DN302 - Ultraprecise Instrumentation Amplifier Makes Robust Thermocouple Interface

Ultraprecise Instrumentation Amplifier Makes
Robust Thermocouple Interface – Design Note 302
Jon Munson
Introduction
The versatile and precise LTC ®2053 instrumentation
amplifier provides an excellent platform for robust, low
power instrumentation products—as exemplified below
by the battery-powered thermocouple amplifier circuit.
The LTC2053 offers exceptionally low 10μV maximum
input offset along with 116dB typical CMRR and PSRR,
a result of a combination of switched capacitor and zerodrift op amp technologies. It is optimized for low voltage
supplies from 2.7V to 11V single ended or up to ±5.5V with
split supplies. The LTC2053 is ideal for battery-powered
instrument applications because of its low 850μA typical
current draw. The gain is easily programmed with two
resistors, as shown in Figure 1, just like a traditional
non-inverting op amp. The LTC2053 also features low
1/f noise and rail-to-rail I/O to maximize dynamic range.
The Requirements of Thermocouple Amplification
A robust thermocouple amplification circuit must meet
several specific requirements. First, a commonly used
type K thermocouple develops 40.6μV/°C, and a standard
readout scale is 10mV/°C, so a precision amplifier with
a nominal gain of 246 is required. Also, thermocouple
leads are generally exposed to the electrical noise of
an industrial environment so the fully differential input
capability of an instrumentation amplifier helps eliminate
errors due to common mode noise pickup. Finally, fault
protection against accidental contact of the thermocouple
to sources of transients or high voltage is needed but the
protection cannot compromise accuracy.
The LTC2053 offers features that help meet all of these
requirements. It can withstand a 10mA of fault current
in any pin so 10kΩ protection resistors allow ±100V
hard faults or Level 4 ESD (8kV contact/15kV air-gap)
on the thermocouple junction without damage to the IC.
The LTC2053 uses a switched-capacitor input topology,
sampling at approximately 2.5kHz. With an internal input
sampling capacitance of ~1000pF, the RC transients of
the 10kΩ protection resistors settle within the ~180μs
01/03/302_conv
sampling window so they do not contribute to offset
errors as they might with a typical IA.
A Battery-Powered Thermocouple Amplifier
Figure 2 shows the LTC2053 used in a battery-powered
thermocouple amplifier. The circuit is used as a plug-in
adapter for common digital multimeters and is completely
portable. This circuit employs the LT1025 thermocouple
compensator to improve accuracy over a wide range of
ambient conditions and is mounted close to the thermocouple connection points for optimal thermal tracking.
It precludes the need to temperature stabilize the thermocouple “cold junctions” and removes the accuracy
penalty of a static room temperature correction value.
The output of the LT1025 provides a 10mV/°C correction
voltage for the ambient temperature difference from
0°C—normally about 250mV at room temperature. The
measured probe temperature is the sum of this compensation voltage and the amplified thermocouple voltage.
Simple connection of the output of the compensator
to the REF input of the LTC2053 is all that is needed to
add these two voltages. The only consideration with
this configuration is that the correction voltage must
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V+
8
V1
3
V2
2
+
7
LTC2053
0.1μF
RG
REF 6
5
–
EN
1
VO
R2
4
VREF
R1
V–
VO = (V1 – V2) 1 + R2 + VREF
R1
Figure 1. Typical Connection of
LTC2053 Instrumentation Amplifier
be capable of either sourcing or sinking the feedback
resistor current that flows. As the LT1025 only sources
current, a precision buffer can be used to drive the REF
node (e.g. using an LTC2050 zero-drift op amp). The
limitation imposed by using a single supply is that both
the probe and amplifier unit temperatures must be above
0°C for valid output. If negative temperatures must be
accommodated, a simple charge-pump inverter, such as
an LTC1046, can be used to develop a minus supply rail.
The excellent PSRR of the LTC2053 precludes the need
for regulated power supplies, and the additional design
and space expense they entail. Four AA alkaline cells
supply the ICs in this circuit with 3.5V to 5V, depending
on state of charge, yielding a minimum full scale output
of 350°C. The total battery draw is typically only 1.8mA.
In a conventional line-powered application, one can use
a single LT1025 and buffer amplifier to correct several
LTC2053 thermocouple amplifier channels, provided all
the thermocouple connections and the LT1025 thermally
track.
Filtering and Protection
Since the LTC2053 operates by sampling the input signal,
the frequencies of interest are generally below a few
hundred Hz so it is useful to rolloff the amplifier response
by adding 0.1μF in the feedback circuit. The capacitors in
the thermocouple input network help absorb RF pickup
and suppress sampling artifacts from appearing on the
thermocouple leads. The resistors connected to the
thermocouple provide a high impedance bias of VS/2
to maximize common mode immunity without inducing
voltage drops in the leads. For short thermocouple lead
lengths, which minimize common-mode signals, the probe
junction may be grounded (note that with split supplies,
grounding would be optimal). The 5.1V Zener is used to
provide fault-induced supply overvoltage and reversebattery protection in conjunction with the 560Ω ballast.
VS
10M
10M
1M
1M
0°C–350°C
TYPE K
THERMOCOUPLE
40.6mV/°C
3
10k
10k
–
ORANGE
0.001μF
0.1μF
8
+
YELLOW
+
7
RG
REF 6
5
0.1μF
LTC2053
2
0.001μF
THERMAL
COUPLING
–
EN
1
4
10mV/°C
51Ω
249k
1%
100Ω
SCALE FACTOR
TRIM
VS
VS
2
1.00k
1%
6
4
0.1μF
LTC2050
LTC1025
3
3
200k
4
VS
–
1
+
560Ω
6V
+
–
CMPZ5231B
5.1V
2
5
Figure 2. Complete Schematic of the Thermocouple Amplifier
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