Texas Instruments | Measuring an RTD Sensor with the TDC1000 and TDC7200 for Ultrasonic Sensing | Application notes | Texas Instruments Measuring an RTD Sensor with the TDC1000 and TDC7200 for Ultrasonic Sensing Application notes

Texas Instruments Measuring an RTD Sensor with the TDC1000 and TDC7200 for Ultrasonic Sensing Application notes
Application Report
SNAA230 – April 2015
Measuring an RTD Sensor with the TDC1000 and TDC7200
for Ultrasonic Sensing
Bahram Mirshab
ABSTRACT
This application note describes the firmware procedure for measuring temperature via two RTD's using
the TDC1000 and TDC7200. Temperature is monitored in heat meters and flow meters.
1
2
3
4
5
6
Contents
Objective ......................................................................................................................
Background ...................................................................................................................
TDC1000 Embedded RTD Interface Circuit ..............................................................................
Temperature Measurement with Multiple RTDs .........................................................................
RTD1 Temperature Measurement .........................................................................................
Software Solution ............................................................................................................
2
2
2
3
3
3
List of Figures
1
Temperature Sensor Interface ............................................................................................. 2
2
Timing Sequence for Temperature Measurements
3
4
.....................................................................
Timing Sequence for RTD1 ................................................................................................
Timing Sequence for RTD2 ................................................................................................
3
3
5
List of Tables
1
RTD1 TDC Clock Counts ................................................................................................... 4
2
RTD1 Conversion Results
3
RTD2 TDC Clock Counts ................................................................................................... 6
4
RTD2 Conversion Results
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.................................................................................................
Measuring an RTD Sensor with the TDC1000 and TDC7200 for Ultrasonic
Sensing
Copyright © 2015, Texas Instruments Incorporated
4
6
1
Objective
1
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Objective
The objective of this application note is to describe a firmware method for monitoring two RTD's for a heat
meter application.
2
Background
Resistance Temperature Detectors (RTD), measure temperature by relating the resistance of the RTD
element with temperature. Typically, an RTD consists of a length of fine coiled wire wrapped around a
ceramic or glass core, placed inside a protective housing. The resistance of the element of the RTD is
provided at various temperatures. The element acts as a temperature senor because with the change of
temperature, the material changes resistance in a predictable manner.
RTDs are characterized by a linear positive change in resistance with respect to temperature. They exhibit
the most linear signal with respect to temperature of any electronic sensing device. Platinum is the most
widely specified RTD element type due to its wide temperature range, accuracy, stability, as well as the
degree of standardization among manufacturers. Nickel, copper, and nickel-iron alloys are also used.
RTDs are often characterized by their base resistance at 0°C. Typical base resistance values available for
platinum thin-film RTDs include 100 Ω, 500 Ω and 1000 Ω. For other element types, typical base values
include 120 Ω for nickel, and 1000 Ω and 2000 Ω for nickel-iron.
3
TDC1000 Embedded RTD Interface Circuit
TDC1000’s embedded interface block supports two external RTD sensors as shown in Figure 1.
Figure 1. Temperature Sensor Interface
The temperature sensor block supports PT1000 or PT500 sensors. The System requires a temperaturestable external reference resistor (R REF). If the RTD type is PT500, then RREF should be 500 Ω. In case
a PT1000 sensor is used, the RREF should be 1000 Ω. The reference resistor needs to have a low
temperature coefficient.
2
Measuring an RTD Sensor with the TDC1000 and TDC7200 for Ultrasonic
Sensing
Copyright © 2015, Texas Instruments Incorporated
SNAA230 – April 2015
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Temperature Measurement with Multiple RTDs
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4
Temperature Measurement with Multiple RTDs
The temperature sensor measurement can be performed without the need of an external ADC. The
temperature sensor block operates by converting the resistance of a reference, RREF, and up to two
RTDs into a series of START and STOP pulses. The interval between the pulses is proportional to the
measured resistance, and therefore, the temperature. As shown in Figure 2, the TDC1000 performs three
measurements per trigger event and generates the corresponding pulses on the START and STOP pins.
Figure 2. Timing Sequence for Temperature Measurements
5
RTD1 Temperature Measurement
In the temperature measurement mode only, short duration pulses can occur after the 2nd and 4th Stop
pulses. These pulses can be detected by TDC7200 and result in invalid temperature measurement.
6
Software Solution
The solution to eliminate the effect of the short duration pulses is summarized as follows:
6.1
Solution for Measuring REF Resistor and RTD1
•
•
•
•
Measure START to STOP1 for REF resistor value
Measure START to STOP2, STOP3 (if it exists), and STOP4
Discard START to STOP3 if too close in time to STOP2
Subtract START to STOP2 from START to STOP4 for RTD1 value
Figure 3. Timing Sequence for RTD1
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Software Solution
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Table 1. RTD1 TDC Clock Counts
Time 1
CLKcount1
Time2
CLCKcount2
Time3
CLKcount3
Time 4
CLKcount4
Time 5
CLKCount5
Time6
CAL1
CAL2
Example RTD1
calculation
based on Data1
2320
1611
298
2768
807
2822
2210
4120
1581
5224
1127
2269
22699
Example RTD1
calculation
based on Data2
1544
1612
2342
2768
1543
4119
191
5224
1242
6485
1493
2268
22695
Table 2. RTD1 Conversion Results
4
calCount
normLSB
Start-Stop1(ns)
Start-Stop2 (ns)
Start-Stop3 (ns)
Start-Stop4 (ns)
Start-Stop5 (ns)
stop2
to
stop3 (ns)
stop2
to
stop4 (ns)
RTD1 (°C)
Remarks
2270
0.055066079
201486.3436
346083.315
352756.0573
515040.6938
653065.6938
6672.742291
168957.3789
50.05718539
2269.666667
0.055074167
201456.0508
346000.0551
514949.5153
653016.6324
2.808782494
168949.4603
307016.5773
50.02509938
Note:
If (stop2 to stop3
(ns) < 20000), use
(stop2 to stop4 (ns)).
Else: use (stop2 to
stop3 (ns)) in RTD1
calculation.
Measuring an RTD Sensor with the TDC1000 and TDC7200 for Ultrasonic
Sensing
Copyright © 2015, Texas Instruments Incorporated
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Software Solution
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6.2
RTD2 Temperature Measurement
This procedure deviates from the steps to measure RTD1 because the TDC7200 can only measure
START to STOP for the first 5 STOP pulses (including short duration pulses).
6.2.1
•
•
•
•
Solution for Measuring REF Resistor and RTD2
Use the blanking feature of the TDC7200 to skip measuring the first three STOP pulses. Use
information from first START to STOP2 to set the appropriate amount of blanking. A STOP mask
period of 400 us (program stop mask register to 0x0C80 for 8 MHz clock) is used in the EVM GUI
software.
Next measure START to STOP1, STOP2, STOP3 (if exists) and Stop 4
Discard START to STOP3 if too close in time to STOP2
Subtract START to STOP2 from START to STOP4 for RTD value
Figure 4. Timing Sequence for RTD2
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Software Solution
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Table 3. RTD2 TDC Clock Counts
Example RTD2
calculation
based on one
shot TOF
Measurement
Result
Time 1
CLKcount1
Time2
CLKcount2
Time3
CLKcount3
Time4
CLKcount4
Time5
CLKcount5
Time6
CAL1
CAL2
606
4120
1980
5224
219
5278
1585
6485
819
0
0
2268
22696
Table 4. RTD2 Conversion Results
calCount
2269.777778
6
normLSB
0.0550071471
Start-Stop1 (ns)
514924.3318
Start-Stop2 (ns)
653021.3127
Start_stop3 (ns)
659696.085
Start_Stop4 (ns)
810613.2698
Start-Stop5 (ns)
0
Measuring an RTD Sensor with the TDC1000 and TDC7200 for Ultrasonic
Sensing
Copyright © 2015, Texas Instruments Incorporated
stop2
to
stop3 (ns)
6674.772371
stop2
to
stop4 (ns)
157591.9571
RTD1 (°C)
REMARKS
72.36831479
Note:
If (stop2 to stop3
(ns) < 20000), use
(stop2 to stop4 (ns)).
Else: use (stop2 to
stop3 (ns)) in RTD2
calculation.
SNAA230 – April 2015
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