AN929 Datasheet, PDF (9/22 Pages) Microchip Technology – Temperature Measurement Circuits for Embedded Applications

AN929
RTD
V_REF
R₁
R₂
Sensor BIT
Dual Power Supply Amplifier Single Power Supply Amplifier
+V
+V
HC14
R₂ >> R₁
Thermocouple
Thermocouple
Instrumentation
Amplifier
-V
FIGURE 15:
Remote Sensor Fault Detection Circuits.
Amplifier Selection Criteria
Two key op amp specifications in a sensor amplifier are
V_OS and CMRR. V_OS is important in sensor applica-
tions when the input signal is of the same magnitude as
V_OS, while the CMRR ratio is critical in reducing the
noise signal induced on the long wires of a remote
sensor. Other op amp specifications, such as the AC
frequency characteristics are less important because
the frequency content of a temperature sensor is
typically less than 100 Hz. The op amps used in
oscillators are relatively immune to DC specifications
such as V_OS. The important parameters for oscillators
are the amplifier’s frequency response and the Gain
Bandwidth Product (GBWP). The op amp’s GBWP
should be at least a factor of 100 times the maximum
oscillation frequency.
SENSOR SELECTION CRITERIA
Table 1 provides a summary of the attributes of
thermocouples, RTDs, thermistors and silicon IC
sensors.
TABLE 1:
ATTRIBUTES OF THERMOCOUPLES, RTDS, THERMISTORS AND SILICON IC
SENSORS
Attribute
Thermocouple (type K)
RTD
Thermistor
Silicon IC
Range
Temperature (t)
Accuracy
Output Signal
Linearity
Precision
Durability
Thermal
Response Time
Cost
Interface Issues
-184°C to 1260°C
Greater of ±2.2°C or
±0.75%
40 µV/°C
Fair
Fair
Good at lower temp.,
Poor at high temp, Open-
circuit vibration failures
Fast (function of probe
material)
Low
Cold junction
compensation, Small ∆V
-200°C to 850°C
-55°C to 150°C
Class B = ± [0.012 + Various,
(0.0019 |t|) − 6 X 10^-7t²] ±0.5°C to 5°C
≈ 0.00385 Ω /Ω/°C
≈ 4% ∆R/°C
for 0°C ≤ t ≤ 70°C
Excellent
Poor
Excellent
Poor
Good, Wire wound
prone to open-circuit
vibration failures
Good, Power
derated with
temperature
Fast (function of probe Moderate
material)
Wire wound – High, Low
Thin-film – Moderate
Small ∆R/°C
Non-linear
resistance
-55°C to 125°C
Various,
±0.5°C to 4°C
Analog, Serial,
Logic, Duty Cycle
Good
Fair
Excellent
Slow
Moderate
Sensor located on
PCB
 2004 Microchip Technology Inc.
DS00929A-page 9