AN70698 PSoC® 3 PSoC 4 and PSoC 5LP Temperature Measurement with an RTD.pdf

AN70698
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
Author: Todd Dust and Praveen Sekar
Associated Part Family: PSoC 3, PSoC 4200, PSoC 5LP
Associated Code Examples: For a complete list, click here.
Related Application Notes: For a complete list, click here.
To get the latest version of this application note, or the associated project file, please
visit http://www.cypress.com/AN70698.
AN70698 explains the theory of temperature measurement using an RTD, and then shows how to do so with a single
®
PSoC 3, PSoC 4 or PSoC 5LP without the need for external ADCs or amplifiers. It also explains how to calculate the
resolution and accuracy of a given system.
Contents
1
2
3
4
5
6
7
8
1
Introduction ...............................................................1
1.1
Using this Document........................................2
RTD – Theory of Operation ......................................3
RTD Resistance Measurement Method....................4
3.1
Two-Wire Measurement...................................4
3.2
Three-Wire Measurement ................................4
3.3
Four-Wire Measurement ..................................5
RTD – Resistance-to-Temperature Conversion........8
4.1
Positive Temperatures .....................................8
4.2
Negative Temperatures ...................................8
4.3
Choosing the Right Polynomial Order..............8
4.4
RTD Component ..............................................9
RTD Temperature Measurement with PSoC .......... 11
Interfacing Multiple RTDs ....................................... 13
Broken RTD Reconfiguration .................................. 13
Performance Measures .......................................... 13
8.1
Temperature Resolution ................................ 13
8.2
Temperature Accuracy .................................. 14
8.3
List of all Errors .............................................. 19
8.4
Test Results................................................... 19
9
Summary ................................................................ 20
10 Related Resources ................................................. 20
10.1
Related Application Notes ............................. 20
10.2
Related Code Examples ................................ 20
A
Appendix A ............................................................. 21
A.1
Broken RTD Reconfiguration ......................... 21
Document History............................................................ 27
Worldwide Sales and Design Support ............................. 28
Products .......................................................................... 28
®
PSoC Solutions ............................................................. 28
Cypress Developer Community....................................... 28
Technical Support ........................................................... 28
Introduction
Temperature is one of the most frequently measured environmental variables. RTD temperature measurement is
typically done using one of four sensors: resistance temperature detector (RTD), thermocouple, thermistor, or diode.
Table 1 compares these four sensor types.
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PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
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 Ω/°C
40 µV/°C (K-type)
416 Ω/°C
250 µV /°C
Accuracy
High
Medium to High
Medium
Low
Linearity
Good
Fair
Poor
Good
Typical cost (US $)
$3 – $80
$3 – $15
$0.2 – $10
<$0.2
<100 meters
Surface mount for
on-board temperature
On-board temperature
Typical distance of
sensing
Surface mount for
on-board temperature
three- and four-wire up
to a few hundred
meters
Leaded for <1 meter
Resource
requirement
Excitation current,
amplifier, ADC,
reference resistor
Amplifier, ADC, voltage
reference, and another
temperature sensor for
cold junction
Excitation current,
ADC, reference
resistor
Excitation current,
amplifier, ADC
Response time
Slow
Fast
Fast
Slow
Computational
complexity (best
possible accuracy)
High
Very high
Very high
Medium
Although they are more expensive than other sensor types, RTDs have the best accuracy over a wide temperature
range.
RTDs are primarily made of platinum, which gives them good linearity and repeatability. Commonly used RTDs
include PT100, PT500, and PT1000. PT stands for platinum, and the number (100/500/1000) indicates the resistance
value at 0 °C. This application note focuses on measurements based on PT100.
PSoC 3, PSoC 4, and PSoC 5LP can fully integrate the hardware required for RTD temperature measurement. Not
only can they integrate the hardware, but PSoC 3 and PSoC 5LP measure RTDs accurately and with high resolution
due to the 20-bit delta-sigma ADC available on those devices.
In addition to the measurement front end you have all of the other resources of the PSoC 3, PSoC 4 and PSoC 5LP
available to complete the rest of your product. PSoC 3, PSoC 4 and PSoC 5LP are attractive devices for reducing
BOM cost and component count in your RTD designs, while maintaining the accuracy and resolution you need.
1.1
Using this Document
This document describes the theory behind temperature measurement with an RTD. If you are looking for code
examples using PSoC to measure RTD temperature there are three associated with this Application Note, links to
these code examples can be found in the Related Code Examples section.
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PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
2
RTD – Theory of Operation
An RTD has a positive temperature coefficient (PTC); resistance increases as temperature increases.
The resistance-temperature relationship is not perfectly linear. Various standards approximate this non-linearity; of
them, IEC 60751 is the most widely used. Equation 1 and Equation 2 define the resistance to temperature
relationship in IEC 60751.
Above 0 °C, RTD temperature is specified by the RTD resistance at 0 °C (R0) and constants A and B.
Equation 1
RT = R0 (1 + AT + BT 2 ) , T > 0
Where, RT is the resistance at T °C
Below 0 °C, in addition to the A and B, a third constant (C) is involved, as shown in Equation 2.
Equation 2
RT = R0 (1 + AT + BT 2 + C (T − 100)T 3 ), T < 0
Equation 1 and Equation 2 are referred to as Callendar–Van Dusen equations, and the A, B, and C coefficients are
known as Callendar–Van Dusen coefficients. The term is named after the scientists who developed the equations.
The values of A, B, and C for a PT100 RTD are specified in IEC 60751 for standard industry-grade platinum:
A = 3.9083 *10 −3 °C −1
B = −5.775 *10 −7 °C −2
C = −4.183 *10 −12 °C −4
The resistance at 0 °C,
R0 = 100 Ω
(PT100 RTD)
Plotting resistance versus temperature yields a nearly linear curve, as Figure 1 shows. 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
450
400
Resistnace in Ω
350
RTD
Curve
Linear
300
250
200
150
100
50
0
-200
0
200
400
Temperature in °C
600
800
RTD temperature measurement involves the following two steps which are described in the consecutive sections:
1.
Measure the RTD resistance accurately
2.
Convert the measured resistance to temperature.
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PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
3
RTD Resistance Measurement Method
The following techniques are commonly used to measure resistance and their effectiveness for RTDs:



3.1
Two-Wire Measurement
Three-Wire Measurement
Four-Wire Measurement
Two-Wire Measurement
In a two-wire measurement, a current is passed through the RTD and the voltage across the RTD is measured, as
Figure 2 shows.
Figure 2. Two-Wire RTD Temperature Measurement
Rw1
Is
VRTD
RRTD
Rw2
The linear approximation shown in Figure 1 is made by assuming the RTD changes by 0.385Ω/°C. A 1-ohm error in
the resistance measurement leads to a temperature error of about 2.6 °C (1/.385). Therefore, the RTD resistance
must be determined with an accuracy of <0.0385 Ω for the temperature error to be <0.1 °C.
Because of the length of the wire connected in series with the RTD, the RTD wire resistances (Rw1 and Rw2) are
added to the RTD resistance. The wire resistances contribute to an error in the final RTD temperature measurement,
as Equation 3 shows.
Equation 3 VRTD = IS * (RRTD + RW1 + RW 2 )
Rw1 and Rw2 vary based on the length of the wire. For very short wires, they could be in the milliohms; for very long
wires, 10s of ohms.
One way to minimize the effect of wire resistance is to use the three-wire method described in Three-Wire
Measurement.
3.2
Three-Wire Measurement
With a three-wire measurement, the error due to the wire resistances is eliminated by measuring the wire resistances
and subtracting it from the measured RTD resistance (see Equation 4 through Equation 6).
In Figure 3, the switch is set to position 1 and the voltage across the RTD, VRTD, is measured. Then, the switch is set
to position 2, and the voltage across the wire resistance, Vwire, is measured.
Equation 4 VRTD = Is * (RRTD + Rw1 + Rw3)
Equation 5 Vwire = Is * (Rw2 + Rw3)
If Rw1 = Rw2,
Equation 6 RRTD = (VRTD – Vwire) / Is
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PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
Figure 3. Three-Wire RTD Temperature Measurement
Rw1
1
2
Is
VRTD
Rw2
RRTD
Vwire
Rw3
Although this method is better than the two-wire measurement method, the three-wire method provides accurate
results only if Rw1 = Rw2. To eliminate all wire resistance errors, use the four-wire measurement described in
Four-Wire Measurement.
3.3
Four-Wire Measurement
The four-wire measurement shown in Figure 4 greatly reduces any error caused by wire resistances.
Figure 4. Four-Wire RTD Temperature Measurement
Rw1
Rw2
IS
VRTD
RRTD
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 is 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.
Equation 7
RRTD =
VRTD
I RTD
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.
3.3.1
Reference Resistor Method
The reference resistor makes the measurement error independent of both the current source accuracy, and the ADC
accuracy. The measurement error depends primarily on a reference resistance.
Figure 5 shows the schematic of this method. 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.
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PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
Equation 8
RRTD =
VRTD
* Rref
Vref
Figure 5. RTD Temperature Measurement
IDAC
0
Rref, 100 ohm
0.1%
Rw1
ADC
Rw3
RRTD
1
Rw4
Rw2
PSoC
The current path is shown by the blue line, and voltage measurement paths are shown in red. 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. The following sections discuss how to choose a reference resistor, and how the offset
error and gain error of the ADC and IDAC become null.
Reference Resistor Selection
When choosing a reference resistor there are several rules of thumb to keep in mind.
1.
Choose a small reference resistor that doesn’t load the IDAC. Figure 5 uses a reference resistance of 100 ohms.
PSoC IDACs have a compliance voltage of VDDA – 1 V. This means that the voltage at the IDAC cannot exceed
VDDA – 1 V.
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 is produced
(1 mA * 400 ohms + 100 ohms + 400 ohms). This is well below the compliance voltage. For more information on
PSoC internal routing resistance, consult AN58827 - PSoC® 3 and PSoC 5LP Internal Analog Routing
Considerations.
2.
Choose a reference resistor that uses the same ADC range as the RTD. In PSoC devices is it possible to have
the ADC measure multiple input ranges, for example +/- 1.024V, or +/-VDDA. Using the same range reduces the
time it takes to measure the RTD, because the ADC does not need to be reconfigured.
If the maximum RTD resistance is 400 ohms, and 1 mA is passed through it, the RTD produces a voltage of
400 mV. The delta-sigma ADC on PSoC 3 and PSoC 5LP 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 must reconfigure the ADC every
sample; reconfiguration takes time.
3.
Choose a reference resistance similar to the resistance of the RTD at the temperature you are measuring. For
example, if the RTD is measuring around 100 °C then pick a reference resistor with the same resistance as the
RTD at 100 °C.
If the reference resistor and RTD are in the same part of the ADC transfer function, non-linearities in the ADC are
canceled out. If you are measuring a wide range of temperatures, keep the reference resistor near the middle of
the range.
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PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
IDAC Current Selection
Another important factor is what current to pass through the RTD. There are two issues to consider.
1.
The more current passed through the RTD and reference resistor, the more of the ADCs range can be used. For
example, on PSoC4 the ADC has a range of +/-1.024 V. As stated earlier the max RTD resistance is ~400Ω thus
to fill the whole range we need 1.024 V / 400 Ω = 2.56 mA.
PSoC 3 and PSoC 5LP have an ADC input range of +/- 512 mV. Thus a smaller current of 0.512 V / 400 Ω =
1.28 mA can be used. Using more of the ADCs range leads to higher temperature resolution.
2.
The more current passed through the RTD the more power it dissipates. This can lead to self-heating which
leads to measurement error. Thus the IDAC current should be kept low. For more information on self-heating see
RTD Self-Heating Error.
Designers of RTD systems need to make tradeoffs between resolution and self-heating error.
One common approach to avoid self-heating is to duty cycle the IDAC current. When not measuring, turn the
IDAC off or disconnect ground. When measuring, turn it back on. In PSoC devices, the IDAC can quickly be
turned off or you can connect the bottom of the RTD to a GPIO pin; set that pin to High-Z when not measuring,
and set it to Strong Drive Low when measuring.
Offset Error Cancellation
In PSoC devices, the ADC offset and signal chain offset can easily be removed through correlated double sampling
(CDS). In CDS the offset is measured, and then in firmware it is subtracted from the other voltage measurements.
®
See AN66444 - PSoC 3 and PSoC 5LP Correlated Double Sampling for details.
For RTD Temperature measurement, the best way to measure system offset is set the IDAC to source 0 mA and
measure the voltage directly across the RTD or Reference Resistor.
With CDS, the equation for resistance measurement becomes:
Equation 9
RRTD =
VRTD − V0
* Rref
Vref − V0
Where, VRTD is the voltage measured across the RTD
Vref is the voltage measured across the 100-Ω 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 is nulled by the difference in Equation 9.
VRTD = (I+I0) * RRTD + ADC offset
V0 = I0 * RRTD + ADC offset
Subtracting V0 from VRTD removes the 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.
Equation 10
RRTD
=
k * k ' * (VRTD − V0 )
* Rref
k * k ' * (Vref − V0 )
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.
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PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
Using the reference resistor method, you can determine the RTD resistance accurately. The next step is to convert
the RTD resistance to temperature, as described in RTD – Resistance-to-Temperature Conversion.
4
RTD – Resistance-to-Temperature Conversion
Once the resistance of the RTD is known, you must then convert it to temperature. The straightforward method to
obtain temperature from resistance is to use the Callendar–Van 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.
4.1
Positive Temperatures
Solving Equation 1 for T,
 R
− A + A 2 − 4 B1 − T
 R0
Equation 11 T =
2B



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
[1]
as the order of the polynomial increases. Using a fifth-order polynomial can reduce the conversion error to
<0.002 °C, but higher-order polynomials require more CPU cycles for execution.
4.2
Negative Temperatures
Solving Equation 2 for T is not straightforward, because it is a fourth-order equation. Again, approximate the
temperature-resistance relationship using a polynomial. Using a fourth-order polynomial reduces the conversion error
to < 0.002 °C.
A single polynomial can apply to both negative and positive temperatures. However, doing so requires a polynomial
order greater than 10 to reduce the conversion error to <0.002 °C. Therefore, use two polynomials, for positive and
negative temperatures.
4.3
Choosing the Right Polynomial Order
The temperature range and accuracy determine the order of the polynomial. As discussed, a fifth-order polynomial
can reduce the error to <0.002 °C in the -200 °C to 850 °C range, but it requires more CPU cycles for computation.
Table 2 provides the number of cycles required for temperature computation and the temperature error resulting from
polynomials of different orders for the -200 °C to 850 °C range.
Table 2. Accuracy and Number of Cycles Required for Computation in Using Polynomials of Different Orders
Number of Cycles
Required for Computation*
Accuracy in °C
(-200 °C to 850 °C Range)**
Accuracy in °C
(-50 °C to 150 °C Range)**
First order
70
< 20.2
<0.55
Second order
110
< 1.7
<0.007
Third order
150
< 0.17
<0.0001
Fourth order
190
< 0.018
0
Fifth order
230
< 0.002
0
Polynomial Order
1
A fifth-order polynomial for temperature in terms of resistance is in the form T = a5R5 + a4R4 + a3R3 + a2R2 + a1R + a0, where a5,
a4,…a0 are constants determined by curve-fitting techniques.
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PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
Note: (*) The number of cycles required for computation is calculated from the PSoC 5LP-based code in the associated code example. The project
uses floating-point arithmetic to compute temperature. You can greatly reduce the number of cycles by using special algorithms for floating-point
multiplications.
(**) This only includes the accuracy of the resistance to temperature conversion, not the whole system accuracy
If your temperature range or accuracy requirements are lower, you can use a lower-order polynomial and decrease
the number of cycles required for temperature computation. If your range is just -50 °C to 150 °C, you can use a
second-order polynomial to achieve a temperature error of <0.007 °C.
Choosing and creating the appropriate polynomial is math-intensive and time-consuming. Cypress simplifies this task
by providing an RTD Component in PSoC Creator which creates a polynomial of the required order based on your
temperature range and the accuracy required.
The Component automatically calculates the required order of the polynomial and polynomial coefficients. For
resistance-to-temperature conversion, the Component provides an API which uses the computed polynomial
coefficients to find the temperature.
4.4
RTD Component
Figure 6 shows the RTD component. The RTD component supports PT100, PT500, and PT1000 RTDs.
Figure 6. RTD Component
Double-clicking the component yields the configuration dialog box shown in Figure 7.
Figure 7. RTD Component Customizer
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PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
Select the RTD type, enter the maximum and minimum temperatures to which the RTD will be subjected, and select
the calculation error budget. The configuration dialog returns an appropriate polynomial order for the chosen error
budget, the temperature calculation error versus temperature graph, the maximum error for the chosen temperature
range, and the number of cycles required for computation.
For example, if you are using a PT100 RTD, your range is -100 °C to 400 °C, and you require 0.05 °C temperature
calculation 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, call the API function RTD_GetTemperature(int32 res) to calculate temperature. The parameter, int32
th
res, is resistance in milliohms, and the return value is temperature in 1/100 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 the Temperature Accuracy section.
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PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
5
RTD Temperature Measurement with PSoC
CE210383 contains several projects demonstrating how to measure RTDs with PSoC 3, PSoC 4, and PSoC 5LP.
Please refer to CE210383 for details on how those projects work. This section briefly describes how to configure a
PSoC device to measure an RTD Temperature.
Figure 9 shows a typical PSoC Creator schematic for a PSoC 3 or PSoC 5LP RTD temperature measurement
project.
Figure 9. PSoC Creator Top Design Schematic for RTD Temperature Measurement
Notice how similar this schematic is to Figure 5. PSoC 3 and PSoC 5LP devices contain the required IDAC and ADC
for RTD Temperature measurement.
PSoC 3 and PSoC 5LP are best suited for RTD Temperature measurement as they both have a high-precision 20-bit
delta-sigma ADC with good linearity.
PSoC 4 is capable of measuring an RTD as shown in Figure 10. However, PSoC 4 has only a 12-bit successive
approximation register (SAR) ADC. A 12-bit ADC reduces the achievable resolution of the RTD Temperature
measurement to around 1 °C, and greatly reduces the accuracy when measuring RTDs. If a wide temperature range
is not required it is recommended that thermistors be used for RTD temperature measurement with PSoC 4. See
AN66477 for more information.
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PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
Figure 10. PSoC Creator Top Design Schematic for PSoC 4 RTD Temperature measurement
One thing to note about PSoC 4 is that maximum current that the IDAC can produce is 612 µA. As mentioned
previously, ~2.5 mA is needed to fill the entire ADC range. Thus the current needs to be increased. This can be
accomplished through a simple current multiplier; two resistors are added to the opamp to increase the current by 3x.
To increase the current by 4x change the 30-Ω resistor to 40 Ω.
Cypress has created a special kit for temperature sensing: the PSoC Precision Analog Temperature Sensor EBK
(CY8CKIT-025). The kit provides four sensors—thermocouple, thermistor, RTD, and diode—for measuring
temperature. In addition, connectors are provided to 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 11 shows the kit. For more details on the kit, go to www.cypress.com/CY8CKIT-025.
Figure 11. PSoC Precision Analog Temperature Sensor EBK
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PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
6
Interfacing Multiple RTDs
The easiest way to interface to multiple RTDs is to wire them in series with one another. However, make sure you do
not 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.
7
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
measure temperature without too much loss of accuracy. Appendix A describes the broken RTD reconfiguration in
detail. A project (Broken RTD reconfiguration) is also found in CE210435.
8
8.1
Performance Measures
Temperature Resolution
The temperature resolution depends on three factors:



The temperature range to be measured
The IDAC current
The ADC resolution
The temperature range being measured is important because it determines the smallest change in resistance per
change in temperature. As stated previously, the linear approximation for an RTD is 0.385 Ω/°C. However this
approximation is not perfect at the extreme ends of the temperature range.
The first step in determining temperature resolution is to determine what the smallest change in Ω/°C is for your
application. This is best done by creating a lookup table of resistance vs temperature in a spreadsheet for your RTD
and temperature range. Use Equation 1 and Equation 2 to construct your table.
For example, if the temperature range is -200 °C to 850 °C, the difference between 850 °C and 849 °C is 0.2926Ω.
The difference between -199 °C and -200 °C is 0.4322Ω. In general the smallest Ω/°C occurs at higher temperatures.
Next, take the minimum Ω/°C and multiply it by the current sourced by the IDAC. For this example 1 mA is used. This
means that we need to resolve 0.001 A * 0.2926 Ω = 292.6 µV to resolve 1 °C. From this number we can assume that
to resolve 0.1 °C we need a voltage resolution of 29.26 µV, and to resolve 0.01 °C we need a voltage resolution of
2.926 µV.
Next, we need to determine the entire voltage range for the temperature range being measured. For the temperature
range is -200 °C to 850 °C the resistances are 18.52Ω to 390.77Ω respectively. Multiply this range by the IDAC
current to get the voltage range. Multiplying this range by 1 mA yields 18.52mV to 390.77mV, the ADC must be able
to measure this range.
For PSoC 3 and PSoC 5LP the delta-sigma ADC has a maximum resolution of 20 bits over a voltage range of +/0.512 V. To determine the voltage resolution, take the voltage range divided by the resolution; 1.024V/(2^20) =
0.976 µV. This shows the PSoC 3 and PSoC 5LP are able to resolve less than 0.01 °C across the entire temperature
range of the RTD.
For PSoC 4 the SAR ADC has a maximum resolution of 12 bits over a voltage range of +/-1.024V. The voltage
resolution is 500 µV. This means that with PSoC 4 the resolution is around 2.5 °C over the entire range.
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PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
8.1.1
Increasing Resolution
There are several methods to increase the temperature resolution.
1.
Increase the current sourced by the IDAC. Increasing the IDAC current increases the minimum voltage required
to resolve 1 °C. For example if the IDAC current is increased to 3mA then the minimum Ω/°C becomes 877.8 µV.
The voltage range increases to 0.55 V to 1.17 V. Now a PSoC 4 is able to resolve down to 1 °C. However, 3mA
can lead to higher self-heating which decreases accuracy.
2.
ADC resolution can be increased by oversampling. This is a common industry practice where multiple ADC
samples are used to create a higher resolution result.
3.
8.1.2

In order to increase the resolution by 1 bit, four ADC samples need to be summed, and then the result right
shifted by 1 (divided by 2).

To increase the resolution by 2 extra bits, 4^2 ADC samples need to be summed and then the result right
shifted by 2.

To increase the resolution by 3 extra bits, 4^3 ADC samples need to be summed and the result shifted right by
3.
The resolution can be extended to any number of extra bits. The tradeoff is conversion speed – the more extra
bits required, the more samples required for each conversion, and the slower the conversion.
Smaller Temperature Range. This allows for more IDAC current, and may increase the minimum Ω/°C if not
measuring very high temperatures.
Noise
Another important part of temperature resolution is the noise present in the acquisition system. If the noise of the
system is too large, it may reduce the resolution of the system.
The PSoC 3 and PSoC 5LP delta-sigma ADC has a specified RMS noise that is approximately 1 count in the +/512 mV range. That is noise of ~0.976 µV, which is well below the resolution required to resolve 0.01 °C.
To reduce the effect of noise, you can use a digital IIR filter. This helps reduce any noise flicker in the final result. For
more information on digital IIR filters, refer AN2099, Single-Pole IIR filter.
8.2
Temperature Accuracy
You can calculate the temperature accuracy by summing all possible individual errors, which fit into one of two
categories:
8.2.1
1.
Error due to the measurement system
2.
Error due to the RTD
E r r o r D u e t o t h e M e a s u r e m e n t S ys t e m
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.
RRTD =
VRTD
* Rref
Vref
VRTD is the voltage measured across the RTD
Vref is the voltage across the 100-Ω 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.
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PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
Error Due to ADC Integral Non-Linearity
The integral non-linearity (INL) of an ADC at any point is the difference between the ideal ADC count and the actual
ADC count.
The delta-sigma ADC has a maximum INL of ±32 LSb in the +/-1.024 V range; 32 LSb corresponds to 64 µV for
20-bit resolution and ±1.024-V range. The ADC INL has the same analog value, 64uV, in ±0.512 V range.
Let us calculate the error due to INL at 850 °C for IDAC current of 1 mA.
VRTD = I * RRTD = 1* 390.481 = 390.481mV (The RTD resistance at 850 °C = 390.481, using Equation 1)
Vref = I * Rref = 1 * 100 = 100mV
Using Equation 8:
Resistance error =
390.481
390.481 + 0.064
*100 −
*100 = 0.3141Ω
100
100 − 0.064
At worst case, the INL causes a measurement error of 0.3141Ω at 850 °C. This corresponds to an error of about
1 °C.
Note that we have taken the worst-case INL across PVT and substituted worst-case positive INL at the numerator
and worst case negative in the denominator. In practical application, the error due to INL is much lower, and most
likely in the same direction. For example, for an INL of +8 LSb at the numerator and +8 LSb at the denominator, the
temperature error at 850 °C is ~0.1 °C.
The PSoC 4 SAR ADC has an INL of ~+/- 1.7 LSB in the 1.024V range at 12 bits. This corresponds to an error of
~850 µV. Plugging this into the equations above yields.
Resistance error =
390.481
390.481 + 0.850
*100 −
*100 = 4.2Ω
100
100 − .850
Which corresponds to an error of ~12 °C.
Again this assumes worst case numbers. If we use more realistic numbers such as 0.8 LSB and in the same direction
the error is ~1.23Ω, which is still an error of 4-5 °C.
So as can be seen PSoC 3 and PSoC 5LP have very good resolution and accuracy for RTD temperature
measurement. PSoC 4 is capable of measuring an RTD however its accuracy and resolution is not good for high
precision applications. If RTD temperature measurement is required for PSoC 4 a thermistor is recommended.
Error Due to Reference Resistance Tolerance
In Equation 8, we substituted 100 Ω 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.
Vref = I * Rref
Substituting the value of Rref with the tolerance and temperature coefficient yields Equation 11.
Equation 12
Vref = I * 100(1 + 0.001 + 0.00001 * ( sysTemp − 25)
The measured value of the RTD resistance, Rmeas, is:
Equation 13
www.cypress.com
Rmeas =
RRTD
(1 + 0.001 + 0.00001 * ( sysTemp − 25))
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PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
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.
Figure 12 shows the temperature error due to the reference resistor error at different RTD temperatures and ambient
temperatures. Ambient temperature 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 12. 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 <1.3 °C for an ambient temperature of
25 °C and RTD temperatures from -200 °C to 850 °C.
The error contributed by the reference resistor can be calibrated out using two methods. For the first method,
measure the value of the reference resistor at 25 °C in the factory using an accurate ohmmeter, and then store it in
the PSoC EEPROM. Use this value (not 100 Ω) for Rref in Equation 8.
For the second method, replace the RTD with a known precision resistor. Measure the value of the resistor. Compute
the ratio of this measured resistance against the actual resistance. Store this ratio in flash. Multiply all subsequent
resistances by this ratio. This method is described in more detail in RTD Calibration.
By calibrating out the initial tolerance of the reference resistor, the remaining error is only due to the temperature
coefficient (see Figure 13).
Figure 13. Temperature Error Due to Reference Resistor Temperature Tolerance
Temperature error in °C
-50
-25
0
50
75
100
125
Temperature in °C
The temperature coefficient of the reference resistor still causes an error if the ambient temperature is much higher or
lower than 25 °C. The ambient temperature can be measured using the on-chip temperature sensor in PSoC 3 /
PSoC 5LP, and the error due to the temperature coefficient of the reference resistor can be corrected.
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PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
For this method, the reference resistor needs to be measured at manufacturing at several temperatures. This
approach will not be explained any further.
8.2.2 RTD Error
This is divided into two factors:
1.
RTD interchangeability error
2.
RTD self-heating error
RTD Interchangeability Error
This error is caused by replacing one RTD with another one of the same part number (process variation). This is
defined in RTD datasheet. IEC 60751 defines two primary classes of RTD tolerances, as shown in Table 3.
Table 3. RTD Tolerance Classes
Tolerance Class
Tolerance °C)
A
0.15 + 0.002 |t|
B
0.30 + 0.005 |t|
At no point does the temperature error of a class B RTD exceed the value 0.3 + 0.005 |t|, Where |t| = Absolute value
of temperature in °C.
Figure 14 shows the graph of RTD tolerance. The errors at 25 °C and 800 °C due to class B RTD are calculated
below.
Figure 14. 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 must be calibrated. For a high-performance RTD (0.1% or 1 °C), calibration is
required.
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PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
RTD Calibration
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 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 Equation 14.
Equation 14
scale =
Ractual
Rmeas
6.
Store the scale error in EEPROM.
7.
Multiply all measured RTD resistances by this scale.
If an accurate and stable temperature cannot be achieved, replace the RTD with a known resistor. Using a known
resistor calibrates out only the error due to the reference resistor. It does 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 is also calibrated. The scale error value can be stored in
PSoC EEPROM and retrieved each time the device goes through a power cycle.
CE210434 demonstrates how this type of calibration is done. Figure 14 and Table 3 show that the RTD error has an
offset and a gain. Thus, it may become necessary to perform a two-point temperature 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 method is not demonstrated.
RTD Self-Heating 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 RTD on the CY8CKIT-025, the value is specified at <= 0.8 °C/mW.
At 150 °C, the RTD resistance is 157.325 Ω. If 1-mA current is passed, the power dissipation is 390 µW or 0.39 mW.
This corresponds to RTD self-heating of <0.13 °C. Note that the power dissipation constant of SMD RTDs are
generally higher (about 1 °C/mW).
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18
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
8.3
List of all Errors
Table 4 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 4. Possible Errors in RTD Temperature Measurement at 150 °C
Error Value at 150 °C (0.1%
Reference Resistor, class B RTD)
Error Source
Error Value at 150 °C (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
<0.13 °C
Error due to reference resistor
(Ambient Temperature = 25 °C)
0.43 °C
Limited only by calibration accuracy and
reference resistor temperature coefficient(very
accurate)
Error due to reference resistor
(Ambient Temperature = 80 °C)
0.6 °C
~0.2 °C unless temperature calibration is
completed
Sensor Error
Error due to RTD interchangeability
(Class B RTD)
1.05 °C
Limited only by calibration accuracy (very
accurate)
Polynomial fit error (fifth-order
polynomial)
0.0003 °C
0.0003 °C
Note(*): worst-case INL is used at the numerator. Typically, the error is < 0.1 °C, Also this is only for PSoC 3 and
PSoC 5LP.
8.4
Test Results
An RTD was simulated by a potentiometer and the signal chain accuracy was tested in the whole temperature range
of the RTD. A potentiometer was connected in the external RTD slot of CY8CKIT-025 and the temperature shown in
the LCD was noted. The potentiometer resistance was measured by a precision multimeter and the resistance value
was noted. The resistance was converted into temperature manually using a fifth-order polynomial. The test results
shown in Table 5 indicate that the RTD signal chain is highly accurate.
Table 5. PSoC 5LP RTD Temperature Measurement Test Results
Resistance Value (Ω)
Expected Temperature (°C)
Measured Temperature (°C)
Error (°C)
27.285
−179.6
−179.6
0.0
32.47
−167.3
−167.4
0.1
66.58
−84.3
−84.4
0.1
80.21
−50.2
−50.3
0.1
118.296
47.1
47
0.1
149.464
129.0
128.9
0.1
218.374
317.9
317.8
0.1
325.314
636.3
636.3
0.0
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Document No. 001-70698 Rev. *I
19
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
9
Summary
When high accuracy is critical in measuring temperatures, an RTD is the sensor of choice. PSoC 3 and PSoC 5LP
have the necessary hardware integrated in the device to achieve high accuracy. The PSoC Creator RTD Component
makes designing with RTDs easier.
10
Related Resources
10.1
Related Application Notes
®
AN2099 - PSoC 1, PSoC 3, PSoC 4, and PSoC 5LP - Single-Pole Infinite Impulse Response (IIR) Filters
®
AN58827 - PSoC 3 and PSoC 5LP Internal Analog Routing Considerations
®
AN60590 - PSoC 3, PSoC 4, and PSoC 5LP – Temperature Measurement with a Diode
AN65977 - PSoC 3 and PSoC 5LP - Creating an Interface to a TMP05/TMP06 Digital Temperature Sensor
®
AN66477 - PSoC 3, PSoC 4, and PSoC 5LP - Temperature Measurement with a Thermistor
AN66444 - PSoC 3 and PSoC 5LP Correlated Double Sampling to Reduce Offset, Drift, and Low Frequency Noise
®
AN75511 - PSoC 3 / PSoC 5LP - Temperature Measurement with a Thermocouple
10.2
Related Code Examples
CE210383 - PSoC 3, PSoC 4, and PSoC 5LP Temperature Sensing with an RTD
CE210434 - PSoC 3 and PSoC 5LP RTD Calibration
CE210435 - PSoC 3 and PSoC 5LP Broken RTD Reconfiguration
About the Author
Name:
Praveen Sekar
Title:
Applications Engineer
Background:
Praveen holds a Bachelor’s degree in Electronics and Communication from the College of
Engineering, Guindy, Chennai. He focuses on analog modules in PSoC
Name:
Todd Dust
Title:
Applications Engineer Sr. Staff
Background:
Todd holds a Bachelor’s degree in Electrical Engineering from Seattle Pacific University.
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Document No. 001-70698 Rev. *I
20
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
A
A.1
Appendix A
Broken RTD Reconfiguration
If one of the four wires of the RTD breaks, PSoC 3 and PSoC 5LP can automatically detect a broken wire,
reconfigure a four-wire RTD connection to a three-wire connection, and measure temperature with minimal
degradation to accuracy.
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 measure temperature.
The PSoC Creator project (Broken RTD Reconfiguration) found in CE210435 demonstrates this behavior. It is only
available with PSoC 3 and PSoC 5LP.
A.1.1
Detecting Broken RTD Wire
Figure 15 shows a four-wire RTD connection to PSoC. 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 15. Four-Wire RTD Connected to PSoC
IDAC
Pin 3_1
Rw1
R1
Pin 4_0
Rw2
ADC +-
R2
RTD
Pin 4_1
Rw3
R3
PSoC
Kit025
External 4-wire
RTD
Rw4
A broken wire can be detected by using PSoC’s GPIO structure. PSoC GPIO can be configured to source VDDIO
through a pull up resistor while simultaneously sensing the pin state through its digital input buffer, as Figure 16
shows.
Figure 16. Pin Configured in Resistive Pull-Up Mode and Digital Input Mode
PSoC
VDDIO
> 3.5k
Pin
State
Digital Input
Buffer
To analog
global
To analog
Mux Bus
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PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
To detect the broken RTD wire connected to pin 3_1, configure pin 3_1 to the resistive pull-up mode, and the pin
state is sensed back see Figure 17.
When wires 1 and 4 are not broken, the RTD resistance forms a resistor divider with the internal pull-up resistor. The
voltage across the RTD is sensed back as the pin state.
The pull-up resistance has a minimum value of 3.5 kΩ, and the RTD can have a maximum resistance of 390 Ω (at
850 °C). Assuming wire resistances (Rw1 and Rw4) = 5 Ω each, we get a maximum value of 400 Ω.
Figure 17. Detecting Wires 1 and 4 for Breakage
PSoC
VDDIO
> 3.5k
Pin
State
Pin 3_1
Rw1
Digital Input
Buffer
To analog
global
Rw2
To analog
Mux Bus
Rw3
RTD
External 4-wire
RTD
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Rw4
22
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
Voltage sensed by pin = Voltage across RTD+Rw1+Rw4
= VDDIO *
RRTD + R w1 + R w 2
RRTD + R w1 + R w 2 + R p
= VDDIO *
400
3900
< 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 is low. When either RTD wire 1 or 4 is broken, the pin state is 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 Table 6.
Table 6: Wire State
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
0
1
Wires 1 and 3 broken
1
1
0
Wires 1 and 2 broken
1
1
1
Wire 4 broken
The 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.
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PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
10.2.1 Reconfiguring Four-Wire RTD to Three-Wire
After detecting the broken RTD wire, we must reconfigure the four-wire RTD to three-wire RTD, eliminating the
broken wire. The flexible analog routing structure of PSoC 3 and PSoC 5LP makes the reconfiguration easy to do.
The reconfiguration routes are: RTD Wire 1 Broken, RTD Wire 2 Broken, RTD Wire 3 Broken, and RTD Wire 4
Broken.
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 18.
Figure 18. RTD Wire 1 Broken
IDAC
Pin 3_1
Rw1
R1
Pin 4_0
Rw2
ADC +-
R2
RTD
Pin 4_1
Rw3
R3
PSoC
Kit025
External 4-wire
RTD
Rw4
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 the one-time wire resistance computation section below.
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 19.
Figure 19. RTD Wire 2 Broken
IDAC
Pin 3_1
Rw1
R1
Pin 4_0
Rw2
ADC +-
R2
RTD
Pin 4_1
Rw3
R3
PSoC
Kit025
External 4-wire
RTD
Rw4
In this case, the ADC measures RTD wire resistance 1 in addition to the RTD resistance.
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PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
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 20.
Figure 20. RTD Wire 3 Broken
IDAC
Pin 3_1
Rw1
R1
Pin 4_0
Rw2
R2
ADC +-
RTD
Pin 4_1
Rw3
R3
PSoC
External 4-wire
RTD
Kit025
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. Onetime 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 21.
Figure 21. RTD Wire 4 Broken
IDAC
Pin 3_1
Rw1
R1
Pin 4_0
Rw2
ADC +-
R2
RTD
Pin 4_1
Rw3
R4
R3
PSoC
Kit025
External 4-wire
RTD
Rw4
In this case, the ADC measures RTD wire resistance 3 in addition to the RTD resistance.
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PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
10.2.2 One-Time Wire Resistance Computation
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 3 °C error in measured temperature. To eliminate the error
due to the wire resistances, perform one-time wire resistance computation before making the RTD measurements.
The following steps are used to compute the wire resistances:
1.
Configure the RTD in four-wire mode as shown in Figure 15 and find the RTD resistance (R0)
2.
Configure the RTD in three-wire mode (wire 1 broken) as shown in Figure 18 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 18, CompRes1 should be subtracted from the measured resistance.
4.
Configure the RTD in three-wire mode (wire 2 broken) as shown in Figure 19 and calculate the resistance (R2).
5.
Compute additional wire resistance, CompRes2 = (R2 – R0). When wire 2 breaks and the RTD is reconfigured as
shown in Figure 19, CompRes2 should be subtracted from the measured resistance.
6.
Configure the RTD in three-wire mode (wire 3 broken) as shown in Figure 20 and calculate the resistance (R3)
7.
Compute additional wire resistance, CompRes3 = (R3 – R0). When wire 3 breaks and the RTD is reconfigured
shown in Figure 20, CompRes3 should be subtracted from the measured resistance.
8.
Configure the RTD in three-wire mode (wire 4 broken) as shown in Figure 21 and calculate the resistance (R4)
9.
Compute additional wire resistance, CompRes4 = (R4 – R0). When wire 4 breaks and the RTD is reconfigured as
shown in Figure 21, CompRes4 should be subtracted from the measured resistance.
CE210435 provides an example of how all of this is done.
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Document No. 001-70698 Rev. *I
26
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
Document History
®
Document Title: AN70698 - PSoC 3, PSoC 4, 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
01/12/2012
MEH Review Feedback in CDT#116240
*B
3520653
PFZ
02/08/2012
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.
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.
*E
3818484
PFZ
11/21/2012
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 9)
Updated Appendix B (Updated Broken RTD reconfiguration (Updated Project
Description (Updated Figure 25))).
Updated Replaced PSoC 5 with PSoC 5LP in all instances across the document.
*F
4057734
TDU
07/11/2013
*G
4152296
TDU
10/09/2013
*H
4202789
TDU
11/26/2013
*I
5075960
TDU
01/07/2016
Added two projects and discussed different performance ranges.
Updated attached Associated Project.
Completing Sunset Review.
Fixed formatting errors.
Moved Example Projects to CEs.
Updated to latest template.
Added PSoC 4.
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Document No. 001-70698 Rev. *I
27
PSoC® 3, PSoC 4, and PSoC 5LP – Temperature Measurement with an RTD
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Document No. 001-70698 Rev. *I
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