4. Calculate the actual RTD resistance at T1, Ractual, using Equations 1 and 2.

5. The scale error is given by

meas

actual

R

Rscale Equation 13

6. Multiply this scale error by the measured RTD resistance.

This process is done in the project RTD_HighEnd. When the project starts, it asks the user if he wants to calibrate. If he does, he must press SW2.

Next, the project asks for the calibration temperature. The CapSense

slider on the CY8CKIT-030 or CY8CKIT-050

can be used to adjust the temperature by 1 C. The CapSense buttons can be used to adjust the temperature by 0.01 C. After the temperature has been entered, press SW2.

The firmware will calculate the resistance for the entered temperature. Next, it will measure the resistance of the RTD. Finally it will compute the ratio and store it in EEPROM.

If an accurate and stable temperature cannot be achieved, replace the RTD with a known resistor. Using a known resistor will calibrate out only the error due to the reference resistor. It will not calibrate out the RTD interchangeability error.

Note that the reference resistance tolerance also causes a fixed scale error in the measured resistance. Hence, when the above six steps are completed, the reference resistance also is calibrated. The scale error value can be stored in PSoC EEPROM and retrieved each time the device goes through a power cycle.

The 100 comes because you measured at 0C and 100C,

for a difference of 100. Now multiply all temperatures by this scale factor.

This process is not implemented in the project.

Figure 15 and Table 4 show that the temperature error has an offset and a gain. Thus, it may become necessary to perform a two-point temperatue calibration. For example, place the RTD at 0 C and measure the temperature (Toffset). Subtract this measured temperature from subsequent temperature readings.

Second, place the RTD at 100 C and measure the temperature (TGain). Next, compute a scale factor using the equation.

Scale = TGain Toffset / 100.

The 100 comes because you measured at 0 C and 100 C, for a difference of 100. Now multiply all temperatures by this scale factor.

This process is not implemented in the project.

RTD Self -Heat ing Error

An RTD self-heating error is the increase in temperature of the RTD due to the current flowing through the RTD.

As a result of self-heating, the RTD can show a temperature slightly higher than ambient. This error can be found in the RTD datasheet. For PTS080501B100RP100, the value is specified at



List of all Errors

Table 5 shows the temperature error due to various components at 150 C. As seen from the table, RTD interchangeability and reference resistance tolerance are the biggest sources of error. By comparison, the other errors are negligible.

Table 5. Possible Errors in RTD Temperature Measurement at 150 C

Error Source Error Value at 150 C (0.1% Reference Resistor, class B

RTD)

Error Value at 150C (Both Reference Resistor and RTD Calibrated)

Signal Chain Error

Offset Error/drift 0 C 0 C

Gain Error/drift 0 C 0 C

ADC INL* 0.2 C 0.2 C

RTD self-heating error (PTS080501B100RP100 SMD RTD)

< 0.13 C



Summary

When high accuracy is critical in measuring temperatures, an RTD is the sensor of choice. PSoC 3 / PSoC 5LP have the necessary hardware integrated in the device to achieve high accuracy. The PSoC Creator RTD Component makes designing with RTD easier.

About the Author

Name: Praveen Sekar

Title: Applications Engineer

Background: Praveen holds a Bachelors degree in Electronics and Communication from the College of Engineering, Guindy, Chennai. He focuses on analog modules in PSoC

Contact: [email protected]

Name: Todd Dust

Title: Applications Engineer Staff

Background: Todd holds a Bachelors degree in Electrical Engineering from Seattle Pacific University.

Contact: [email protected]



Appendix A

This section describes the effect of having the RTD reference resistance in a separate path (not in series with the RTD).

The IDAC current droops by about 1 percent as the IDAC load voltage increases from 0 V to Vdda-1 V. Each of the four IDACs in PSoC 3 / PSoC 5LP can connect to one pin directly and to other pins through the analog globals. The direct connection is through a single switch, which has a resistance of about 200 . When the IDAC connects to a pin using an analog global, it connects through two switches, each with a resistance of about 200 . For a detailed description of the PSoC 3 or PSoC 5LP analog routing, see AN58827 Internal Routing Considerations for PSoC

3 and PSoC 5LP Analog Designs.

For IDAC3, the direct connection is to pin 3_1. The RTD on the EBK is connected to pin 3_1 of PSoC, and the calibration channel is connected to pin 3_4 of PSoC (see Figure 16).

The Top Design schematic, shown in Figure 11 requires separate IDAC multiplexer channels: the RTD channel and the calibration channel. In building the project, PSoC Creator can route the RTD channel through the direct switch and the calibration channel through the analog global. In this case,

IDAC load resistance in RTD channel = 200 + RTD resistance

IDAC load resistance in calibration channel = 500 (400 + calibration resistance 100 ).

The different load resistances result in different IDAC load voltages, which produce different IDAC currents in both channels. The error in RTD resistance caused by the different currents is calculated by Equation 16.

f

RTDRTD

I

IRR

Re

1* Equation 9

The resistance error increases as the RTD resistance (temperature) rises. If the ratio, IRTD/IRef, is equal to 1, the resistance error falls to zero, as expected.

Lets calculate the error in temperature at 25 C due to the different IDAC currents.

The IDAC current used = 1 mA

The IDAC load voltage across the RTD channel = Load resistance * IDAC current = (200 + 109.732) * 1 mA = 309.732 mV

The IDAC load voltage across the calibration channel = 500 * 1 mA = 500 mV

As the load voltage rises from 309.732 mV to 500 mV, the IDAC current drops by about 0.5 A (See IDAC user module datasheet graph)

From Equation 16,

055.0

5.999

10001*734.109R

This corresponds to a temperature error of 0.14 C at 25 C.



Figure 16. IDAC to Pin Routing Resistances

IDAC3

Pin 3_4

Pin 3_1

AGR 4 AGR 5

200

200

RTD

Calibration

Resistance

Temp Sensor EBK

PSoC

200

200

200

To avoid that error, use manual analog routing to force the router to use the analog global path for both channels. Now the load resistance in both channels remains about the same. The only factor that would affect the load resistance is the change in RTD resistance due to temperature. At 850C, the RTD resistance would be 400 , while the calibration resistance would be 100 . That 300- difference causes a 300-mV difference in the load voltages. For that voltage difference of 300 mV, the current change would be about 0.5 A. (The current droop with load voltage is available in the IDAC user module datasheet) As a result, the worst-case error would be 0.7 C at 850 C.

Also, you should force the IDAC to IDAC3 by using a force placement directive in the directives tab of the design-wide resources (.cydwr) file, as shown in Figure 17.

Figure 17. Force Placement Directive on IDAC



Appendix B

Broken RTD reconfiguration

If one of the four wires of the RTD breaks, PSoC can automatically detect a broken wire and reconfigure four-wire RTD connection to three-wire connection and display temperature with minimal degradation to accuracy (< +/- 0.5C). Broken RTD reconfiguration involves three steps.

1. Detect broken wire connection

2. Reconfigure the analog routing to change four-wire connection to three-wire connection

3. Compensate for the additional wire resistance due to the three wire mode and display temperature.

The PSoC Creator project (Broken RTD Reconfiguration) associated with this application note demonstrates this feature.

Detect ing Broken RTD wire

A four-wire RTD connection to PSoC is shown in Figure 18. The current is passed through pin 3_1 into RTD wire 1 and it is grounded through RTD wire 4 (RTD wire 4 is not connected to a PSoC pin). The ADC differential inputs are connected to pins 4_0 and 4_1, which are connected to wires 2 and 3 of the RTD. The pin choices are made according to the connections in CY8CKIT-025 PSoC precision analog temperature sensor EBK

Figure 18. Four-Wire RTD Connected to PSoC

IDAC

ADC +-

PSoC Kit025External 4-wire

RTD

R1

R2

R3

RTD

Pin 3_1

Pin 4_0

Pin 4_1

Rw1

Rw2

Rw3

Rw4

A broken wire can be detected by using PSoCs GPIO structure. PSoC GPIO can be configured to source vdd through a pull up resistor while simultaneously sensing the pin state through its digital input buffer. For example, pin 3_1 can be configured as shown in Figure 19.

Figure 19. Pin Configured in Resistive Pull-Up Mode and Digital Input Mode

VDDIO

Pin

State

> 3.5k

PSoC

To analog

global

Digital Input

Buffer

To analog

Mux Bus

To detect the broken RTD wire connected to pin 3_1, pin 3_1 is configured in the resistive pull-up mode and the pin state is sensed back.



When wires 1 and 4 are not broken, the RTD resistance forms a resistor divider with the internal pull-up resistor and the voltage across the RTD is sensed back as the pin state.

The pull-up resistance has a minimum value of 3.5 k and the RTD can have a maximum resistance of 390 (at 850 C). Assuming wire resistances (Rw1 and Rw4) = 5 each, we get a maximum value of 400 .

Figure 20. Detecting Wires 1 and 4 for Breakage

VDDIO

Pin

State

> 3.5k

External 4-wire

RTD

RTD

Rw1

Rw2

Rw3

Rw4

Pin 3_1

PSoC

To analog

global

Digital Input

Buffer

To analog

Mux Bus

Voltage sensed by pin = Voltage across RTD+Rw1+Rw4

=

pwwRTD

wwRTD

RRRR

RRR*VDDIO

21

21

= 3900

400*VDDIO

< 0.1 * VDDIO

VIL of the pin < 0.3 * VDDIO

Therefore, when pin 3_1 is configured as resistive pull up (with a high voltage forced through the pin) and when no RTD wire is broken, the pin state will be low. When either RTD wire 1 or 4 is broken, the pin state will be high.

Similarly, we can detect if RTD wires 2 and 3 are broken by configuring the respective pins to resistive pull-up modes and reading the pin state back.

To find which RTD wire is broken, follow these steps:

1. Disconnect pins 3_1, 4_0 and 4_1 from ADC and DAC

2. Configure pin 3_1 in resistive pull up mode

3. Drive high through pin 3_1

4. Read the pin state of pin 3_1

5. Repeat steps 2, 3, and 4 for pin 4_0 and pin 4_1

Let the pin states of pin 3_1, pin 4_0 and pin 4_1 be stored in variables A, B, and C respectively. Based on different values of A, B, and C, we can have eight states, as shown in the following table.



A B C Result

0 0 0 No Wire Broken

0 0 1 Wire 3 broken

0 1 0 Wire 2 broken

0 1 1 Wires 2 and 3 broken

1 0 0 Wire 1 broken



1 1 1 Wire 4 broken

The previous table also shows the result of each combination of A, B, and C.

If any of A, B and C is equal to 0, then wire 4 is not broken.

If A, B and C are all equal to 1, wire 4 is definitely broken. Apart from wire 4 any other wire can also be broken. But in such a case reconfiguration is not possible. Reconfiguration is possible only if one of the wires is broken.

Reconf iguring four-wire RTD to three-wire

After detecting the broken RTD wire, we have to reconfigure the four-wire RTD to three-wire RTD eliminating the broken wire. The flexible analog routing structure of PSoC makes it very easy to reconfigure four-wire RTD to three-wire RTD eliminating the broken wire. The reconfiguration routes are shown below.

RTD Wire 1 Broken

If RTD wire 1 is broken, the current path is opened. To close the current path, the routing is reconfigured such that the current is forced through the ADC pin 4_0 as shown in Figure 21.

Figure 21. RTD Wire 1 Broken

IDAC

ADC +-

PSoC Kit025

R1

R2

R3

Pin 3_1

Pin 4_0

Pin 4_1

Rw1

Rw2

Rw3

Rw4External 4-wire

RTD

RTD

R1 = Routing resistance from IDAC to pin

R2 = Routing resistance from ADC (positive) to pin

R3 = Routing resistance from ADC (negative) to pin

In this case, the IDAC current flows through R1, Rw2, through RTD to ground. Since Rw2 is in the measurement path of the ADC, the RTD resistance and wire 2 resistance is also measured. This wire resistance can be eliminated through calibration explained in one time wire resistance computation section.



RTD Wire 2 Broken

If RTD wire 2 is broken, the path from RTD to ADC positive terminal is opened. To close the path, we connect ADC positive terminal to pin 3_1 as shown in Figure 22 .


IDAC

ADC +-


RTD

R1

R2

R3

RTD

Pin 3_1

Pin 4_0

Pin 4_1

Rw1

Rw2

Rw3

Rw4

In this case, the ADC measures RTD wire resistance 1 in addition to the RTD resistance.

RTD Wire 3 Broken

If RTD wire 3 is broken, the path from RTD to ADC negative terminal is opened. To close the path, we connect ADC negative terminal to ground as shown in Figure 23.


IDAC

ADC +-


RTD

R1

R2

R3

RTD

Pin 3_1

Pin 4_0

Pin 4_1

Rw1

Rw2

Rw3

Rw4

In this case, the ADC measures RTD wire resistance 4 in addition to the RTD resistance. Also, any difference in potential between the two grounds (kit-025 ground and the chip internal ground) adds to measurement error. One time offset correction eliminates both the wire resistance error and the ground difference error.

RTD Wire 4 broken

If RTD wire 4 is broken, the current path from RTD to ground is opened. The ADC input terminal is Hi-Z and no current flows through the ADC input. To close the path, we provide the ground path by configuring the pin in open drain low mode as shown in Figure 24.


IDAC

ADC +-


RTD

R1

R2

R3

RTD

Pin 3_1

Pin 4_0

Pin 4_1

Rw1

Rw2

Rw3

Rw4R4

In this case, the ADC measures RTD wire resistance 3 in addition to the RTD resistance.



One-t ime Wire Resistance Computat ion

When reconfiguring a four-wire RTD into a three-wire RTD, wire resistances affect the RTD temperature measurement accuracy. 1-ohm wire resistance can cause 3C error in measured temperature. To eliminate the error due to the wire resistances, we perform a one-time wire resistance computation before making the RTD measurements.

The steps followed to compute the wire resistances are given below

1. Configure the RTD in four-wire mode as shown in Figure 18 and find the RTD resistance (R0)

2. Configure the RTD in three-wire mode (wire 1 broken) as shown in Figure 21 and calculate the resistance (R1)

3. Compute additional wire resistance, CompRes1 = (R1 R0). When wire 1 breaks and the RTD is reconfigured as shown in Figure 21, CompRes1 should be subtracted from the measured resistance.




7. Compute additional wire resistance, CompRes3 = (R3 R0). When wire 3 breaks and the RTD is reconfigured shown in Figure 23, CompRes3 should be subtracted from the measured resistance.



Project Descript ion

The Project (Broken RTD reconfiguration) has been built to work with CY8CKIT-025. The project detects a broken RTD wire and automatically reconfigures a four-wire RTD connection to a three-wire RTD connection and continues to display temperature with a broken alert. The project also performs one-time wire resistance compensation so that the temperature displayed is accurate.

The PSoC Creator schematic of the project is shown in Figure 25.

Figure 25. PSoC Creator Schematic of Broken RTD Reconfiguration Project



This project is similar to the RTD temperature measurement project except that there are more IDAC and ADC channels to support four-wire to three-wire reconfiguration. Manual analog routing has been used to ensure that the ADC positive connection always happens through analog mux bus. This way we ensure that the ADC and IDAC connections are through two different paths until the pin. Each pin connects to the analog network through two paths: Analog mux bus and Analog globals (see Figure 19 and Figure 20). See PSoC3 Datasheet I/O System and Routing section for a detailed description of pin structure. By making sure that the ADC connects to the pin through the analog mux bus and that the DAC connects to the pin through one of the analog globals, we eliminate any additional resistance that can add to RTD resistance.

Figure 21 shows separate IDAC and ADC paths until pin. Figure 26 shows IDAC and ADC paths merged before pin. In this we have resistance R4 in addition to Rw2. This additional resistance will also get subtracted out when we perform resistance compensation. But having separate current and voltage paths until the pin is a better way of doing a three wire RTD. If the compensation fails due to some reason, the additional resistance error will be in the order of ohms when you have separate IDAC and ADC paths whereas it will be in the order of hundreds of ohms when you do not have separate IDAC and ADC paths until the pin.

Figure 26. Wire 1 Broken IDAC and ADC Paths Merged Before Pin

IDAC

ADC +-

PSoC Kit025

R1

R2

R3

Pin 3_1

Pin 4_0

Pin 4_1

Rw1

Rw2

Rw3

Rw4External 4-wire

RTD

RTD

R4

Firmware Flowchart

The firmware flowchart is shown in Figure 27. There is a step in which the firmware reconfigures the routing to four-wire mode on a switch press. Whenever a wire is broken, the RTD reconfigures the routing and remains in that routing even if the broken wire is fixed. The user can press the Switch, SW2, in the DVK and the firmware reconfigures the routing to four wire mode. When one of the RTD wire breaks, the RTD resistance will drop down to less than 10 . This condition is used in the project to trigger the checking of wires.



Figure 27. Firmware Flowchart

Start

Find additional wire resistances in

each of the four cases, wires 1, 2, 3,

and 4 broken

Calculate RTD resistance, R, and

subtract additional wire resistance

Is R



Document History

Document Title: AN70698 PSoC 3 and PSoC 5LP Temperature Measurement with an RTD

Document Number: 001-70698

Revision ECN Orig. of Change

Submission Date

Description of Change

** 3458038 PFZ 12/12/2011 New Application note

*A 3490797 PFZ 1/12/2012 MEH Review Feedback in CDT#116240

*B 3520653 PFZ 2/8/2012 Updated project. No change to document

*C 3689958 PFZ 08/09/2012 RTD component includes support for PT100, PT500 and PT1000 RTDs

A new section on broken RTD reconfiguration has been added

Other minor changes

*D 3740378 PFZ 09/11/2012 Updated associated project files.

*E 3818484 PFZ 11/21/2012 Updated title to read as PSoC 3 and PSoC 5LP Temperature Measurement

with an RTD.

Updated Associated Part Family as All PSoC 3 and PSoC 5LP Parts.

Updated Related Application Notes as AN75511, AN66477, AN60590.

Updated Introduction.

Updated RTD Resistance-to-Temperature Conversion (Updated Choosing the Right Polynomial Order (Updated description), updated RTD Component (Updated Figure 6, Figure 7, Figure 8)

Updated Project Description (Updated Figure 11 and Figure 12)

Updated Appendix B (Updated Broken RTD reconfiguration (Updated Project Description (Updated Figure 25))).

Replaced PSoC 5 with PSoC 5LP in all instances across the document.

*F 4057734 TDU 07/11/2013 Added two projects and discussed different performance ranges.

*G 4152296 TDU 10/09/2013 Updated attached Associated Project.

Completing Sunset Review.

*H 4202789 TDU 11/26/2013 Fixed formatting errors.



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