DN5 - Temperature Measurement Using Data Acquisition Systems

Temperature Measurement Using Data Acquisition Systems
Design Note 5
William Rempfer and Guy Hoover
Introduction
Accurate temperature measurement is a difficult and
very common problem. Whether recording a temperature, regulating a temperature or modifying a process
to accommodate a temperature, the LTC®1090 family
of data acquisition systems can provide an important
link in the chain between the blast furnace temperature
and the microcontroller. Features of the LTC1090 family
can make temperature measurement easier, cheaper
and more accurate.
High DC input resistance and reduced span operation
allow direct connection to many standard temperature
sensors. Multiplexer options allow one chip to measure
up to 8 channels of temperature information. Single
supply operation, modest power requirements (~5mW)
and serial interfaces make remote location possible.
Switching power on and off lowers power consumption (560μW) even more for battery applications.
Finally, because few sensors have accuracies as good
as 0.1%, the 10-bit resolution and 0.05% accuracy of
the LTC1090 family are just right for most temperature
sensing applications.
Thermocouple Systems
The circuit of Figure 1 measures exhaust gas temperature in a furnace. The 10-bit LTC1091A gives 0.5°C
resolution over a 0°C to 500°C range. The LTC1052
amplifies and filters the thermocouple signal, the
LT1025A provides cold junction compensation and
the LT1019A provides an accurate reference. The J
type thermocouple characteristic is linearized digitally
inside the MCU. Linear interpolation between known
temperature points spaced 30°C apart introduces less
than 0.1°C error. The code for linearizing is available
from LTC. The 1024 steps provided by the LTC1091 (24
more than the required 1000) insure 0.5°C resolution
even with the thermocouple curvature.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
9V
2
2
–
8
0.1μF
6
+
10μF
VIN
4
J LT1025A
1N4148
GND COMMON
4
5
J TYPE
LT1019A-5
+
3
+
1μF
LTC1050
2
–
1k
0.1%
LTC1091A
7
6
1μF
4
VCC
CH0
CLK
TO
MCU
CH1 DOUT
10k
GND
DIN
DN005 F01
0.33μF
3.4k
1%
47Ω
CS
178k
0.1%
Figure 1. 0°C-500°C Furnace Exhaust Gas Temperature Monitor with Low Supply Detection
12/87/5_conv
Offset error is dominated by the LT1025 cold junction
compensator which introduces 0.5°C maximum. Gain
error is 0.75°C max because of the 0.1% gain resistors
and to a lesser extent the output voltage tolerance of
the LT1019A and the gain error of the LTC1091A. It may
be reduced by trimming the LT1019A or gain resistors.
The LTC1091A keeps linearity better than 0.25°C. The
LTC1050’s 5μV offset contributes negligible error (0.1°C
or less). Combined errors are typically 0.5°C or less.
These errors don’t include the thermocouple itself. In
practice, connection and wire errors of 0.5°C to 1°C
are not uncommon. With care, these errors can be
kept below 0.5°C.
(REF+ pin) of the LTC1090 are set by the precision resistor string to directly digitize the roughly 0.2V to 1V
sensor output. The LT1006 buffers the 10kΩ reference
resistance of the LTC1090. 0°C and 100°C correspond
to unipolar output codes of 0 and 1000 (decimal), respectively with an overrange of 102.3°C.
The 20k/10k divider on CH1 of the LTC1091 provides
low supply voltage detection (the LT1019A reference
requires a minimum supply of 6.5V to maintain accuracy). Remote location is easy, with data transferred
from the MCU to the LTC1091 via the 3 wire serial port.
Thermistors
A thermistor is a cheaper alternative to thermilinear
components in narrower temperature range applications. In Figure 2, CH7 is being used to digitize the
output of a 5kΩ thermistor. The resistor shown linearizes
the output voltage around the 30°C point. The output
remains linear to 0.1°C over a 20°C to 40°C range but
gets nonlinear rapidly outside this range. By correcting for the non-linearity in software this range can be
extended to 0°C to 60°C. Beyond that, the repeatability
error of the thermistor increases above 0.2°C making
correction difficult.
Thermilinear Networks
Figure 2 shows an 8 channel 0°C to 100°C temperature
measurement system with 0.1°C resolution. The high
DC input resistance and adjustable span of the LTC1090
allow it to measure the outputs of the YSI thermilinear
components directly. Accuracy is limited by the sensor
repeatability and precision resistors to 0.25°C.
Silicon Sensors
Because of its high DC input impedance and reduced
span capability, the LTC1090 family can directly measure the output of most industry standard, silicon
temperature sensors, both voltage and current mode.
Popular sensors of this type include the LM134 and
AD590 (current output) and silicon diodes.
Sensor input voltage (VIN), not critical because of ratiometric operation, is set to around 1.5V to minimize
self heating. The zero scale (COM pin) and full-scale
Figure 3 shows a simple connection between the
LTC1092 and industry standard 1μA/°K current output
sensors. Resolution is 0.25°C and accuracy is limited
by the sensor and resistors. Standard 10mV/°K voltage
output sensors can also be connected directly to the
LTC1092 input in a similar manner.
5V
4.7μF
2N3904
10k ±10%
YSI 44201
0°C-100°C
15k ±10%
CH0
VCC
CH1
ACLK
CH2
SCLK
CH3 LTC1090 DIN
YSI 44201
5k AT 25°C
20°C-40°C
2954Ω
5000Ω
CH4
TO
MCU
DOUT
CH5
CS
CH6
REF+
CH7
REF–
COM
V–
DGND
4.7μF 5V
9V
LM134 OR OTHER
1μA/°K SENSOR
+
AGND
CS
–
www.linear.com
Linear Technology Corporation
TO
MCU
VCC
SCLK
DOUT
DN005 F03
DN005 F02
Figure 2. 0°C-100°C 0.25°C Accurate Thermistor
Based Temperature Measurement System
Data Sheet Download
+
–
GND VREF
1491Ω
YISI 44007 OR 44034
3Ω
LTC1092
226Ω
LT1006
11.5k
4562Ω
10μF
LT1019-2.5
Figure 3. –55°C to +125°C Thermometer Using
Current Output Silicon Sensors
For applications help,
call (408) 432-1900
dn5f_conv IM/GP 1287 160K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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© LINEAR TECHNOLOGY CORPORATION 1987
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