Analog Front End (AFE) for Sensing

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Analog Front End (AFE) for Sensing Temperature in Smart
Grid Applications Using RTD
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TIDA-00110
Design Page
ADS1248
TPS7A1633
TCA6408A
TS3A5017D
CSD17571Q2
ADS1148
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WEBENCH® Calculator Tools
Based on the ADS1248 24-Bit Delta-Sigma (ΔΣ)
ADC With Internal PGA and Selectable Gain
up to 128
Can Measure 2-, 3-, or 4-Wire RTD Inputs
Meets Requirements for Smart Grid Applications
Uses Ratiometric Measurement for Higher
Accuracy
Matched Current DACs for RTD Excitation
Multiplexer (Analog Switch) to Switch Excitation
Currents for Four RTD Inputs
Accuracy < ±2°C Without Calibration for Pt100
I2C I/O Expander for ADC Interface Control and
Excitation Current Switching Provided (No External
I/Os Required)
Featured Applications
•
•
•
Protection Relays
RTD Extension Modules for Protection Relay
Remote Terminal Units
Ch1
Ch2
Ch3
Analog
Switch
TS3A5
017D
Ch4
IEXC1_Ch1
RTD
I
E
X
C
1
I
E
X
C
2
Control Lines
I2C
I2C I/O
Expander
TCA6408A
Vrtd_Ch1
Vrtd_Ch1
IEXC2_Ch1
Vrtd_Ch4
IEXC1_Ch2
Control
Lines
AIN1
AIN2
2 LEDs: RTD
channel
being
sampled
AIN3
24 bit û ADC
ADS1248
Vrtd_Ch2
IEXC2_Ch2
AIN0
Vrtd_Ch2
AIN4
Vrtd_Ch3
AIN6
SPI, I2C
Interface
Connector
AIN5
SPI
AIN7
IEXC1_Ch3
Vrtd_Ch3
IEXC2_Ch3
IEXC1_Ch4
VDC
VDC
LDO
TPS7A1633DGNR
Power Supply
Connector
GND
+3.3V
Vrtd_Ch4
IEXC2_Ch4
An IMPORTANT NOTICE at the end of this TI reference design addresses authorized use, intellectual property matters and other
important disclaimers and information.
All trademarks are the property of their respective owners.
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Analog Front End (AFE) for Sensing Temperature in Smart Grid Applications
Using RTD
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1
System Description
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1
System Description
1.1
Resistance Temperature Detector
Temperature is one of the oldest known physical quantities. Temperature is the most essential factor that
needs continuous measurement and monitoring in smart grid. Today, the industry demands accurate,
repeatable, and reliable measurement of temperature, because temperature can have a significant impact
on product cost, quality, efficiency, and safety.
Temperature sensors types include:
• Resistance temperature detector (RTD)
• Thermistor
• Thermocouple
Figure 1. Comparison of Different Temperature Sensors Used for Smart Grid Applications
The focus of this design is to measure temperature using RTD, a sensing element whose resistance
changes with the temperature. The relationship between the resistance and temperature of an RTD is
highly predictable, which allows accurate and repeatable temperature measurement over a wide range.
2
Analog Front End (AFE) for Sensing Temperature in Smart Grid Applications
Using RTD
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1.1.1
RTD Measurement
The basic principle of RTD measurement is based on the Ohm’s law equation:
V
R=
I
where
•
•
•
R = Resistance of the RTD element
I = Known excitation current
V = Voltage across RTD element
(1)
RTDs require constant current source for its excitation to produce a voltage output proportional to the
resistance of the RTD. The resulting voltage output is measured by the analog-to-digital converter (ADC).
The RTD voltage is amplified based on the requirement. Based on the measured voltage, the RTD
resistance or temperature is calculated. Depending on the RTD type, different excitation currents can be
used. The RTDs are available in different lead wire configurations: 2-, 3-, and 4-wire.
1.1.2
Ratiometric Measurement
A ratiometric approach guarantees more effective number of bits (ENOBs) as the noise in the IDAC
reflects in the reference and as well as in the input and hence tends to cancel off. The effect of the IDAC
current temperature drift also gets canceled off in this ratiometric topology.
ADC requires a reference voltage to convert the input voltage into a digital output. In most applications,
this reference is fixed and generated either internal or external to the ADC. The voltage reference has
direct influence on the accuracy of output. If the measurement can be configured such that the ADC result
is a ratio of the input and a precision element such as a resistor, then much higher precision results can
be obtained. In ratiometric configuration, the excitation current that flows through the RTD returns to
ground through a low-side reference resistor, RREF. The voltage developed across RREF is fed into the
positive and negative reference pins (REFP and REFN) of the ADC and ADS1248 is configured to use this
external reference voltage VREF for the analog-to-digital conversions. Select RREF as a low-tolerance, lowdrift resistor for accurate results.
The voltage drop across the RTD and RREF resistors is produced by the same excitation source and the
ADC output code is a relationship between the input voltage and the reference voltage. Therefore, errors
as a result of the absolute accuracy of the excitation current and the errors because of excitation drift are
virtually eliminated. In addition, the noise of the excitation source at the inputs is also reflected on the
reference path of the ADC and, in this manner, cancels the noise. Therefore, the system becomes
immune to variations in the excitation.
I1
VIN+
RRTD
VDIFF
VIN-
ΔΣ
ADC
PGA
REFP
REFN
RREF
Figure 2. Simplified Circuit for RTD Ratiometric Measurement
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System Description
ADC _ Output = ADC _ Output = ADC _ Output = www.ti.com
VIN ´ GAIN ´ (223 - 1)
VREF
IDAC ´ RTD ´ GAIN ´ (2
IDAC ´ RREF
RTD ´ GAIN ´ (2
RREF
23
(2)
23
- 1)
(3)
- 1)
(4)
IDAC1
RLEAD
AIN2
AIN1
Input
Mux
RLEAD
RTD
Main ADC
RLEAD
COM
RLEAD
Int Ref
Advantages:
+ Most accurate implementation
+ RLEAD compensation
+ RLEAD does not need to match
+ No drift mismatch between references
REFP
R REF
REFN
Reference MUX
Disadvantages/Error Sources:
- Precision R REF required
Figure 3. Ratiometric 4-Wire Operation
4
Analog Front End (AFE) for Sensing Temperature in Smart Grid Applications
Using RTD
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1.1.3
Connecting 2-, 3-, and 4-Wire RTD Inputs
This module is compatible with 2-, 3-, and 4-wire RTD inputs. The connection diagrams for connecting
them to the module are shown in Figure 4. The user just needs to connect jumper wires externally as
indicated by the red-colored wires. This arrangement does not call for any change in the hardware on the
module and is quite useful when user can access only the interface connectors.
Two-wire RTD
RTD
Three-wire RTD
RTD
Four-wire RTD
RTD
Figure 4. Different RTD Input Connections
Table 1. RTDs Used in Smart Grid
RTD TYPE
TEMPERATURE COEFFICIENT OF RESISTANCE (TCR) / °C
100-Ω platinum
0.00385
250-Ω platinum
0.00385
100-Ω nickel
0.00618
120-Ω nickel
0.00618
10-Ω copper
0.00427
Table 2 shows the resistance versus temperature for different types of RTDs.
Table 2. RTD Resistance versus Temperature
TEMPERATURE (°C)
RTD TYPE
Pt100
Ni100
Ni120
Cu10
300
212.02
—
439.44
—
200
175.84
223.20
303.46
16.78
100
138.50
161.80
200.64
12.90
90
134.70
154.90
191.64
12.51
80
130.89
148.30
182.84
12.12
70
127.07
141.70
174.25
11.74
60
123.24
135.30
165.90
11.35
50
119.40
129.10
157.74
10.97
40
115.54
123.00
149.79
10.58
30
11.67
117.10
142.06
10.19
20
107.79
11.20
134.52
9.81
10
103.90
105.60
127.17
9.42
0
100.00
100.00
120.00
9.04
–10
96.09
94.60
113.00
8.65
–20
92.16
89.30
106.15
8.26
–30
88.22
84.10
99.41
7.88
–40
84.27
79.10
92.76
7.49
–50
80.31
—
86.17
7.10
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System Description
1.2
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Protection Relay and Need for Temperature Sensing
Smart grid consists of the following sections:
1. Generation
2. Transmission
3. Distribution
Transmission Lines
500, 345, 230, and 138 kV
Subtransmission Customer
26 kV and 69 kV
Substation
Step-Down
Transformer
Primary Customer
13 kV and 4 kV
Generating Station
Generator Step Up
Transformer
Transmission Customer
138 kV or 230 kV
Subtransmission Customer
120 V and 240 V
Generation
Transmission
Distribution
Figure 5. Smart Grid — Generation to Distribution
A typical smart grid system consists of generators for power, step-up transformers for transmission, stepdown transformers for distribution, and loads consisting mainly of motors. The voltage and the power
levels across the grid is very high, and any electrical faults on the system can lose a huge amount of
capacity and revenue. To ensure the systems are protected against different electrical faults, use
protection relays at different stages of the transmission system, such as
• Generator protection
• Transformer protection
• Distance protection
• Feeder protection
• Motor protection
• Bus bar protection
The basic purpose of a protection relay is to protect the grid in the event of a malfunction by monitoring
the current and voltage on specific lines on the grid. The inputs into a protection relay are typically
currents and voltages from a sensor on the line plus any communication from other related equipment on
the grid network. The output would be signals to a circuit breaker (to turn open or close) and
communication to the grid network. In case the protection relay detects a fault, the delay commands a
breaker to open the line where the fault is detected, which protects everything down the line from the
protection relay. The accurate measurement of the voltage, current, or other parameter like temperature
pressure or vibration of a power system is a prerequisite to any form of control, ranging from automatic
closed-loop control to the recording of data for statistical purposes. Measuring these parameters can be
accomplished in a variety of ways, including the use of direct-reading instruments and electrical measuring
transducers.
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Analog Front End (AFE) for Sensing Temperature in Smart Grid Applications
Using RTD
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Protections relays measure the following parameters and based on the set threshold they protect:
1. Currents
2. Voltages
3. Temperature
4. Power direction
Most protection relays monitor temperature of the systems they protect.
Generator or Motor Protection
Generators are designed to run at a high load factor for a large number of years and permit certain
incidences of abnormal working conditions. The machine and its auxiliaries are supervised by monitoring
devices to keep the incidences of abnormal working conditions down to a minimum. Despite the
monitoring, electrical and mechanical faults can occur, and the generators must be provided with
protective relays, which, in case of a fault, quickly disconnect the machine from the system and, if
necessary, completely shut down the machine. Thermal overload protection is one such protection. For
motor protection, the relay monitors temperature of the following: motor winding, motor bearing, load
bearing, and auxiliary winding.
Transformer Protection
Transformers are a critical and expensive component of the power system. Due to the long lead time for
repair of and replacement of transformers, a major goal of transformer protection is to limit the damage to
a faulted transformer. Temperature-based protection can aid this goal by identifying operating conditions
that may cause transformer failure. Transformer protection relay monitors temperature of primary or
secondary winding hot-spots, the oil at the bottom and top of the transformer, and the ambient air. An
RTD input can also be used as a direct resistance measuring input for position tracking of an on-load tap
changer.
The number of sensors depends on the size of the motor, generator, and transformer. Protection relays
provide a certain number of RTDs. Many applications may need to monitor more RTDs and multiple
motors, generators, or transformers using one protection relay. An RTD expansion module is used along
with the protection relay to sense the temperature inputs, compute the temperatures and communicate the
temperature values to the relay for protection. Different types of RTD can be used based on the
applications. The accuracy of measurement for different sensors is expected to be the same and high. An
accurate ADC is required to measure the temperature. An ADC with internal PGA ensures multiple types
of RTD connection. A current source is required to excite the RTDs for measured. If the current source is
integrated with the ADC, the complexity of design reduces and ensured better accuracy.
Some of the protections required in each segment are:
• For power generation: Generator protection, breaker protection, and transformer protections
• For transmission: Transformer protection, line voltage differential protection, and line distance
protections
• For distribution: Transformer protection, motor protection, air circuit breakers, and molded case circuit
breakers
Since a number of RTDs are connected to one expansion module or protection relay the conversion time
of the ADC is important. The temperature is a slow varying signal, so the number of samples to be
measured per second will be less. It is preferred that all the RTD inputs are samples at least once a
second in a module that has 12 RTDs. Higher resolution ADCs with PGA, matched current source and
radiometric measurement techniques, are used to improve accuracy.
TI has a large portfolio of ΔΣ ADCs that suits the requirements for RTD measurements. Additional to the
resolution, TI ΔΣ ADCs have a high level of integration including current source, PGA, and reference. The
ADCs consume a low amount of power.
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System Description
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Other advantages of ΔΣ ADCs include:
• Better noise performance for DC applications
• High resolution
• No active anti-aliasing filter required
• Good for "slow" signals
• Lower cost
• Lower power
• Small size
• Integration with:
– PGA
– Current sources
– Sensor burn out detection
– Temperature sensor
This design focuses on the following:
• Using TI ΔΣ ADCs for measuring temperature using RTD
• Measuring four RTD inputs
• Multiplexing current source to measure four RTD inputs
• Using internal PGA to achieve higher accuracy
• Using SPI to configure and read data from the ADC
• Using I2C I/O expander for /CS, START, /DRDY, excitation current selection, and LED indications
NOTE: This design can be used inside a protection relay or in expansion modules. For safety, the
user may need to isolate the RTD measurement sub system from the main processing
system.
When there is a need for isolation, this TI design can be interfaced with the TI Design
TIDA-00300. The TIDA-00300 provides isolation for SPI, I2C, and power inputs. The
interface connectors are screw-type connectors enabling the boards to connect easily.
All the relevant design files such as schematics, BOM, layer plots, Altium files, firmware, and Gerber have
also been provided to the user in Section 8.
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Using RTD
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Design Specifications
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2
Design Specifications
Typical requirements for TIDA-00110 are:
Table 3. System Specifications for TIDA-00110
PARAMETERS
SPECIFICATIONS AND FEATURES
Temperature sensing range
–50°C to 250°C
Measurement accuracy
< ±2°C
ADC resolution and type
24-bit, ΔΣ ADC with differential input
ADC interface for digital data
SPI compatible
RTD sensor type
2-, 3-, and 4-wire inputs
Number of RTD inputs
Four (4)
Current sources and excitation current range
Excitation current selection
Using dual single-pole quadruple-throw (4:1) analog switch
Multiplexer (analog switch) selection control
Display of measured values
Resistance measurement method
DC input voltage
Dual-matched current source with a current range programmable in
defined steps in the range of 50 µA to 1.5 mA
Using an I2C I/O expander
GUI
Ratiometric
4 to 6 V
ADC power supply
Indication
3.3 V
LED indications for RTD input being sampled
Interface connectors
4-pin screw-type terminal block for each RTD input
4-pin screw-type terminal block for input power supply
8-pin screw-type terminal block for SPI and I2C interface
NOTE: For cost sensitive applications and applications that do not require wide temperature
measurement, the ADS1148 16-bit ΔΣ ADC can be used. The ADS1148 is pin and footprint
compatible with the ADS1248. Modify the firmware accordingly to use the ADS1148.
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Block Diagram
3
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Block Diagram
Ch1
Ch2
Ch3
Analog
Switch
TS3A5
017D
Ch4
I
E
X
C
1
I
E
X
C
2
IEXC1_Ch1
Control Lines
I2C
I2C I/O
Expander
TCA6408A
Vrtd_Ch1
RTD
Vrtd_Ch1
IEXC2_Ch1
Vrtd_Ch4
IEXC1_Ch2
Control
Lines
AIN1
AIN2
2 LEDs: RTD
channel
being
sampled
AIN3
24 bit û ADC
ADS1248
Vrtd_Ch2
IEXC2_Ch2
AIN0
Vrtd_Ch2
AIN4
Vrtd_Ch3
AIN6
SPI, I2C
Interface
Connector
AIN5
SPI
AIN7
IEXC1_Ch3
Vrtd_Ch3
IEXC2_Ch3
IEXC1_Ch4
VDC
VDC
LDO
TPS7A1633DGNR
+3.3V
Power Supply
Connector
GND
Vrtd_Ch4
IEXC2_Ch4
Figure 6. System Block Diagram
3.1
ADC
This design demonstrates measurement of four RTDs using a single ADS1248 ADC. ADS1248 is a highlyintegrated, precision, 24-bit ADC.
ADS1248 has following features:
• Four differential inputs
• Matched current source for RTD excitation
• PGA with selectable gain up to 128
• Internal reference with provision to configure for external reference
• SPI for configuration and ADC samples reading
• /CS and START (conversion start) for control of sampling
• GPIOs for user usage
To communicate with the ADS1248, an SPI is provided on 8-pin screw-type terminal blocks. Four-pin
screw-type terminal blocks are available for connecting the RTD inputs.
3.2
Dual 4:1 Analog Switch
This design uses dual-matched current source. This current is switched between four RTDs. TS3A5017D
is used to switch excitation current between RTDs. The TS3A5017 is a dual single-pole quadruple-throw
(4:1) analog switch that operates from 2.3 to 3.6 V and can handle analog signals.
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Analog Front End (AFE) for Sensing Temperature in Smart Grid Applications
Using RTD
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Block Diagram
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3.3
I2C I/O Expander
I2C I/O expander is used for following:
• Switching of excitation current for RTD inputs
• For ADC control lines like /DRDY, START, /CS, /RESET
• To control LEDs (for visual indication)
This design uses TCA6408A, a low-voltage, 8-bit I2C I/O expander.
To communicate with the TCA6408A, the required I2C signals are extended to the 8-pin screw-type
terminal block.
3.4
Power Supply
This design requires a 3.3-V supply. TPS7A1633 is used to generate 3.3 V. The TPS7A1633 is an ultralow power, low-dropout (LDO) voltage regulator that offers the benefits of ultra-low quiescent current, high
input voltage, and a miniaturized, high thermal-performance packaging. A 4-pin screw-type terminal block
is provided to connect the external DC input.
3.5
LED Indicators
Two LEDs are provided to indicate the RTD input channel currently being scanned.
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Circuit Design
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4
Circuit Design
4.1
ADC
Figure 7 and Figure 8 display the ADS1248 features:
DVDD
1
28
SCLK
DGND
2
27
DIN
CLK
3
26
DOUT/DRDY
RESET
4
25
DRDY
REFP0/GPIO0
5
24
CS
REFN0/GPIO1
6
23
START
REFP1
7
22
AVDD
ADS1248
REFN1
8
21
AVSS
VREFOUT
9
20
IEXC1
VREFCOM
10
19
IEXC2
AIN0/IEXC
11
18
AIN3/IEXC/GPIO3
AIN1/IEXC
12
17
AIN2/IEXC/GPIO2
AIN4/IEXC/GPIO4
13
16
AIN7/IEXC/GPIO7
AIN5/IEXC/GPIO5
14
15
AIN6/IEXC/GPIO6
Figure 7. Pin Configuration of ADS1248
+3.3V
U4
1
C26
+3.3V
10µF
SGND
SGND
R18
10k
R41
C52
0.1µF
2
10k
3
4
ADC_Reset/
TP7
C25
0.1µF
TP6
SGND
TP2
RESET1
/INT1
R40
100
5
R39
100
6
7
RTD_REFP0
TP5
8
RTD_REFN0
9
C48
10µF
SGND
RTD_AINP1
RTD_AINN1
RTD_AINP2
RTD_AINN2
C50 10
0.1µF
11
12
13
14
DVDD
SCLK
DGND
DIN
CLK
DOUT/DRDY
RESET
DRDY
REFP0/GPIO0
CS
REFN0/GPIO1
START
REFP1
AVDD
REFN1
AVSS
VREFOUT
IOUT1
VREFCOM
IOUT2
AIN0/IEXC
AIN3/IEXC/GPIO3
AIN1/IEXC
AIN2/IEXC/GPIO2
AIN4/IEXC/GPIO4
AIN7/IEXC/GPIO7
AIN5/IEXC/GPIO5
AIN6/IEXC/GPIO6
28
27
26
25
24
R44
100
R43
100
R47
100
R42
100
23
SCLK
SDO
SDI
ADC_Rdy/
ADC_CS/
ADC_Start
R46
10k
SGND
R50
10k
+3.3V
22
+3.3V
21
20
19
18
17
16
15
IEXC1
C31
C29
10µF
0.1µF
IEXC2
RTD_AINN4
RTD_AINP4
RTD_AINN3
SGND
RTD_AINP3
ADS1248IPW
Figure 8. ADS1248 Pin Configuration
The four RTD inputs are connected to four differential inputs of the ADS1248.
The ADS1248 is a highly-integrated, precision, 24-bit ADC. The ADS1248 features an onboard, low-noise,
programmable gain amplifier (PGA), a precision ΔΣ ADC with a single-cycle settling digital filter, and an
internal oscillator. The ADS1248 also provides a built-in, very low-drift voltage reference with a 10-mA
output capacity, and two matched programmable current digital-to-analog converters (DACs). The
ADS1248 provides a complete front-end solution for temperature sensor applications including thermal
couples, thermistors, and RTDs.
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Analog Front End (AFE) for Sensing Temperature in Smart Grid Applications
Using RTD
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An input multiplexer supports four differential inputs for the ADS1248. In addition, the multiplexer has a
sensor burnout detect, system monitoring, and general-purpose digital I/Os. The onboard, low-noise PGA
provides selectable gains of 1 to 128. The ΔΣ modulator and adjustable digital filter settle in only one
cycle, for fast channel cycling when using the input multiplexer, and support data rates up to 2 kSPS.
Internal reference with provision to configure for external reference is available in ADS1248.
The voltage reference for the ADS1248 is the differential voltage between REFP and REFN:
VREF = VREFP – VREFN
For the ADS1248, there is a multiplexer that selects the reference inputs. The reference input uses a
buffer to increase the input impedance as with the analog inputs, REFP0 and REFN0 can be configured
as digital I/Os on the ADS1248. This design uses external reference.
The ADS1248 is rated over the extended specified temperature range of –40°C to 105°C.
Some of the highlighted features of ADS1248 are:
• 24 bits, no missing codes
• Data output rates up to 2 kSPS
• Single-cycle settling for all data rates
• Four differential or seven single-ended inputs
• Low-noise PGA: 48 nV at PGA = 128
• Matched current source DACs
• Very low drift internal voltage reference: 10 ppm/°C (max)
• Sensor burnout detection
• Eight general-purpose I/Os
• Internal temperature sensor
• Power supply and VREF monitoring
• Self and system calibration
• SPI compatible
• Analog supply: unipolar (2.7 to 5.25 V)
• Digital supply: 2.7 to 5.25 V
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Circuit Design
4.1.1
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3-Wire RTD Calculations
The ADS1248 integrates all necessary features (such as dual-matched programmable current sources,
buffered reference inputs, PGA, and so forth) to ease the implementation of ratiometric 2-, 3-, and 4-wire
RTD measurements. The 3-wire RTD configuration is most commonly used for industrial temperature
sensors. Figure 9 shows a typical implementation of a ratiometric 3-wire RTD measurement.
RBIAS
Line 3
VREF1P
MUX
Line 2
RTD
Line 1
VREF1N
AVSS
IDAC
Current
ADS1247/48
AIN0
PGA
AIN1
24-Bit
DS ADC
Digital
Filter
1:128
Serial
Interface
and
Control
SCLK
DIN
DOUT
DRDY
CS
START
RESET
Line Resistance
IDAC
Current
CLK
Figure 9. 3-Wire RTD Measurement Circuit Diagram
The ADS1248 features two IDAC current sources capable of outputting currents from 50 μA to 1.5 mA.
IDAC1 is routed to one of the excitation leads of the RTD while IDAC2 is routed to the second excitation
lead as shown in Figure 9 by appropriate setting of IDAC1 and IDAC2 in the firmware. Both currents have
the same value, which is programmable. The design of the ADS1248 ensures that both IDAC values are
closely matched, even across temperature.
14
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4.1.1.1
RREF and PGA Gain
The resistance of the Pt100 changes from 80.31 Ω at –50°C to 194.07 Ω at 250°C. The line resistance
RLEAD depends on the distance of the sensor from the measurement setup. Assuming RLEAD equals 5 Ω,
the positive resistance swing is from 100 to 194.07 Ω, which is about 90.07 Ω. The negative resistance
swing is from 100 to 80.31 Ω, which is about 19.69 Ω. The IDAC current must be 1 mA or less to minimize
the self-heating error. The IDAC current chosen here is 500 μA. Then, maximum and minimum input
voltages to the PGA are 194.07 Ω × 500 μA = 97.04 mV and 80.31 Ω × 500 μA = 40.15 mV, respectively.
The external reference resistor RREF serves two purposes firstly it decides the external reference voltage
for ADC and secondly, it also determines the input common mode voltage of the PGA. Set the common
mode voltage around mid-supply (AVDD – AVSS) / 2 = (3 V – 0 V) / 2 = 1.65 V. Therefore, the reference
voltage chosen here is 2 V, which also depends on easily available resistance value and excitation
current. The sum of both currents flows through a precision low-drift reference resistor, RREF. The voltage,
VREF, generated across the reference resistor is given in Equation 5:
VREF = (IDAC1 + IDAC2 ) ´ RREF
(5)
Because IDAC1 = IDAC2 = 500 μA:
VREF = 2 ´ IDAC1 ´ RREF
(6)
Solving for RREF:
VREF
2V
RREF = = = 2 KW
2 ´ IDAC1 2 ´ 500 µA
(7)
For the required gain:
VREF
2V
GAINPGA = = = 20.6
VINMAX
97.04mV
(8)
The nearest gain that can be programmed is 16.
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Common-Mode Voltage Compliance Check
The signal of an RTD is of a pseudo-differential nature, where the negative input must be biased at a
voltage other than 0 V and the positive input can then swing up to 97.04 mV above the negative input.
The allowed common-mode input voltage range is as highlighted in Figure 10 (taken from the ADS1248
datasheet [8]):
Figure 10. Common-Mode Input Range Equation
Assume that IDAC1 = IDAC2 and RL (RLEAD) = 5 Ω (depending on length of lead wires).
Calculating VCMI from the equations highlighted in Figure 10:
Placing AVSS = 0 V, VIN = 97.04 mV, Gain = 16, and AVDD = 3.3 V in the equations shown in Figure 10:
VCMI_MIN = 0.876 V and VCMI_MAX = 2.423 V
Now, the common-mode input voltage actually set by the design can be given as:
IDAC ´ RRTD
VCMI = (IDAC ´ RLEAD ) + + 2 ´ I DAC ´ (RLEAD + RREF )
2
(9)
Placing IDAC = 500 µA, RLEAD = 5 Ω, RRTD = 194.07 Ω, and RREF = 2 kΩ in Equation 9:
VCMI_MIN_APPLIED = 2.027 V
Placing IDAC = 500 μA, RLEAD = 5 Ω, RRTD = 194.07 Ω, and RREF = 2 kΩ in Equation 9:
VCMI_MAX_APPLIED = 2.056 V
Here, VCMI_MIN_APPLIED > VCMI_MIN and VCMI_MAX_APPLIED < VCMI_MAX
This value is well within the maximum allowed common-mode input voltage range.
16
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4.1.2
Noise Considerations and Input Filter
RTD voltage output signals are typically in millivolt range which makes them susceptible to noise. A firstorder differential and common-mode RC filter (RF1, RF2, CDIF1, CCM1, and CCM2) is placed on the ADC inputs,
as well as on the reference inputs (RF3, RF4, CDIF2, CCM3, CCM4) to eliminate high-frequency noise in RTD
measurements. For best performance, it is recommended to match the corner frequencies of the input and
reference filters. More detailed information on matching the input and reference filters can be found in
application report RTD Ratiometric Measurements and Filtering Using the ADS1148 and ADS1248.[2]
The differential filters chosen for this application were designed to have a –3-dB corner frequency at least
10 times larger than the bandwidth of ADC. The selected ADS1248 sampling rate of 20 SPS results in a
–3-dB bandwidth of 13.1 Hz. The cut off frequency chosen for this design is higher to account for faster
sampling rate. For proper operation, the differential cutoff frequencies of the reference and input low-pass
filters must be well matched. Matching the frequencies and filters can be difficult because as the
resistance of the RTD changes over the span of the measurement, the filter cutoff frequency changes as
well. To mitigate this effect, the two resistors used in the input filter (RI1 and RI2) were chosen to be more
than an order of magnitude larger than the RTD. Limiting the resistors to at most 20 kΩ to keep DC offset
errors low due to input bias current.
I1
CI_CM1
RI1
CI_DIFF
RRTD
ΔΣ
ADC
GAIN
RI2
RZERO
CI_CM2
REFN
REFP
I2
CR_CM1
CR_DIFF
RR1
CR_CM2
RR2
RREF
Figure 11. Common Mode and Differential Mode Filters on RTD Input and Reference
RI1 = RI2 = 4.12 kΩ and CI_DIFF = 0.047 μF
The –3-dB cutoff frequency of differential input filter at a 186-Ω RTD resistance (at mid-scale temperature)
can be calculated as given in Equation 10.
1
F-3dB _ I _ DIFF = 2 ´ p ´ CI _ DIFF ´ (RI1 + RRTD + RI2 )
F-3dB _ I _ DIFF » 402.1 Hz TIDU575 – December 2014
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To ensure that mismatch of the common-mode filtering capacitors is not translated to a differential voltage,
the common-mode capacitors (CI_CM1 and CI_CM2) were chosen to be 10 times smaller than the differential
capacitor. This results in a common-mode cutoff frequency that is roughly 10 times larger than the
differential filter, making the matching of the common-mode cutoff frequencies less critical.
CI_CM1 = CI_CM2 = 4700 pF
Although it is not always possible to exactly match the corner frequencies of all the filters, a good
compromise is to attempt to balance the corner frequencies of the input path differential filter and the
reference path differential filter because these filters have a dominant effect in the performance.
RR1 = RR2 = 4.7 kΩ and CR_DIFF = 0.033 μF
The –3-dB cutoff frequency of differential reference filter can be calculated as given in Equation 11:
1
F-3dB _ R _ DIFF = 2 ´ p ´ CR _ DIFF ´ (RR1 + RREF + RR2 )
F-3dB _ R _ DIFF » 405.83 Hz
(11)
To ensure that mismatch of the common-mode filtering capacitors is not translated to a differential voltage,
the common-mode capacitors (CR_CM1 and CR_CM2) were chosen to be 10 times smaller than the differential
capacitor. This results in a common-mode cutoff frequency that is roughly 10 times larger than the
differential filter, making the matching of the common-mode cutoff frequencies less critical.
CR_CM1 = CR_CM2 = 3300 pF
C35
+3.3V
RTD4_IEXC2
0.1µF
C36
RTD4_IEXC2
RTD4_IEXC1
R52
C
D25
DESD1P0RFW-7
2
C
21
R55
3
282834-4
4.12k
1
2
SGND
SGND
D30
C58
1000pF
P4SMA13CA
SGND1
2
C56
1000pF
21
1
C38
4700pF
1
D26
DESD1P0RFW-7
249
J6
1
2
3
4
21
R53
P4SMA13CA
C34
0.047µF
K
RTD_AINN4
D36
DFLS1200-7
D27
4.12k
A
RTD_AINN4
C57
1000pF
R54
3
A
249
SGND1
C37
4700pF
2
2
K
RTD_AINP4
1
RTD_AINP4
D35
DFLS1200-7
SGND
1
0.1µF
SGND
RTD4_IEXC1
D28
P4SMA13CA
SGND1
C11
+3.3V
0.1µF
C13
0.1µF
SGND
RTD_REFP0
R15
C
249
RATIO
D11
DESD1P0RFW-7
C10
3300pF
R10
3
RATIO
4.70k
R17
249
D9
DESD1P0RFW-7
C
C43
1000pF
R4
3
21
R3
2k
2
RTD_REFN0
D4
P4SMA13CA
4.70k
A
RTD_REFN0
C12
0.033µF
K
2
1
1
A
RTD_REFP0
K
2
SGND
1
R9
C9
3300pF
SGND
SGND1
0
SGND
SGND
SGND
Figure 12. Common-Mode and Differential-Mode Filters Implemented in Design for RTD
18
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Furthermore, before taking sensor measurement, the user must ensure that the external RC filters settle
down to ½ LSB after activating the excitation current sources. It may be ensured by implementing
software delay for several RC time constants. For 24-bit resolution measurement, after exciting the
sensor, the user must wait up to 17-RC filter time constants for consistent measurements.
Table 4. 4-Pin Terminal Block TH Connector for External RTD Input
RTD
CONNECTOR ON THE BOARD
RTD1
J1
RTD2
J2
RTD3
J4
RTD4
J6
Figure 13 shows the RTD connectors on the board.
J1
1
2
3
4
282834-4
RATIO
Figure 13. J1 Connector of RTD1
The ADS1248 has a simple SPI-compatible serial interface to communicate with the host. In this design,
the SPI is communicating at 2Mbps.
Figure 14 shows the 8-pin terminal block for the SPI and I2C interface.
J3
1
2
SCLK
3
SDO
4
SDI
5
6
7 SDA
8 SCL
TP8
SGND
SDA
R57
2.0k
+3.3V
SCL
1725711
R56
2.0k
Figure 14. J3 Connector for SPI and I2C Interface With External Devices
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Circuit Design
4.2
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Multiplexer
TS3A5017D is used to switch excitation current for all the RTDs. The TS3A5017 is a dual single-pole,
quadruple-throw (4:1) analog switch that is designed to operate from 2.3 to 3.6 V. This device can handle
both digital and analog signals, and signals up to V+ can be transmitted in either direction.
Logic
Control
Logic
Control
V+
1EN
1
IN2
2
15 2EN
1S4
3
14 IN1
1S3
4
13 2S4
1S2
5
12 2S3
1S1 6
11 2S2
1D
7
10 2S1
GND
8
9
2D
Figure 15. Pin Configuration View of TS3A5017D
Figure 16 shows excitation current multiplexing connections:
U3
R25
SGND
RTD_Sel1
RTD4_IEXC1
RTD3_IEXC1
RTD2_IEXC1
RTD1_IEXC1
IEXC1
R23
10k
1
100
2
R22
0
3
R21
0
4
R20
0
5
R19
0
6
R24
0
7
SGND
8
+3.3V
1EN
IN2
1S4
1S3
V+
2EN
IN1
2S4
1S2
2S3
1S1
2S2
1D
GND
2S1
2D
16
R38
15
10k
SGND
14
R37
100
13
R36
0
12
R35
0
11
R34
0
10
R33
0
9
R29
0
RTD_Sel0
RTD4_IEXC2
C20
10µF
C19
0.1µF
SGND
RTD3_IEXC2
RTD2_IEXC2
RTD1_IEXC2
IEXC2
TS3A5017D
Figure 16. Multiplexer Section
Some of the highlighted features of TS3A5017D are:
• Isolation in the powered-down mode, V+ = 0
• Low on-state resistance
• Low charge injection
• Excellent on-state resistance matching
• Low total harmonic distortion (THD)
• 2.3- to 3.6-V single-supply operation
• Latch-up performance exceeds 100 mA per JESD 78, Class II
20
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4.3
I2C I/O Expander
This design uses the TCA6408A, a low-voltage 8-bit I2C I/O expander.
VCCI
ADDR
RESET
P0
P1
P2
P3
GND
1
16
2
15
3
14
4
13
5
12
6
11
7
10
8
9
VCCP
SDA
SCL
INT
P7
P6
P5
P4
Figure 17. Pin Configuration of TCA6408A
The TCA6408A performs the following actions in this design:
• Controls switching of excitation current between four channels of RTD
• Communicates with ADC control lines /DRDY, START, /CS, and /RESET
• Controls LEDs (for visual indication)
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TP3
U2
SCL
SDA
SCL
SDA
+3.3V
14
15
2
R27
10k
R16
10k
/INT1
RESET1
R11
10k
SGND
RESET1
/INT1
+3.3V
13
3
1
16
SCL
SDA
ADDR
INT
RESET
P0
P1
P2
P3
P4
P5
P6
P7
VCCI
VCCP GND
4
5
6
ADC_Start
7
ADC_CS/
9
10
LED1
11
LED2
12
RTD_Sel0
RTD_Sel1
ADC_Rdy/
ADC_Start
ADC_CS/
ADC_Reset/
8
TCA6408APWR
C17
0.1µF
SGND
SGND
Figure 18. I2C I/O Expander
This 8-bit I/O expander for the I2C provides general-purpose remote I/O expansion through the I2C
interface [serial clock (SCL) and serial data (SDA)].
Some of the highlighted features of the TCA6408A are:
• Operating power-supply voltage range of 1.65 to 5.5 V
• I2C to parallel port expander
• Low standby current consumption of 1 μA
• Schmitt-trigger action allows slow input transition and better switching noise immunity at the SCL and
SDA inputs VHYS = 0.33 V typical at 3.3 V
• 5-V tolerant I/O ports
• Active-low reset (RESET) input
• Open-drain active-low interrupt (INT) output
• 400-kHz fast I2C bus
• I/O configuration register
• Polarity inversion register
• Internal power-on reset
• Power up with all channels configured as inputs
• No glitch on power up
• Noise filter on SCL/SDA inputs
• Latch-up performance exceeds 100 mA per JESD 78, Class II
• ESD protection exceeds JESD 22
– 2000-V human-body model (A114-A)
– 1000-V charged-device model (C101)
22
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4.4
Power Supply
The TPS7A1633 is an ultra-low power, LDO voltage regulator that offers the benefits of ultra-low
quiescent current, high input voltage and miniaturized, high thermal-performance packaging.
The TPS7A1633 is designed for continuous or sporadic (power backup) battery-powered applications
where ultra-low quiescent current is critical to extending system battery life.
The TPS7A1633 offers an enable pin (EN) compatible with standard CMOS logic and an integrated open
drain active-high power good output (PG) with a user-programmable delay. These pins are intended for
use in microcontroller-based, battery-powered applications where power-rail sequencing is required.
Not only can this device supply a well-regulated voltage rail, but it can also withstand and maintain
regulation during voltage transients. These features translate to simpler and more cost-effective, electrical
surge-protection circuitry
Table 5. Critical Parameters of TPS7A1633
PARAMETER
VALUE
Iout (Max) (A)
0.1
Output options
Fixed output
Vin (Min) (V)
3
Vin (Max) (V)
60
Fixed output options (V)
3.3
Vout (Min) (V)
3.3
Vout (Max) (V)
3.3
Iq (Typ) (mA)
0.005
Vdo (Typ) (mV)
60
Accuracy (%)
2
PSRR at 100 KHz (dB)
26
OUT
1
8
IN
FB/DNC
2
7
DELAY
PG
3
6
NC
GND
4
5
EN
Figure 19. Pin Configuration of TPS7A1633
Figure 20 shows the implementation of a 3.3-V power supply using the TPS7A1633 LDO.
TP1
+6V
TP4
TPS7A1633DGNR
U1
8
5
7
EN
OUT
DELAY
NC
DNC
3
L1
1
1000 OHM
6
2
R14
0
EP GND
R26
1.2k
C14
4
10µF
C8
C5
0.1µF 1000pF
PG
9
C4
+3.3V
IN
C16
4.7µF
10µF
C15
0.1µF
D14
PTZTE253.6B
3.8V
D13
Green SGND
SGND
SGND
SGND
SGND
SGND
SGND
Figure 20. TPS7A1633 Section of the TIDA-00110 Schematic
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Figure 21 shows the 4-pin terminal block for the power supply input.
+6V TP9
TP10
J5
1
2
3
4
282834-4
SGND
Figure 21. DC Input Connector
4.5
LED Indicators
The LEDs indicate the present status of the RTD channel that is beg scanned. The indication logic is as
shown in Table 6.
Table 6. LED Indicators
24
PRESENT SCANNING
LED1 (D8) STATUS
LED2 (D10) STATUS
Channel 1
Toggle
Toggle
Channel 2
OFF
ON
Channel 3
ON
OFF
Channel 4
ON
ON
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4.6
Tiva™ C Series LaunchPad™ Interface
The Tiva C Series LaunchPad (EK-TM4C123GXL) is a low-cost evaluation platform for ARM® Cortex™M4F-based microcontrollers. The Tiva C Series LaunchPad design highlights the TM4C123GH6PMI
microcontroller USB 2.0 device interface, hibernation module, and motion control pulse-width modulator
(MC PWM) module. The Tiva C Series LaunchPad also features programmable user buttons and an RGB
LED for custom applications. The stackable headers of the Tiva C Series LaunchPad BoosterPack™ XL
interface demonstrate how easy it is to expand the functionality of the Tiva C Series LaunchPad when
interfacing to other peripherals on many existing BoosterPack add-on boards as well as future products.
Figure 22 shows a photo of the Tiva C Series LaunchPad.
Power Select
Switch
USB Connector
(Power/ICDI) Green Power LED
Tiva
TM4C123GH6PMI
Microcontroller
USB Micro-A/-B
Connector
(Device)
Reset Switch
RGB User LED
Tiva C Series
LaunchPad
BoosterPack XL
Interface (J1, J2, J3,
and J4 Connectors)
Tiva C Series
LaunchPad
BoosterPack XL
Interface (J1, J2, J3,
and J4 Connectors)
Tiva
TM4C123GH6PMI
Microcontroller
MSP430
LaunchPad-Compatible
BoosterPack Interface
MSP430
LaunchPad-Compatible
BoosterPack Interface
User Switch 1
User Switch 2
Figure 22. Tiva C Series TM4C123G LaunchPad Evaluation Board
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Circuit Design
4.7
PCB Design Guidelines
•
•
•
•
•
26
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An SMD ceramic bypass capacitor of approximately 0.1 μF in value is recommended for all the digital
ICs. Should it be required to use leaded components, keep leads as short as possible to minimize lead
inductance.
A continuous ground plane is ideal for providing a low-impedance signal return path as well as
generating the lowest EMI signature by reducing phenomena such as unintended current loops.
Should a continuous ground plane not be possible, it is important to minimize the length of the trace
connecting VCC and ground.
PCB material: Standard Flame Retardant 4 (FR-4) epoxy-glass as printed-circuit board (PCB) material
is preferred for industrial applications with a speed < 100 Mhz. FR-4 meets the requirements of
Underwriters Laboratories UL94-V0 and is preferred over cheaper alternatives due to its lower
dielectric losses at high frequencies, less moisture absorption, greater strength and stiffness, and its
self-extinguishing, flammability characteristics.
Trace routing: Use 45° bends (chamfered corners), instead of right-angle (90°) bends. Right-angle
bends increase the effective trace width, and thus the trace impedance. This creates additional
impedance mismatch, which may lead to higher reflections.
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5
Software Description
For software description and code examples for TIDA-00110, please see TIDU575: Software Code
Examples for TIDA-00110.
6
Test Setup
RTD1
RTD4
SPI and I2C
GND
6V
Tools and equipment used to test ADC measurement accuracy:
• Yokogawa Model GS610 Source Measure Unit with accuracy: ±0.02% (DC voltage generation)
• Agilent 34401A 6½-Digit Multimeter for measuring resistance in four-wire method and measuring mV
• 0.01% tolerance high precision resistor to simulate RTD resistance
RTD2
RTD3
Figure 23. Test Setup for TIDA-00110
For Pt100 RTD, Table 7 shows resistance and respective voltage (mV) for different temperatures.
Table 7. Temperature versus Voltage Across RTD
TEMPERATURE (°C)
RESISTANCE (Ω)
EXPECTED VOLTAGE DROP WITH 500-µA EXCITATION CURRENT (mV)
–50
80.3068
40.15
0
100.0000
50.00
50
119.3951
59.70
100
138.5000
69.25
150
157.3149
78.66
200
175.8396
87.92
250
194.0743
97.04
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Test Results
7
Test Results
7.1
ADC Linearity
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To check the linearity of ADS1248, a DC mV input signal is applied using the Yokogawa Model GS610
Source. ADC bit counts (ADC_CODE) are read for RTD channels 1 to 4. ADC counts are converted to VIN
using Equation 12:
2 ´ VREF ´ (ADC _ CODE )DEC
VIN = 1 LSB ´ (ADC _ CODE )DEC = GAIN ´ 223 - 1
(
)
(12)
Use VREF = 2.048 V, GAIN = 16, and ADC_CODE = ADC readings for each RTD channel.
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Table 8. Channel 1: Linearity Performance
VAPPLIED (mV)
VCHANNEL1 (mV)
(WITHOUT GAIN MULTIPLICATION)
CHANNEL1ERROR
(AFTER GAIN MULTIPLICATION)
24.972
24.95766
0.07%
29.972
29.94252
0.03%
34.969
34.9265
0.01%
36.969
36.92699
0.01%
38.970
38.9221
0.01%
40.969
40.92225
0.01%
42.969
42.9146
0.00%
44.969
44.90884
–0.01%
46.968
46.91064
0.01%
48.967
48.90668
0.01%
50.970
50.90246
0.00%
52.970
52.89703
–0.01%
54.967
54.89291
–0.01%
56.968
56.88804
–0.01%
58.967
58.885
–0.01%
60.968
60.88195
–0.01%
62.967
62.87782
–0.01%
64.966
64.87613
–0.01%
69.965
69.86636
–0.01%
74.965
74.85669
–0.02%
79.964
79.85213
–0.01%
84.963
84.84324
–0.01%
89.963
89.83919
–0.01%
94.963
94.835
–0.01%
96.960
96.83171
0.00%
99.960
99.82369
–0.01%
102.962
102.8255
0.00%
105.962
105.8196
–0.01%
107.960
107.8193
0.00%
109.960
109.8188
0.00%
111.959
111.8131
0.00%
114.958
114.8095
0.00%
119.957
119.8032
0.00%
124.957
124.7988
0.00%
126.958
126.7952
0.00%
129.956
128.0000
–1.38%
NOTE: Applied gain multiplication factor is 1.00128.
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Table 9. Channel 2: Linearity Performance
VAPPLIED (mV)
VCHANNEL2 (mV)
(WITHOUT GAIN MULTIPLICATION)
CHANNEL2ERROR
(AFTER GAIN MULTIPLICATION)
24.972
24.95338
0.06%
29.972
29.93974
0.02%
34.969
34.92714
0.01%
36.969
36.92403
0.01%
38.970
38.91727
0.00%
40.969
40.91727
0.00%
42.969
42.91207
0.00%
44.969
44.90584
–0.01%
46.968
46.91152
0.01%
48.967
48.90452
0.00%
50.970
50.89978
–0.01%
52.970
52.89782
–0.01%
54.967
54.8893
–0.01%
56.968
56.88943
–0.01%
58.967
58.88457
–0.01%
60.968
60.88365
–0.01%
62.967
62.87493
–0.02%
64.966
64.87406
–0.01%
69.965
69.86466
–0.01%
74.965
74.85849
–0.01%
79.964
79.85144
–0.01%
84.963
84.8479
0.00%
89.963
89.83695
–0.01%
94.963
94.83193
–0.01%
96.960
96.82842
0.00%
99.960
99.82069
–0.01%
102.962
102.822
–0.01%
105.962
105.8159
–0.01%
107.960
107.8166
0.00%
109.960
109.8165
0.00%
111.959
111.8078
0.00%
114.958
114.8069
0.00%
119.957
119.8038
0.00%
124.957
124.7929
0.00%
126.958
126.7918
0.00%
129.956
128.0000
–1.38%
NOTE: Applied gain factor is 1.001310.
30
Analog Front End (AFE) for Sensing Temperature in Smart Grid Applications
Using RTD
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Table 10. Channel 3: Linearity Performance
VAPPLIED (mV)
VCHANNEL3 (mV)
(WITHOUT GAIN MULTIPLICATION)
CHANNEL3ERROR
(AFTER GAIN MULTIPLICATION)
24.972
24.95697
0.07%
29.972
29.94044
0.02%
34.969
34.92873
0.01%
36.969
36.92693
0.01%
38.970
38.92284
0.01%
40.969
40.91598
0.00%
42.969
42.91109
–0.01%
44.969
44.90875
–0.01%
46.968
46.90954
0.00%
48.967
48.9026
0.00%
50.970
50.9006
–0.01%
52.970
52.89588
–0.01%
54.967
54.89344
–0.01%
56.968
56.88652
–0.01%
58.967
58.88598
–0.01%
60.968
60.88221
–0.01%
62.967
62.87755
–0.01%
64.966
64.8734
–0.01%
69.965
69.86158
–0.02%
74.965
74.85734
–0.02%
79.964
79.8507
–0.01%
84.963
84.84557
–0.01%
89.963
89.8371
–0.01%
94.963
94.83109
–0.01%
96.960
96.83072
–0.01%
99.960
99.82227
–0.01%
102.962
102.8224
–0.01%
105.962
105.8192
–0.01%
107.960
107.8165
0.00%
109.960
109.8157
0.00%
111.959
111.8106
0.00%
114.958
114.8075
0.00%
119.957
119.8013
0.00%
124.957
124.7962
0.00%
126.958
126.7951
0.00%
129.956
128.0000
–1.38%
NOTE: Applied gain factor is 1.001284.
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Table 11. Channel 4: Linearity Performance
VAPPLIED (mV)
VCHANNEL4 (mV)
(WITHOUT GAIN MULTIPLICATION)
CHANNEL4ERROR
(AFTER GAIN MULTIPLICATION)
24.972
24.95606
0.07%
29.972
29.94122
0.03%
34.969
34.92886
0.01%
36.969
36.92489
0.01%
38.970
38.92012
0.00%
40.969
40.91657
0.00%
42.969
42.91369
0.00%
44.969
44.90719
–0.01%
46.968
46.9107
0.01%
48.967
48.90607
0.00%
50.970
50.90158
–0.01%
52.970
52.89598
–0.01%
54.967
54.89154
–0.01%
56.968
56.88658
–0.01%
58.967
58.88463
–0.01%
60.968
60.88013
–0.02%
62.967
62.87574
–0.02%
64.966
64.87512
–0.01%
69.965
69.86325
–0.02%
74.965
74.85668
–0.02%
79.964
79.85253
–0.01%
84.963
84.8452
–0.01%
89.963
89.8364
–0.01%
94.963
94.83339
–0.01%
96.960
96.83004
0.00%
99.960
99.82408
–0.01%
102.962
102.8224
–0.01%
105.962
105.8189
–0.01%
107.960
107.8186
0.00%
109.960
109.814
0.00%
111.959
111.8099
0.00%
114.958
114.8075
0.00%
119.957
119.7999
0.00%
124.957
124.796
0.00%
126.958
126.7941
0.00%
129.956
128.0000
–1.38%
NOTE: Applied gain factor is 1.001292.
32
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Using RTD
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When an 130-mV input is applied, for a gain of 16 the output is 2080 mV. The ADC range up to which the
linearity performance is guaranteed is 2048 mV. The ADC measurement saturates above 2048 mV.
0.3%
0
Error
-0.3%
-0.6%
-0.9%
-1.2%
-1.5%
20
Channel1ERROR (%)
Channel2ERROR (%)
Channel3ERROR (%)
Channel4ERROR (%)
40
60
80
100
VAPPLIED (mV)
120
140
D001
Figure 24. ADC Linearity Performance After Applying Gain Factor
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7.2
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ADS1248 Characterization in Temperature Measurement Configuration
To test the accuracy of the acquisition circuit alone, a series of high-precision discrete resistors were used
as the input to the system. The offset error can be attributed largely due to the offset of the internal PGA
and ADC, while the gain error can be attributed to the accuracy of the RREF resistor and gain error of the
internal PGA and ADC. The ADC error characterization includes corrections for any mismatch in excitation
currents, offset, and gain errors.
Figure 25. 4-Wire Resistance Measurement Using 6½-Digit Multimeter
The design team followed this procedure for ADD characterization:
1. Chose different resistor values representing the RTD temperature inputs
2. Selected the resistance range equivalent to the temperature range of interest
3. Combined multiple resistors in series and parallel to get the required resistance values
4. Measured the resistance values with a multi-meter using the 4-wire resistance measurement technique
5. Connected the resistors to the RTD input terminals with care to ensure there was no additional
resistance being introduced from the contact by tightening the screws
6. Used a GUI to display the measured values
34
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Using RTD
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Table 12. RTD1 Measurement
RCONNECTED (Ω)
TEMPERATURE (°C)
EQUIVALENT—ACTUAL
RTD1 (Ω)
TEMPERATURE (°C)
EQUIVALENT—MEASURED
ERROR (°C)
81.0755
–48.06
81.1769
–47.81
–0.25
89.3629
–27.11
89.48028
–26.81
–0.32
99.9778
–0.06
100.1098
0.28
0.34
119.9989
51.57
120.1415
51.93
0.36
145.8278
119.38
146.0030
119.83
0.45
163.1770
165.74
163.3946
166.29
0.56
179.8721
210.99
180.0964
211.55
0.55
Table 13. RTD2 Measurement
RCONNECTED (Ω)
TEMPERATURE (°C)
EQUIVALENT—ACTUAL
RTD2 (Ω)
TEMPERATURE (°C)
EQUIVALENT—MEASURED
ERROR (°C)
81.0755
–48.06
81.16632
–47.83
–0.20
89.3629
–27.11
89.46603
–26.84
–0.27
99.9778
–0.06
100.1127
0.29
0.35
119.9989
51.57
120.1418
51.93
0.36
145.8278
119.38
146.0063
119.84
0.46
163.1770
165.74
163.3907
166.28
0.54
179.8721
210.99
180.0973
211.55
0.56
Table 14. RTD3 Measurement
RCONNECTED (Ω)
TEMPERATURE (°C)
EQUIVALENT—ACTUAL
RTD3 (Ω)
TEMPERATURE (°C)
EQUIVALENT—MEASURED
ERROR (°C)
81.0755
–48.06
81.17819
–47.80
–0.20
89.3629
–27.11
89.47675
–26.82
–0.29
99.9778
–0.06
100.1217
0.31
0.37
119.9989
51.57
120.1531
51.96
0.41
145.8278
119.38
146.0188
119.87
0.51
163.1770
165.74
163.3938
166.29
0.55
179.8721
210.99
180.0995
211.56
0.57
Table 15. RTD4 Measurement
RCONNECTED (Ω)
TEMPERATURE (°C)
EQUIVALENT—ACTUAL
RTD4 (Ω)
TEMPERATURE (°C)
EQUIVALENT—MEASURED
ERROR (°C)
81.0755
–48.06
81.16966
–47.83
–0.17
89.3629
–27.11
89.46527
–26.85
–0.26
99.9778
–0.06
100.1159
0.30
–0.36
119.9989
51.57
120.1417
51.93
0.36
145.8278
119.38
146.0065
119.84
0.46
163.1770
165.74
163.3884
166.27
0.53
179.8721
210.99
180.094
211.55
0.56
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Figure 26 shows error for the four RTS after multiplying the measured mV with gain factor.
0.6
0.5
0.4
Error (°C)
0.3
0.2
0.1
0
-0.1
RTD1ERROR (°C)
RTD2ERROR (°C)
RTD3ERROR (°C)
RTD4ERROR (°C)
-0.2
-0.3
-0.4
-60
-30
0
30
60
90 120 150 180 210 240
Temperature (°C) Equivalent of the Connected Resistor D002
Figure 26. RTD Measurement Accuracy
7.3
Interfacing With Isolated Synchronous Serial Communication Module (TIDA-00300)
This design is a sub-system for sensing multiple RTD channels inside a protection relay or an RTD
expansion module. For safety in some of the applications, the RTD inputs are isolated from the measuring
system. This design, when interfaced with the Isolated Synchronous Serial Communication Module
(TIDA-00300) is configured as an isolated RTD measurement module. TIDA-00300 provides isolation for
SPI, I2C, and DC voltage inputs. The interface connectors are simple screw-type connectors enabling
easy connection between the two boards.
Isolated RTD functionality is verified with the TIDA-00300 board.
Table 16. Summary
SERIAL NUMBER
36
TITLE
OBSERVATION
1
Sensing of RTD inputs
ADC measured the inputs as expected
2
ADC, PGA configuration
Measurement follows the programmed gain
3
I2C I/O expander
All I/Os functions were as expected
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Using RTD
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8
Design Files
8.1
Schematics
To download the schematics, see the design files at TIDA-00110.
C7
+3.3V
RTD1_IEXC2
0.1µF
C6
RTD1_IEXC2
C
2
K
D6
DESD1P0RFW-7
R6
RTD_AINN1
21
1
2
3
4
R1
3
C
249
J1
4.12k
C1
0.047µF
282834-4
RATIO
4.12k
2
SGND
SGND
1
D1
C41
1000pF
P4SMA13CA
SGND1
D2
21
C40
1000pF
2
C2
4700pF
21
1
1
A
RTD_AINN1
D31
DFLS1200-7
D3
P4SMA13CA
R2
3
A
249
C42
1000pF
2
2
K
R5
RTD_AINP1
SGND1
C3
4700pF
D7
DESD1P0RFW-7
1
RTD_AINP1
D5
DFLS1200-7
SGND
1
0.1µF
SGND
RTD1_IEXC1
RTD1_IEXC1
P4SMA13CA
SGND1
C18
+3.3V
RTD2_IEXC2
0.1µF
C24
RTD2_IEXC2
R32
C
2
C
249
21
J2
1
2
3
4
R30
3
282834-4
4.12k
1
2
SGND
SGND
C47
1000pF
RATIO
P4SMA13CA
SGND1
D15
21
D19
2
C49
1000pF
21
1
C22
4700pF
1
D17
DESD1P0RFW-7
R31
D32
DFLS1200-7
D12
P4SMA13CA
4.12k
K
RTD_AINN2
C46
1000pF
C23
0.047µF
A
RTD_AINN2
SGND1
R28
3
A
249
C21
4700pF
2
2
K
RTD_AINP2
1
RTD_AINP2
D18
DFLS1200-7
SGND
D16
DESD1P0RFW-7
1
0.1µF
SGND
RTD2_IEXC1
RTD2_IEXC1
P4SMA13CA
SGND1
C27
RTD3_IEXC2
RTD3_IEXC1
2
K
C
2
R49
D24
DESD1P0RFW-7
C
J4
1
2
3
4
R51
3
282834-4
RATIO
4.12k
SGND
SGND
SGND1
D23
C53
1000pF
P4SMA13CA
2
C54
1000pF
21
C32
4700pF
2
1
1
249
D33
DFLS1200-7
4.12k
K
RTD_AINN3
D20
P4SMA13CA
C30
0.047µF
A
RTD_AINN3
C51
1000pF
R45
3
A
249
SGND1
RATIO
1
R48
C28
4700pF
21
RTD_AINP3
1
RTD_AINP3
D34
DFLS1200-7
SGND
D22
DESD1P0RFW-7
RTD3_IEXC1
1
0.1µF
SGND
21
RTD3_IEXC2
2
+3.3V
0.1µF
C33
D21
P4SMA13CA
SGND1
Figure 27. RTD1 to RTD3 Schematic
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C35
+3.3V
RTD4_IEXC2
0.1µF
C36
RTD4_IEXC2
RTD4_IEXC1
R52
C
2
C
21
J6
1
2
3
4
R55
3
282834-4
4.12k
1
2
SGND
SGND
D30
C58
1000pF
P4SMA13CA
SGND1
2
C56
1000pF
21
1
C38
4700pF
1
D26
DESD1P0RFW-7
249
P4SMA13CA
21
R53
D36
DFLS1200-7
4.12k
K
RTD_AINN4
D27
C34
0.047µF
A
RTD_AINN4
C57
1000pF
R54
3
A
249
D25
DESD1P0RFW-7
SGND1
C37
4700pF
2
2
K
RTD_AINP4
1
RTD_AINP4
D35
DFLS1200-7
SGND
1
0.1µF
SGND
RTD4_IEXC1
D28
P4SMA13CA
SGND1
C11
+3.3V
0.1µF
C13
0.1µF
SGND
RTD_REFP0
R15
C
249
RATIO
D11
DESD1P0RFW-7
C10
3300pF
D29
R10
3
J7
SMBJ18CA
C
C43
1000pF
R4
3
21
1
249
R3
2k
2
1
2
R17
K
RTD_REFN0
C12
0.033µF
D4
282834-2
C55
1000pF
C39
1
R9
C9
3300pF
SGND
1
2
P4SMA13CA
4.70k
A
RTD_REFN0
D9
DESD1P0RFW-7
Chessis ground
RATIO
4.70k
A
RTD_REFP0
K
2
SGND
SGND1
0
1000pF
SGND
SGND1
SGND
SGND
Figure 28. RTD4 and Ratiometric Measurement
38
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+3.3V
U4
1
C26
+3.3V
10µF
SGND
SGND
R18
10k
R41
C52
0.1µF
2
10k
3
TP7
C25
0.1µF
TP6
SGND
TP2
RESET1
/INT1
R40
100
5
R39
100
6
TP5
9
C48
C50 10
0.1µF
10µF
SGND
12
RTD_AINN1
13
RTD_AINP2
14
RTD_AINN2
DRDY
REFP0/GPIO0
CS
REFN0/GPIO1
START
AVDD
REFN1
AVSS
VREFOUT
IOUT1
VREFCOM
11
RTD_AINP1
DOUT/DRDY
REFP1
8
RTD_REFN0
DIN
RESET
7
RTD_REFP0
SCLK
DGND
CLK
4
ADC_Reset/
DVDD
IOUT2
AIN0/IEXC
AIN3/IEXC/GPIO3
AIN1/IEXC
AIN2/IEXC/GPIO2
AIN4/IEXC/GPIO4
AIN7/IEXC/GPIO7
AIN5/IEXC/GPIO5
AIN6/IEXC/GPIO6
R44
100
R43
100
R47
100
R42
100
28
27
26
25
24
SCLK
1
2
SCLK
3
SDO
4
SDI
5
6
7 SDA
8 SCL
SDI
ADC_Rdy/
ADC_CS/
23
J3
SDO
ADC_Start
R46
R50
10k
SGND
10k
+3.3V
1725711
+3.3V
22
TP8
SGND
SDA
R57
2.0k
+3.3V
SCL
R56
2.0k
21
20
IEXC1
19
C31
C29
10µF
0.1µF
IEXC2
18
RTD_AINN4
17
RTD_AINP4
16
SGND
RTD_AINN3
15
RTD_AINP3
ADS1248IPW
U3
R25
SGND
RTD_Sel1
RTD4_IEXC1
RTD3_IEXC1
RTD2_IEXC1
RTD1_IEXC1
IEXC1
R23
10k
1
100
2
R22
0
3
R21
0
4
R20
0
5
R19
0
6
R24
0
7
SGND
8
+3.3V
1EN
IN2
V+
2EN
1S4
IN1
1S3
2S4
1S2
2S3
1S1
2S2
1D
2S1
GND
2D
16
R38
15
10k
SGND
14
R37
100
13
R36
0
12
R35
0
11
R34
0
10
R33
0
9
R29
0
RTD_Sel0
RTD4_IEXC2
C20
10µF
C19
0.1µF
SGND
RTD3_IEXC2
RTD2_IEXC2
RTD1_IEXC2
IEXC2
TS3A5017D
Figure 29. ADS1248 and Analog Switch Circuit
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Design Files
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TP3
+6V TP9
U2
SCL
SDA
SCL
SDA
+3.3V
14
15
2
R27
10k
R16
10k
/INT1
13
RESET1
3
+3.3V
SGND
1
16
RESET1
/INT1
R11
10k
SCL
SDA
P0
P1
P2
P3
P4
P5
P6
P7
ADDR
INT
RESET
VCCI
VCCP GND
4
5
6
ADC_Start
7
ADC_CS/
9
10
LED1
11
LED2
12
TP10
J5
RTD_Sel0
RTD_Sel1
ADC_Rdy/
ADC_Start
ADC_CS/
ADC_Reset/
1
2
3
4
282834-4
SGND
8
TCA6408APWR
C17
0.1µF
SGND
SGND
+3.3V
+3.3V
R8
300
R12
300
TP1
+6V
TP4
TPS7A1633DGNR
U1
C8
C5
0.1µF 1000pF
10µF
NC
DNC
D10
Green
1
1000 OHM
6
2
R14
0
EP GND
R26
1.2k
C14
C16
4.7µF
10µF
C15
0.1µF
D14
PTZTE253.6B
3.8V
D13
Green SGND
Q2
CSD17571Q2
30V
3
R13
300
7
4
SGND
8
6
1
2
5
OUT
DELAY
D8
Green
L1
SGND
C45
0.1µF
LED1
Q1
CSD17571Q2
30V
3
R7
LED2
300
7
4
C4
EN
3
8
6
1
2
5
7
+3.3V
PG
4
5
IN
9
8
C44
0.1µF
SGND
SGND
SGND
SGND
SGND
SGND
Figure 30. I2C I/O Expander, Power Supply, and LEDs
40
Analog Front End (AFE) for Sensing Temperature in Smart Grid Applications
Using RTD
Copyright © 2014, Texas Instruments Incorporated
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Design Files
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8.2
Bill of Materials
To download the bill of materials (BOM), see the design files at TIDA-00110.
Table 17. BOM
QTY
REFERENCE
PART DESCRIPTION
MANUFACTURER
MANUFACTURER
PARTNUMBER
PCB
FOOTPRINT
1
!PCB1
Printed Circuit Board
Any
TIDA-00110
4
C1, C23, C30, C34
CAP, CERM, 0.047 µF, 50
V, ±10%, X7R, 0603
TDK
C1608X7R1H473K
0603
8
C2, C3, C21, C22, C28,
C32, C37, C38
CAP, CERM, 4700 pF, 50 V,
±10%, X8R, 0603
TDK
C1608X8R1H472K
0603
6
C4, C14, C20, C26, C31,
C48
CAP, TA, 10 µF, 16 V,
±10%, 2 Ω, SMD
AVX
F931C106KBA
3528-21
1
C5
CAP, CERM, 1000 pF, 100
V, ±5%, X7R, 0603
AVX
06031C102JAT2A
0603
0
C6, C7, C11, C13, C18,
C24, C25, C27, C33, C35,
C36
CAP, CERM, 0.1 µF, 50 V,
±10%, X7R, 0603
AVX
06035C104KAT2A
0603
7
C8, C15, C17, C19, C29,
C50, C52
CAP, CERM, 0.1 µF, 50 V,
±10%, X7R, 0603
AVX
06035C104KAT2A
0603
2
C9, C10
CAP, CERM, 3300 pF, 50 V,
±10%, X7R, 0603
Kemet
C0603C332K5RAC
TU
0603
1
C12
CAP, CERM, 0.033 µF, 50
V, ±10%, X7R, 0603
MuRata
GRM188R71H333
KA61D
0603
1
C16
CAP, CERM, 4.7 µF, 50 V,
±10%, X5R, 0805
TDK
C2012X5R1H475K
125AB
0805
2
C39, C55
CAP, CERM, 1000 pF, 2 KV
10% X7R 1206
Johanson
Dielectrics Inc
202R18W102KV4E
1206
13
C40, C41, C42, C43, C46,
C47, C49, C51, C53, C54,
C56, C57, C58
CAP CER, 1000 pF, 100 V,
10% X7R 1206
Yageo
CC1206KRX7R0B
B102
1206
2
C44, C45
CAP, CERM, 0.1 µF, 25 V,
±5%, X7R, 0603
AVX
06033C104JAT2A
0603
13
D1, D2, D3, D4, D12, D15,
D19, D20, D21, D23, D27,
D28, D30
TVS Diode 11.1VWM
18.2VC SMD
Littelfuse Inc
P4SMA13CA
SMA
8
D5, D18, D31, D32, D33,
D34, D35, D36
Diode, Schotky, 200 V, 1 A,
PowerDI123
Diodes
Incorporated
DFLS1200-7
PowerDI123
0
D6, D7, D9, D11, D16, D17,
D22, D24, D25, D26
TVS Diode,70VVVM, 8VC,
SOT-323
Diodes
Incorporated
DESD1P0RFW-7
SOT-323
3
D8, D10, D13
LED, SMARTLED, GREEN,
570 NM, 0603
OSRAM Opto
Semiconductors
Inc
LG L29K-G2J1-24Z
0603
1
D14
Diode Zener, 3.8 V, 1 W,
PMDS
Rohm
Semiconductor
PTZTE253.6B
DO-214AC,
SMA
1
D29
TVS 18-V, 600-W BI-DIR
SMB
Littelfuse Inc
SMBJ18CA
SMB
0
H1, H2, H3, H4
Machine Screw, Round, #440 × ¼, Nylon, Philips
panhead
B&F Fastener
Supply
NY PMS 440 0025
PH
Screw
5
J1, J2, J4, J5, J6
Receptacle, 100-mil, 4×1 TH
TE Connectivity
282834-4
10.62 × 10 ×
6.5 mm
1
J3
Terminal Block, 8×1, 2.54
mm, TH
Phoenix Contact
1725711
8POS
Terminal
Block
1
J7
Terminal Block, 2×1,
2.54mm, TH
TE Connectivity
282834-2
Terminal
Block, 2×1,
2.54 mm, TH
TIDU575 – December 2014
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Copyright © 2014, Texas Instruments Incorporated
41
Design Files
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Table 17. BOM (continued)
QTY
PART DESCRIPTION
MANUFACTURER
MANUFACTURER
PARTNUMBER
PCB
FOOTPRINT
TDK Corporation
MMZ1608B102C
0603
DNI
1
L1
Ferrite Chip 1000 Ω, 300
MA, 0603
0
LBL1
Thermal Transfer Printable
Labels, 0.650" W × 0.200" H
- 10,000 per roll
Brady
THT-14-423-10
PCB Label
0.650"H ×
0.200"W
2
Q1, Q2
MOSFET, N-CH, 30 V, 22 A,
SON 2X2 MM
Texas Instruments
CSD17571Q2
DQK
8
R1, R2, R28, R30, R45,
R51, R54, R55
RES, 4.12 kΩ, 0.1%, 0.1 W,
0603
Susumu Co Ltd
RG1608P-4121-BT5
0603
1
R3
RES 2 KΩ, ¼ W, 0.1% 1206
Vishay-Dale
TNPW12062K00B
EEA
1206
2
R4, R10
RES, 4.70 kΩ, 0.1%, 0.1 W,
0603
Susumu Co Ltd
RG1608P-472-BT5
0603
10
R5, R6, R15, R17, R31,
R32, R48, R49, R52, R53
RES, 249, 1%, 0.1 W, 0603
Vishay-Dale
CRCW0603249RF
KEA
0603
4
R7, R8, R12, R13
RES, 300 Ω, 5%, 0.1 W,
0603
Vishay-Dale
CRCW0603300RJ
NEA
0603
1
R9
RES, 0 Ω, 5%, 0.125 W,
0805
Yageo America
RC0805JR-070RL
0805
0
R11, R46, R50
RES, 10 k, 5%, 0.1 W, 0603
Vishay-Dale
CRCW060310K0J
NEA
0603
DNI
0
R14
RES, 0, 5%, 0.1 W, 0603
Vishay-Dale
CRCW06030000Z0
EA
0603
DNI
6
R16, R18, R25, R27, R38,
R41
RES, 10 k, 5%, 0.1 W, 0603
Vishay-Dale
CRCW060310K0J
NEA
0603
10
R19, R20, R21, R22, R24,
R29, R33, R34, R35, R36
RES, 0, 5%, 0.1 W, 0603
Vishay-Dale
CRCW06030000Z0
EA
0603
8
R23, R37, R39, R40, R42,
R43, R44, R47
RES, 100, 1%, 0.1 W, 0603
Vishay-Dale
CRCW0603100RF
KEA
0603
1
R26
RES, 1.2 k, 5%, 0.1 W, 0603
Vishay-Dale
CRCW06031K20J
NEAHP
0603
2
R56, R57
RES, 2.0 k, 5%, 0.1 W, 0603
Yageo America
RC0603JR-072KL
0603
10
TP1, TP2, TP3, TP4, TP5,
TP6, TP7, TP8, TP9, TP10
Test Point 40-mil pad, 20-mil
drill
STD
STD
U1
Single Output LDO, 100 mA,
Fixed 3.3-V Output, 3- to 60V Input, with Enable and
Power Good, 8-pin MSOP
(DGN), –40°C to 125°C,
Green (RoHS and no Sb/Br)
Texas Instruments
TPS7A1633DGNR
DGN0008C
1
U2
Low-Voltage 8-Bit I2C and
SMBus I/O Expander, 1.65
to 5.5 V, –40°C to 85°C, 16pin TSSOP (PW), Green
(RoHS and no Sb/Br)
Texas Instruments
TCA6408APWR
PW0016A
1
U3
IC, Dual, 14 Ω, SP4T Analog
Switch
Texas Instruments
TS3A5017D
SO16
1
U4
IC, 24-Bit A-D Converters for
Temperature Sensors
Texas Instruments
ADS1248IPW
TSSOP-28
1
42
REFERENCE
Analog Front End (AFE) for Sensing Temperature in Smart Grid Applications
Using RTD
Copyright © 2014, Texas Instruments Incorporated
DNI
TIDU575 – December 2014
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Design Files
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8.3
Layer Plots
To download the layer plots, see the design files at TIDA-00110.
Figure 31. Top Overlay
Figure 32. Top Solder
Figure 33. Top Layer
Figure 34. GND Plane
Figure 35. PWR Plane
Figure 36. Bottom Layer
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Design Files
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Figure 37. Bottom Solder
8.4
Figure 38. Bottom Overlay
Altium Project
To download the Altium project files, see the design files at TIDA-00110.
Figure 39. Multilayer Composite Print
Figure 40. Top Layer
44
Figure 41. Bottom Layer
Analog Front End (AFE) for Sensing Temperature in Smart Grid Applications
Using RTD
Copyright © 2014, Texas Instruments Incorporated
TIDU575 – December 2014
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Design Files
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8.5
Gerber Files
To download the Gerber files, see the design files at TIDA-00110.
Figure 42. Fabrication Drawing
TIDU575 – December 2014
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Using RTD
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Design Files
8.6
www.ti.com
Assembly Drawings
Figure 43. Top Assembly Drawing
8.7
Figure 44. Bottom Assembly Drawing
Software Files
To download the software files, see the design files at TIDA-00110.
9
References
1. Texas Instruments, RTD Temperature Transmitter for 2-Wire, 4 to 20-mA Current Loop Systems,
TIDA-00095 Design Guide (TIDU182)
2. Texas Instruments, RTD Ratiometric Measurements and Filtering Using the ADS1148 and ADS1248
Family of Devices Application Report (SBAA201)
3. TI Precision, Hardware-Compensated Ratiometric 3-Wire RTD System, 0°C – 100°C, 0.005°C Error
Design Guide (TIDU045)
4. TI Precision, 3-Wire RTD Measurement System Reference Design, –200°C to 850°C (SLAU520)
5. Robert Burnham and Nagaraj Ananthapadamanabhan, Example Temperature Measurement
Applications Using the ADS1247 and ADS1248, Application Report (SBAA180)
6. Collin Wells, Signal Conditioning and Linearization of RTD Sensors, 2011 Texas Instruments
Technology Day Presentation (TIDU433)
7. Texas Instruments, Advanced Debugging Using the Enhanced Emulation Module (EEM) With Code
Composer Studio Version 6, Application Report (SLAA393)
8. Texas Instruments, 24-Bit Analog-to-Digital Converters for Temperature Sensors, ADS1248 Datasheet
(SBAS426G)
10
About the Author
PRAHLAD SUPEDA is a systems engineer at Texas Instruments India where he is responsible for
developing reference design solutions for Smart Grid within Industrial Systems. Prahlad brings to this role
his extensive experience in power electronics, EMC, analog, and mixed signal designs. Prahlad earned
his bachelor of instrumentation and control engineering from Nirma University, India. He can be reached
at [email protected]
VIVEK GOPALAKRISHNAN is a firmware architect at Texas Instruments India where he is responsible
for developing reference design solutions for Smart Grid within Industrial Systems. Vivek brings to his role
his experience in firmware architecture design and development. Vivek earned his master’s degree in
sensor systems technology from VIT University, India. He can be reached at [email protected]
46
Analog Front End (AFE) for Sensing Temperature in Smart Grid Applications
Using RTD
Copyright © 2014, Texas Instruments Incorporated
TIDU575 – December 2014
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