AN929 Datasheet, PDF (13/22 Pages) Microchip Technology – Temperature Measurement Circuits for Embedded Applications

AN929
SINGLE POWER SUPPLY CIRCUIT
Figure 19 provides a low-cost, single power supply
thermocouple amplifier circuit using a quad op amp.
The buffered input differential amplifier topology is
similar to an instrumentation amplifier and offers the
feature of equal and high input impedance at the ampli-
fier inputs. An instrumentation amplifier with integrated
gain resistors can also be used to implement this cir-
cuit. The gain of the amplifier was selected to be 249,
providing a temperature coefficient of 10 mV/°C. The
thermocouple inputs are biased to V_DD/2 through
10 MΩ resistors, providing the ability to detect a failed
open-circuit thermocouple.
Connector
Type K
Shielded
Thermocouple ^Cable
R₃
R₁
EMI Filter
C₂
EMI Filter
+5V ^C1
R₄
+5V
R₂
U_1A
U_1B
V_IN1 ^R5
V_IN2
R₆
R₈
R₇
+5V
C₃
U_1C
MCP619
IN_1
ADC
R₁ = R₂ = 1 MΩ
R₃ = R₄ = 10 MΩ
R₅ = R₆ = 1 kΩ
R₇ = R₈ = 249 kΩ
C₁ = C₂ = 1 nF
C₃ = 0.1 µF
TC1047A
Cold Junction Compensation
IN_2
V_OUT
=
(V_IN2 – V_IN1 )_{ R}-^R---₅-⁷-_
+ V_REF
=
(
∆V_I
N
)


2_---1-4_---9k_---k_{-}
Temp. Coef. = 249 × 40µV ⁄ °C ≅ 10mV ⁄ °C
FIGURE 19:
Schematic of Single Supply Voltage Thermocouple Amplifier.
RTD Oscillator Circuits
Oscillator circuits can be used to provide an accurate
temperature measurement with an RTD sensor. The
state variable oscillator provides an output frequency
that is proportional to the square root of the product of
two temperature-sensing resistors and is a good circuit
for precision applications. The astable multi-vibrator or
relaxation oscillator provides a square wave output
with a single amplifier and is a good alternative for
cost-sensitive applications.
The components must be chosen carefully so that the
change in the oscillation frequency results primarily
from the RTD and not from variation due to the compo-
nent tolerance, temperature coefficient and drift rate.
Metal film resistors, metal foil resistors and NPO
porcelain capacitors are recommended to minimize the
component error. Capacitors are relatively poor in
performance when compared to resistors. Typically, the
capacitor limits the accuracy of the oscillator. Further-
more, precision capacitors are only available in
relatively small values. The state variable oscillator
requires two 100 nF capacitors, while the relaxation
oscillator uses a 0.68 µF capacitor to produce a nomi-
nal oscillation of 1 kHz. The state variable and relax-
ation circuits have an uncalibrated measurement
accuracy of approximately 1°C and 3°C, respectively.
The difference is primarily due to the capacitor error.
An application that requires an accuracy of better than
±1°C may require a temperature calibration and burn-
in procedure. A temperature compensation algorithm
can easily be implemented using the E² non-volatile
memory of a microcontroller to store temperature
correction data in a look-up table. The temperature
coefficients are obtained by calibrating the circuit over
temperature and comparing the measured temperature
against the actual temperature. A burn-in or tempera-
ture-cycling procedure can significantly reduce the drift
of the resistors and capacitors. Burn-in procedures are
useful because the majority of the change in magnitude
of resistors and capacitors occurs within the 500 hours
of a life test.
 2004 Microchip Technology Inc.
DS00929A-page 13