AN929 Datasheet, PDF (12/22 Pages) Microchip Technology – Temperature Measurement Circuits for Embedded Applications

AN929
The microcontroller computes the actual temperature
by subtracting the cold junction temperature from the
temperature determined from the thermocouple
amplifier. The cold junction temperature is measured
with a TC1047A silicon IC analog output sensor that is
located on the PCB. The actual cold junction occurs
where the thermocouple wires meet the copper wires,
which is typically at a connector. Placing the TC1047A
adjacent to the connector can minimize the cold
junction error (T_{CJC_Location}) that results by placing the
temperature sensor on the PCB. The voltage output of
the TC1047A is listed below.
V_OUT = [(Temp. (°C)) x (10 mV/°C)] + 500 mV
The TC1047A provides an output voltage of 10 mV/°C
with an offset of 500 mV.
The accuracy of the thermocouple amplifier and cold
junction circuits were estimated to be ±5.4°C by using
the root-squared-sum (RSS) equation. The common
mode voltage noise signal entering the circuit from the
thermocouple leads was assumed to be equal to 10V.
It is important to use tight tolerance resistors for the
differential amplifier in order to achieve a high CMRR.
0.1% resistors were chosen for R₃, R₄, R₅ and R₆. The
analysis shows that the majority of the measurement
error is due to the error of the cold junction
compensation.
CMRR
Resistors
≈
-¹----+-----^A----^m----^p---^l--ⁱ-^f--ⁱ--^e--^r----^G----^a---ⁱ--ⁿ-
4 × Tolerance
=
---¹----+-----²---⁴---⁹----
4 × 0.001
=
62500
=
95.9db
Error CMRR Resistors
=
-----^V----^C----^M-----^–---^N----^O---^I--^S---^E------
CMRR Resistors
=
---¹---⁰---^V-----
62500
=
160µV ≅ 4°C
CMRR Amplifier (TC913A) = 116db(typ.)
Noise Attenuation = ---------¹---------
630,957
Error CMRR Amplifier (TC913A)
=
------------------^V----^C----^M-----^–---^N----^O---^I--^S---^E-------------------
CMRR Amplifier (TC913A)
=
-----¹---⁰----^V------
630,957
=
15.8µV ≅ 0.4°C
V_OS error =
--------^V----^O----^S----⁽--^T---^C----⁹---¹---³---^A----⁾-------
Temp. Coef. (type K)
=
-₄---₀--¹-_µ--⁵--_V-^µ----_⁄^V--_°---_C--- ≅ 0.4°C
T_{CJC_Sensor} (TC1047A) error = 3°C (max.)
T_{CJC_Location}error = 2°C (est.)
Total error_{Worst Case} = CMRR_RESISTORS + CMRR_{AMPLIFIER (TC913A)} + V_OS + T_{CJC_Sensor} + T_{CJC_Location}
= 4 + 0.4 + 0.4 + 3 + 2 = 9.8°C
Total error_RSS = (4)² + (0.4)² + (0.4)² + (3)² + (2)² = 5.4°C
DS00929A-page 12
 2004 Microchip Technology Inc.