CN-0221

Circuit Note

Rev. C | Page 4 of 5

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

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

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 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.

EVAL-ADuCM360TCZ

WAVETEK 4808

MULTIFUNCTION

CALIBRATOR

PC

J5

J1

AIN7/VBIAS

THERMOCOUPLE

JUNCTION

SEE TEXT

USB

CABLE

Figure 7. Test Setup Used to Calibrate and Test the Circuit Over Full

Thermocouple Output Voltage Range