TECHNICAL ARTICLE
Mary McCarthy
and Aine McCarthy
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ADC REQUIREMENTS
FOR RTD TEMPERATURE
MEASUREMENT SYSTEMS
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There are several types of temperature sensors that can be used in a
temperature system. The temperature sensor to use depends on the
temperature range being measured and the accuracy required. Along
with the sensor, the accuracy of the temperature system depends on the
performance of the analog-to-digital converter (ADC) to which the sensor
is interfaced. A high resolution ADC is required in many cases as the magnitude of the signal from the sensor is quite small. Sigma-delta (Σ-Δ)
ADCs are suitable for these systems as they are high resolution devices.
They also have additional circuitry embedded on-chip, which is required in
a temperature system such as excitation currents and reference buffers.
This article describes 3-wire and 4-wire resistance temperature detectors (RTDs) that are commonly used. It describes the circuitry needed
to interface a sensor to an ADC along with explaining the performance
requirements needed from the ADC.
RTDs
RTDs are useful for measuring temperatures in the range of –200°C
to +800°C and these devices have a near linear response over this
temperature range. Typical elements used for RTDs are nickel, copper,
and platinum, with 100 Ω and 1000 Ω platinum RTDs being the most
common. An RTD is made up of either 2-wire, 3-wire, or 4-wire versions,
with 3-wire and 4-wire versions being the most commonly used. These
are passive sensors that require an excitation current to produce an
output voltage. The output voltage levels of such RTDs vary from tens of
millivolts to hundreds of millivolts depending on the RTD chosen.
3-Wire RTD Interface and Building Blocks
Figure 1 shows a 3-wire RTD system. The AD7124-4/AD7124-8 includes
all the building blocks needed for the system. To fully optimize this
system, two identically matched current sources are needed. These two
current sources are used to cancel the lead resistance errors produced
by RL1 and RL2 of the RTD. One excitation current flows through both the
precision reference resistor, RREF, and the RTD. The second current flows
through lead resistance RL2 and develops a voltage that cancels the
voltage drop across RL1. The voltage generated across the precision reference resistor is used as the reference voltage REFIN1(±) to the ADC. Since
one excitation current is used to generate both the reference voltage and
the voltage across the RTD, the current source accuracy, mismatch, and
mismatch drift has a minimal effect on the overall ADC transfer function.
The AD7124-4/AD7124-8 offers a choice of excitation current values,
which allows the user to tune the system so that most of the ADC input
range is used, resulting in increased performance.
AVDD
REFIN1(+)
REFIN1(–)
REFIN2(+)
REFIN2(–)
Band Gap Reference
AVDD
AVSS
AVDD
VBIAS
Reference
Detect
AIN0/IOUT0
AVDD
REFIN1(+)
RREF
REFIN1(–)
PGA
X-Mux
RL1
RTD
RL2
AIN2
AIN3
AIN1/IOUT1
AVSS
RL3
IOUT0
PSW
Σ-Δ
ADC
Digital
Filter
Serial
Interface
and
Control
Logic
Channel
Sequencer
Temp
Sensor
VDD
Diagnostics
DIN
SCLK
CS
IOVDD
Internal
Clock
CLK
Excitation
Currents
SYNC
AD7124-4/AD7124-8
IOUT1
REGCAPA
Figure 1. 3-wire RTD temperature system.
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DOUT/RDY
REGCAPD
AVSS
DGND
ADC Requirements for RTD Temperature Measurement Systems
2
AD7124-4/AD7124-8
Temperature
–40ºC
1. Measure the two individual currents using the cross multiplexer functionality of the AD7124-4/AD7124-8, the precision reference resistor
and the ADC’s internal low drift reference.
Pt100
+25ºC
2. Perform system chopping where the currents are swapped to the
different sides of the RTD and the average of the two results is used in
the overall calculation of the temperature.
+105ºC
1
0
4-Wire RTD Interface and Building Blocks
4-wire RTD measurements require one excitation current source only.
Figure 3 shows a 4-wire RTD system. Like the 3-wire RTD system, the
reference input used is REFIN1(±) and the reference buffers are enabled
to allow unlimited antialiasing or EMC filtering. The current through the
RTD also flows through the precision reference resistor, RREF, which is
used to generate the reference voltage for the ADC. This configuration is
resulting in a ratiometric measurement between the reference voltage
and the voltage generated across the RTD. The ratiometric configuration
ensures that variation in the excitation current value has no influence on
the overall system accuracy. Figure 4 shows the RTD temperature error
measured for a 4-wire Class B RTD after an internal zero-scale and fullscale calibration. Similar to the 3-wire configuration, the overall error
recorded is much less than ±1°C.
–1
–2
–3
–50
–25
0
25
50
75
100
125
150
175
200
Pt100 Temperature (°C)
Figure 2. 3-wire RTD temperature system.
The low level output voltage from the RTD needs to be amplified so that
most of the ADC’s input range is used. The AD7124-4/AD7124-8’s PGA
is programmable from a gain of 1 to 128, allowing the customer to trade
off excitation current value vs. gain and performance. Filtering is required
between the sensor and the ADC for antialiasing and EMC purposes.
Reference buffers allow unlimited values for the R and C components of
the filter; for example, these components do not impact the accuracy of
the measurement.
AD7124-4/AD7124-8
Temperature
–40ºC
2
+25ºC
Pt100
+105ºC
Calibration is also required in the system to eliminate gain and offset
errors. Figure 2 shows the temperature error measured for this 3-wire
Class B RTD following an internal zero-scale and full-scale calibration,
the overall error being much less than ±1°C.
1
Error (°C)
2
Error (°C)
Having the precision reference resistor on the high side of the RTD works
well for systems using a single RTD. When multiple RTDs are needed, the
precision resistor should be placed on the low side so the reference resistor is shared among all the RTD sensors. For this implementation, better
excitation current matching and matching drift is required. To minimize
the errors due to the mismatches in the excitation current sources, two
different techniques can be used:
0
–1
–2
–3
–50
–25
0
25
50
75
100
125
150
175
200
Pt100 Temperature (°C)
Figure 4. 4-wire RTD temperature system.
AVDD
REFIN1(+)
REFIN1(–)
REFIN2(+)
REFIN2(–)
Band Gap Reference
AVDD
AVSS
AVDD
VBIAS
Reference
Detect
AIN0/OUT0
RL1
RL2
RTD
RL3
RL4
RREF
AVDD
AIN2
AIN3
PGA
X-Mux
Digital
Filter
Σ-Δ
ADC
REFIN1(+)
REFIN1(–)
AVSS
RHEADROOM
IOUT0
PSW
Serial
Interface
and
Control
Logic
Channel
Sequencer
Temp
Sensor
VDD
Diagnostics
DOUT/RDY
DIN
SCLK
CS
IOVDD
Internal
Clock
CLK
Excitation
Currents
SYNC
AD7124-4/AD7124-8
IOUT1
REGCAPA
REGCAPD
Figure 3. 4-wire RTD temperature system.
AVSS
DGND
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Other ADC Requirements
For temperature systems, the measurements are mainly low speed (up to
100 samples per seconds typically). Therefore, a low bandwidth ADC is
required, but the ADC must have high resolution. Σ-Δ ADCs are suitable
for these applications since low bandwidth, high resolution ADCs can be
developed using the Σ-Δ architecture.
Power
With Σ-Δ converters, the analog input is continuously sampled, and the
sampling frequency is considerably higher than the band of interest. They
use noise shaping also, which pushes noise out of the band of interest into
a region not being used by the conversion process, further reducing the
noise in the band of interest. The digital filter attenuates any signal outside
the band of interest.
The digital filter has images at the sampling frequency and multiples
of the sampling frequency. Hence, some external antialiasing filters are
required. However, due to the oversampling, a simple first-order RC filter
is sufficient for most applications. The Σ-Δ architecture allows 24-bit
ADCs with a peak-to-peak resolution of up to 21.7 bits to be developed
(21.7 stable or flicker free bits).
Filtering (50 Hz/60 Hz Rejection)
Diagnostics
Diagnostics are becoming increasingly important in industrial applications.
Typical diagnostic requirements are
XX
Power supply/reference voltage/analog input monitoring
XX
Open wire detection
XX
Conversion/calibration checks
XX
Signal chain functionality check
XX
Read/write monitoring
XX
Register content monitoring
For systems that are being designed for fail-safe applications, on-chip
diagnostics save the customer in design time, external components,
board space, and cost. A part such as the AD7124-4/AD7124-8 includes
the above diagnostics. The failure modes effects and diagnostic analysis
(FMEDA) of a typical temperature application using this device has shown
a safe failure fraction (SFF) greater than 90% according to IEC 61508. Two
traditional ADCs are normally required to provide this level of coverage.
0
0
–10
–10
–20
–20
–30
–30
Filter Gain (dB)
Filter Gain (dB)
Along with rejecting the noise as discussed previously, the digital filter is
also useful to provide 50 Hz/60 Hz rejection. Interference occurs at 50 Hz
or 60 Hz when systems are operated from the mains power supply. There
are mains-generated frequencies at 50 Hz and its multiples in Europe,
and 60 Hz and its multiples in the U.S. The low bandwidth ADCs mainly
use sinc filters that can be programmed to set notches at 50 Hz and/or
60 Hz along with multiples of 50 Hz and 60 Hz, thereby providing rejection at 50 Hz/60 Hz and its multiples. There is an increasing requirement to
provide 50 Hz/60 Hz rejection using filtering methods that have low settling
time. In a multichannel system, the ADC sequences through all enabled
channels, generating a conversion on each. When a channel is selected, it
requires the filter settling time to generate a valid conversion. The number
of channels converted in a given period of time is increased if the settling
time is reduced. The AD7124-4/AD7124-8 includes post filters or FIR filters,
which provide simultaneous 50 Hz/60 Hz rejection at lower settling times
compared to a sinc3 or sinc4 filters. Figure 5 shows one digital filter option,
this post filter has a settling time of 41.53 ms and provides simultaneous
50 Hz/60 Hz rejection of 62 dB.
The current consumed in a system is dependent on the end application. In
some industrial applications, such as temperature monitoring in factories,
the complete temperature system containing the sensor, ADC, and microcontroller are contained on a standalone board, which is powered from the
4 mA to 20 mA loop. Therefore, the standalone board has a current budget
of 4 mA maximum. In portable equipment such as gas analyzers used to
analyze the gases present in mines, the temperature must be measured
along with the gas analysis. These systems are operated from a battery,
the aim being to maximize the lifetime of the battery. In these applications,
low power is essential but high performance is still required. In process
control applications, more current can be allowed for the system. For this
type of application, the requirement may be to sequence through a higher
channel count in a certain period of time while still achieving a certain
level of performance. The AD7124-4/AD7124-8 contains three power
modes, user selectable via 2 bits in one of its registers. The power mode
chosen determines the range of output data rates along with the current
consumed by the analog blocks on-chip. Therefore, the part can be
operated in mid power or low power mode for loop-powered or batterypowered systems. In process control systems, the part can be operated
in full power mode, where higher current consumption leads to improved
performance.
–40
–50
–60
–40
–50
–60
–70
–70
–80
–80
–90
–90
–100
40
100
200
300
Frequency (Hz)
(a)
400
500
600
–100
40
45
50
55
Frequency (Hz)
(b)
Figure 5. Frequency response, post filter, 25 sps a) dc to 600 Hz, b) 40 Hz to 70 Hz.
60
65
70
3
Conclusion
The ADC and system requirements for a temperature measurement
system are quite stringent. The analog signals generated by these
sensors are small and must be amplified by a gain stage whose noise
is low to ensure that the gain stage’s noise does not swamp the signal
from the sensor. Following the amplifier, a high resolution ADC is
required so that the low level signal from the sensor can be converted
into digital information. ADCs, which use a Σ-Δ architecture, are
suitable for such applications since high resolution, high precision ADCs
can be developed using these architectures. Along with the ADC and
gain stage, a temperature system requires other components such as
excitation currents and reference buffers. Finally, the end application
dictates the current budget allowed for the system. Portable or looppowered systems must use low power components, and with redundancy
being included for fail-safe systems, this reduces the current consumption
allowance per component further. For systems such as input modules,
there is a desire for a certain level of performance at higher throughputs, leading to increased channel density. Using a device with multiple
power modes eases the burden on the user in that one ADC, which can
be designed into multiple end systems, reducing design times.
About The Authors
Mary McCarthy is an applications engineer at Analog Devices. She
joined ADI in 1991 and works in the Linear and Precision Technology
Applications Group in Cork, Ireland, focusing on precision Σ-Δ
converters. Mary graduated with a bachelors degree in Electronic and
electrical engineering from University College Cork in 1991.
Aine McCarthy is an applications engineer at Analog Devices. Aine
joined ADI in 2006 and works in the Linear and Precision Technology
Applications Group in Cork, Ireland. Aine graduated with a B.Eng
in electronic engineering from Cork Institute of Technology and an
M.Eng.Sc. micro electronic design from University College Cork.
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