Circuit Note

CN-0300

Rev. A | Page 4 of 7

When calibrating the DAC, connect the VLOOP+outputthrough

an accurate current meter.The first part of the DAC calibration

routine calibrates the DACto set a 4 mA output, and the second

part of the DAC calibration routinecalibrates the DAC to set a

20 mA output. The DAC code usedto set a 4 mA and 20 mA

output is stored to flash. Thevoltagemeasuredat AIN9 for the

final 4 mA and 20 mA settings is alsorecorded and saved toflash.

Because the voltageat AIN9 is linearly related to the current

flowing across RLOOP, these valuesareused to calculate the

adjustment factor for the DAC. This closed-loopschememeans

any linearity errors on the DAC and transistor based circuit are

fine-tuned out using the on-chip 24-bit Σ-Δ ADC.

The UART is configured for a baud rateof 9600, 8 data bits, no

parity, and no flow control. If the circuit is connected directly to

a PC, a communication port viewing application, such as

HyperTerminal, can be used to view the results sent by the

program to the UART, as shown in Figure 7.

To enter the charactersrequiredby the calibration routines,

type the required character in the viewing terminal and this

character will be received by the ADuCM360 UART port.

Figure 7. Output of HyperTerminal when Calibrating the DAC

Temperature Measurement Section of Code

To get a temperaturereading, measure the temperatureof the

thermocoupleand the RTD. TheRTDtemperatureis converted to

its equivalentthermocouplevoltagevia a look-uptable (seetheISE,

Inc., ITS-90 Tablefor TypeT Thermocouple). These twovoltages

are added together togivetheabsolutevalueatthethermocouple.

First, the voltage measured between the two wiresof the

thermocouple(V1). The RTD voltage is measured, converted to

a temperaturevia a look-up table, andthen, this temperatureis

converted to its equivalent thermocouplevoltage(V2). V1 and

V2 are then added to give the overall thermocouplevoltage, and

this is then converted to the final temperaturemeasurement.

For the thermocouple, temperatures fora fixednumber of voltages

are storedin an array. Temperaturevaluesin between arecalculated

using a linear interpolation between the adjacent points.

Figure 8 shows theerror obtained when using ADC1 onthe

ADuCM360 tomeasure52 thermocouplevoltagesover the full

thermocoupleoperating range. The overall worst-caseerror is

less than 1°C.

Figure 8. Error when Using Piecewise Linear Approximation Using

52 Calibration Points Measured by ADuCM360/ADuCM361

The RTD temperatureis calculated using lookup tablesand is

implementedfortheRTDthe sameway asforthethermocouple.

Note that the RTD has a different polynomial describing its

temperaturesas a function of resistance.

For details on linearization and maximizing the performance of

the RTD, refer to Application NoteAN-0970, RTD Interfacing

and Linearization Usingan ADuC706x Microcontroller.

Temperature-to-Current Output Section of Code

Once the final temperaturehasbeenmeasured, settheDACoutput

voltage to the appropriatevalue thatgives the requiredcurrent

to +350°C. The code sets the outputcurrent to 4 mA for −200°C

and 20 mAfor+350°C.Thecodeimplementsa closed-loopscheme,

as shown in Figure 9, where the feedbackvoltage on AIN9 is

measuredby ADC0, and this value is used to compensatethe

DAC outputsetting. The FineTuneDAC(void)function performs

this correction.

For best results, calibratetheDACbeforebeginning performance

testing of this circuit.

–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)