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Analog Front-End Design Considerations
for RTD Ratiometric Temperature
Measurements
By Barry Zhang and Alex Buda
Introduction
Many system designers use Σ-Δ ADCs together with RTDs
(resistance temperature detectors) for temperature measurements, but have difficulties achieving the high performance
as specified by the data sheet of the ADC they are using.
For example, some designers may only be able to get 12 to
13 noise-free bits from a 16-bit to 18-bit ADC. The front-end
techniques introduced in this article will enable designers to
achieve 16+ noise-free bits in their system designs.
Using RTDs in a ratiometric measurement has the advantage
in that it eliminates sources of error such as the accuracy and
drift of the excitation current source. Below is a typical circuit
for a 4-wire RTD ratiometric measurement circuit. The 4-wire
configuration has the advantage that the error due to lead
resistance can be cancelled.
When engineers design their products using this type of
circuit, they will add some resistors and capacitors before the
analog input, external reference pins for low-pass filtering,
and overvoltage protection as shown in Figure 2. In this article,
we will show what should be considered in choosing suitable
resistors and capacitors for better noise performance.
ADC
IEXC
R1
RRTD
RRef
ADC
RLEAD
R2
REF+
R4
C6
C5
REF–
Note: Many ADI Σ-∆ ADCs
integrate excitation current internally
Figure 2. Typical 4-wire RTD ratiometric measurement circuit.
VRTD
AIN2
RLEAD
REF+
RRef
AIN2
C4
AIN1
RRTD
C3
C2
R3
IEXC
4-Wire
RTD
AIN1
C1
VRef
From Figure 2 we can see that R1, R2, C1, C2, and C3 are used as
a first-order, low-pass RC filter that provides attenuation for
both differential and common-mode voltage signals. The values
of R1 and R2 should be the same and similarly for the values of
C1 and C2. Similarly, R3, R4, C4, C5, and C6 are used as a low-pass
filter for the reference path.
REF–
Common-Mode Low-Pass RC Filter
Figure 1. 4-wire RTD ratiometric measurement circuit.
From the circuit above, we can derive the following
two equations:
VRTD = R RTD × IEXC
VRef = R Ref × IEXC
The general expression used to calculate the RTD resistance
(RRTD) when the ADC is operating in bipolar differential mode
is given by:
R RTD =
CodeRTD × R Ref
CodeADC_Fullscale
where:
CodeRTD is the ADC code.
Figure 3 shows the common-mode, low-pass filter equivalent
circuit.
+
Vin
–
R1
C1
R2
a
C2
C3
b
AIN1
AIN2
Ccm = C 1 = C 2
R = R1 = R2
Figure 3. Common-mode low-pass filter.
The measured resistance value of the RTD is theoretically
only related to the precision and drift of the reference resistor. Normally RREF is an accurate and low drift resistor with
0.1% precision.
Because the common-mode voltage at Point a is equal to
the voltage at Point b, there is no current flowing through
C3. Therefore, the common-mode cutoff frequency can be
expressed as
1
1
f=
=
2π R1C 1 2π RCcm
Analog Dialogue 50-03, March 2016
analog.com/analogdialogue
CodeADC_Fullscale is the ADC full-scale code.
1
Differential Mode Low-Pass RC Filter
Resistor and Capacitor Considerations
To better understand the low-pass RC filter cutoff frequency for
differential signals, the C3 capacitor in Figure 4 can be thought
of as two separate capacitors Ca and Cb in Figure 5.
Other than being part of the low-pass filter, R1 and R2 can
also provide overvoltage protection. If 3 kΩ resistors are
used before the AD7124-4 AIN pins in Figure 6, these can
protect against up to 30 V miswiring. It’s not recommended
to use larger resistors before the AIN pins for the following
two reasons. First of all, they will generate more thermal
noise. Secondly, the AIN pins will have input currents that will
flow across these resistors and introduce errors. These input
currents do not have a constant value and when combined
with a mismatch between them they will generate noise that
will increase with the size of the resistors.
R1
+
AIN1
C1
Vin
R2
–
C3
C2
AIN2
Figure 4. Differential mode low-pass filter.
R1
+
Vin
R2
–
C1
Ca
C2
Cb
AIN1
AIN2
Measured Noise Performance on ADuCM360 with
Ratiometric Measurement
Ccm = C 1 = C 2
Ca = Cb = 2C 3
R = R1 = R2
Figure 5. Differential mode low-pass filter equivalent circuit.
From Figure 5, the differential mode cutoff frequency is:
f=
1
1
=
2π R1 (C 1 + Ca) 2π R (Ccm + 2C3 )
Normally the value of C3 is 10× larger than the value of Ccm.
The purpose of this is to decrease the effects that are introduced
by the mismatch of C1 and C2. For example, with an analog
front-end design used in the Analog Devices circuit note
CN-0381 as seen in Figure 6, the cutoff frequency for differential signals is around 800 Hz and the cutoff frequency for
common-mode signals is 16 kHz.
The ADuCM360 is a fully integrated, 3.9 kSPS, 24-bit data
acquisition system that incorporates dual high performance,
multichannel Σ-Δ ADCs, a 32-bit ARM® Cortex®-M3 processor,
and Flash/EE memory on a single chip. It also integrates
programmable gain instrumentation amplifiers, a precision
band gap reference, programmable excitation current sources,
a flexible multiplexer, and many other features. It allows a
direct interface to resistive temperature sensors.
When using the ADuCM360 for RTD measurements, the
REF– pin is normally connected to ground so R4 and C5 from
Figure 2 can be removed as there is no current flowing across
them. C4 and C6 are in parallel so these two can be added
together. However, because C4 is much smaller than C6, it can
be ignored. This results in the simplified analog front-end
circuit as shown in Figure 7.
ADuCM360
AD7124-4/
AD7124-8
Pt100
RL1
RL2
RL3
IEXC
IOUT0 (AIN0)
1 kΩ
AIN2
0.01 µF
RL4
The resistor and capacitor values play a vital role in determining the performance of the final circuit. Designers need to
understand their field requirements and calculate the resistor
and capacitor values according to the equations above. For ADI
Σ-Δ ADC parts and precision analog microcontrollers with an
integrated excitation current source, it is recommended to use
the same resistor and capacitor values before the AIN and reference pins. This design ensures that the analog input voltage
remains ratiometric to the reference voltage and any errors
in the analog input voltage due to the temperature drift and
noise of the excitation current are compensated by the variation of the reference voltage.
R1
R2
AIN2
R3
AIN3
0.01 µF
C3
C2
0.1 µF
1 kΩ
AIN1
C1
RRTD
RRef
REF+
C6
REF–
Figure 6. Analog input configuration for RTD measurement
using AD7124.
2
C 1 = C 6 = 0.1 µF
C 1 = C 2 = 10 nF
Figure 7. ADuCM360 analog front-end circuit for RTDs measurement.
Analog Dialogue 50-03, March 2016
Table 1 shows the noise level with matched and unmatched
filters in front of the analog and reference input paths. A 100 Ω
precision resistor is used instead of RRTD to measure the noise
voltage on the ADC input pins. The value of RRef is 5.62 kΩ.
Table 1. Noise Test Results
ADC
Gain
16
16
16
16
Noise Voltage on 100 Ω Resistor (μV)
ISOURCE
(μA)
R1 = R2 = R3 = 1k
R1 = R2 = 10k
R3 = 1k
100
200
300
400
1.6084
1.6311
1.6117
1.6279
1.8395
1.7594
1.9181
1.9292
From Table 1 we can see that using a matched analog frontend circuit where the values of R1 and R2 are the same as R3,
the noise decreases by around 0.1 μV to 0.3 μV as compared to
the unmatched circuit, which means that the number of ADC
noise-free bits increases by about 0.25 bit to 16.2 bits with an
ADC PGA gain of 16.
Conclusion
Using matched RC filter circuits and choosing the right resistor
and capacitor values based on field requirements according to
the considerations introduced in this article, the RTDs in ratiometric measurement applications can achieve optimum results.
References
CN-0381 Circuit Note. “Completely Integrated 4-Wire RTD
Measurement System Using a Low Power, Precision, 24-Bit,
Sigma-Delta ADC.” Analog Devices, Inc.
CN-0267 Circuit Note. “Complete 4 mA to 20 mA Loop-Powered
Field Instrument with HART Interface.” Analog Devices, Inc.
Barry Zhang
Barry Zhang [[email protected]] is an applications engineer at
Analog Devices in Beijing, China. He joined Analog Devices in 2011 and works
for the Integrated Precision Group. Prior to joining ADI, Barry worked
for Rigol and Putian as a hardware engineer. In 2006, Barry earned his
master’s degree in mechatronic engineering from University of Science
and Technology Beijing.
Alex Buda
Alex Buda [[email protected]] is an applications engineer with
the Integration Precision Group within ADI. He joined ADI in 2012,
where he has been working with precision analog microcontrollers.
Alex graduated in 2012 with a first class honours bachelor's degree
in electronic engineering and computers from the National University
of Ireland, Maynooth. As part of his degree program he underwent a
six month placement in ADI with the Integration Precision Group.
Analog Dialogue 50-03, March 2016
3
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