AN929 Datasheet, PDF (11/22 Pages) Microchip Technology – Temperature Measurement Circuits for Embedded Applications

AN929
SOT-23-5A
TC77
Mold Compound
Die Attach Adhesive
Die Attach Pad
Copper Lead
Die Gold Wire
FIGURE 17:
Cross-section of the TC77
Silicon IC Sensor.
Noise immunity can be provided to silicon sensors by
using a decoupling capacitor and a good PCB layout. A
0.1 µF to 1 µF decoupling capacitor should be inserted
between the power supply and ground pins. The PCB
should be designed with the standard layout guidelines
used for a low-noise circuit. The temperature sensor’s
main thermal path to the PCB is through the ground
connection. Thus, the size of the ground pad at the
sensor should be as large as possible. A good PCB
layout keeps high-frequency clock and switching power
supply PCB traces away from the sensor pins.
DESIGN EXAMPLES
Thermocouple Circuits
DUAL POWER SUPPLY CIRCUIT
Figure 18 shows a circuit that can be used for a remote
thermocouple-sensing application. A TC913A auto-
zeroed op amp was selected as the amplifier because
of its low V_OS of 15 mV (max.) and high CMRR of
116 dB (typ.). Auto-zero, chopper and instrumentation
amplifiers are good thermocouple amplifiers because
of their good V_OS and CMRR specifications. The
thermocouple inputs are tied to a positive and negative
supply through 10 MΩ resistors, so that the circuit can
detect a failed open-circuit thermocouple. The gain of
the amplifier was selected to be 249, which provides a
temperature coefficient (TC) of 10 mV/°C.
Type K
Thermocouple
Connector
Shielded
Cable
R₁ = R₂ = 10 MΩ
R₃ = R₄ = 1 kΩ
R₅ = R₆ = 249 kΩ
C₁ = C₂ = C₃ = 0.1 µF
+5V
R₅
R₁
EMI Filter
EMI Filter
R₂
+5V
V_IN1 ^R3
V_IN2
R₄
R₆
-5V
+5V
C₁
IN_1
TC913A
C₂
ADC
-5V
C₃
TC1047A _{Cold Junction Compensation} IN_2
V_OUT
=
(V_IN2 – V_IN1 )_{ R}-^R---₃-⁵-_
+ V_REF
=
(
∆V_I
N⁾


2_---1-4_---9k_---k_{-}
Temp. Coef. = 249 × 40µV ⁄ °C ≅ 10mV ⁄ °C
FIGURE 18:
Schematic of Dual Supply Voltage Thermocouple Amplifier.
 2004 Microchip Technology Inc.
DS00929A-page 11