CN-0221 Datasheet, PDF (4/5 Pages) Analog Devices – USB-Based Temperature Monitor Using the ADuCM360 Precision Analog Microcontroller

CN-0221
Figure 6 shows the error obtained when using ADC1 on the
ADuCM360 to measure 52 thermocouple voltages over the full
thermocouple operating range. The overall worst-case error
is <1°C.
0.5
0.4
0.3
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
–210 –140 –70
0
70 140 210 280 350
TEMPERATURE (°C)
Figure 6. Error When Using Piecewise Linear Approximation Using
52 Calibration Points Measured by ADuCM360/ADuCM361
The RTD temperature is calculated using lookup tables and is
implemented for the RTD the same way as for the thermocouple.
Note that the RTD has a different polynomial describing its
temperatures as a function of resistance.
For details on linearization and maximizing the performance of
the RTD, refer to Application Note AN-0970, RTD Interfacing
and Linearization Using an ADuC706x Microcontroller.
COMMON VARIATIONS
The ADP1720 regulator can be replaced with the ADP120, which
has the same operating temperature range (−40°C to +125°C)
and consumes less power (typically 35 µA vs. 70 µA) but has a
lower maximum input voltage. Note that the ADuCM360/
ADuCM361 can be programmed or debugged via a standard
serial wire interface.
For a standard UART to RS-232 interface, the FT232R transceiver
can be replaced with a device such as the ADM3202, which
requires a 3 V power supply. For a wider temperature range, a
different thermocouple can be used, such as a Type J. To minimize
the cold junction compensation error, a thermistor can be placed in
contact with the actual cold junction instead of on the PCB.
Instead of using the RTD and external reference resistor for
measuring the cold junction temperature, an external digital
temperature sensor can be used. For example, the ADT7410 can
connect to the ADuCM360/ADuCM361 via the I²C interface.
For more details on cold junction compensation, refer to Sensor
Signal Conditioning, Analog Devices, Chapter 7, “Temperature
Sensors.”
If isolation between the USB connector and this circuit is required,
the ADuM3160/ADuM4160 isolation devices must be added.
Circuit Note
CIRCUIT EVALUATION AND TEST
To test and evaluate the circuit, the thermocouple measurements
and the RTD measurements were evaluated separately.
Thermocouple Measurement Test
The basic test setup is shown in Figure 7. The thermocouple is
connected to J5, and Jumper J1 must be installed to allow the
AIN7/VBIAS pin to set the thermocouple common-mode
voltage. The circuit board receives its power from the USB
connection to the PC.
Two methods were used to evaluate the performance of the
circuit. Initially, the circuit was tested with the thermocouple
attached to the board and it was used to measure the temperature
of an ice bucket. Then, it was used to measure the temperature
of boiling water.
A Wavetek 4808 Multifunction Calibrator was used to fully
evaluate the error, as shown in Figure 4 and Figure 6. In this
mode, the thermocouple was replaced with the calibrator as the
voltage source, as shown in Figure 7. To evaluate the entire range
of a Type T thermocouple, the calibrator was used to set the
equivalent thermocouple voltage at 52 points between −200°C
to +350°C for the negative and positive ranges of the T-type
thermocouple (see the ISE, Inc., ITS-90 Table for Type T
Thermocouple).
To evaluate the accuracy of the lookup algorithm, 551 voltage
readings, equivalent to temperatures in the range of −200°C to
+350°C spaced at +1°C, were passed onto the temperature
calculation functions. Errors were calculated for the linear
method and the piecewise linear approximation method as is
shown in Figure 4 and Figure 5.
THERMOCOUPLE
JUNCTION
EVAL-ADuCM360TCZ
J5
SEE TEXT
WAVETEK 4808
MULTIFUNCTION
CALIBRATOR
J1
AIN7/VBIAS
USB
CABLE
PC
Figure 7. Test Setup Used to Calibrate and Test the Circuit Over Full
Thermocouple Output Voltage Range
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