Circuit Note

CN-0300

Rev. A | Page 5 of 7

Figure 9. Closed-Loop Control 4 mA to 20 mA DAC Output

For debug purposes, the following strings are sent to the UART

during normal operation (see Figure 10).

Figure 10. UART Strings Used for Debug

COMMONVARIATIONS

For a standardUART-to-RS-232interface,theFT232R transceiver

can be replaced with a device such as the ADM3202, which

requires a 3 V power supply. For a wider temperaturerange, a

different thermocouple can beused, such asa TypeJ. To minimize

the cold junction compensationerror,a thermistorcan be placedin

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

temperaturesensor canbeused. Forexample, theADT7410can

For more details on cold junction compensation, referto Sensor

Signal Conditioning, Analog Devices, Chapter 7, “Temperature

Sensors.”

If isolation between theUSBconnector and this circuitis required,

the ADuM3160/ADuM4160 isolation devicesmustbe added.

CIRCUIT EVALUATIONANDTEST

Current Output Measurements

The DAC and external-voltage-to-current-convertor circuit

performance testswereall completed together.

A currentmeter was placed in serieswiththeVLOOP+connection,

as shown in Figure 1. The meter used wasa HP 34401A. The

circuit performance when the initial calibration is performed

and when using the closed-loop control of the VDAC output

resultsin temperaturevaluesof0.5°Cbeing reportedbytheDAC

output circuit. Nonlinearity errorsfromtheDACandtheexternal

transistor circuit are adjustedout thanks to the 24-bit ADC.

Because temperatureis a slow changing input parameter,this

closed scheme is ideal for this application. Figure 11 shows the

ideal DAC outputin blue andthereal DACoutput with noclosed-

loop control (ADC0 is not used tocompensatetheDACoutput).

The error can be >10°C without closed-loop control.

Figure 11. Temperature in °C vs. Current Out in mA (Blue = Ideal Value, Open

Loop Operation: DAC Output Uncompensated)

Figure 12 shows the sameinformation when the closed-loop

control is used as is recommended. The error is tiny, less than

0.5°C from the ideal value.

Figure 12.Temperature in °C vs. Current out in mA (Blue = Ideal Value,

Closed-Loop Operation: DAC Output Compensated by ADC0 Measurement

Thermocouple Measurement Test

The basictest setup is shown in Figure 13. The thermocoupleis

connected to J2.

Two methodswere used toevaluatetheperformanceof thecircuit.

Initially, the circuit was tested with the thermocoupleattached

to the board,andit wasusedtomeasurethe temperatureof an ice

bucket. Then, it was used to measurethe temperature of boiling

water.

100kΩ

100kΩ

RLOOP

47Ω

VLOOP+

VLOOP–

AIN9

AIN8

DAC

ADC0

NPN

BC548

0

–200 –150

–100

–50

0

TEMPERATURE (°C)

50

100

150

200

250

350

5

10

15

20

25

IDEAL CURRENT

ACTUAL CURRENT

0

5

10

15

20

25

–200

–100

0

100

200

300

400

ACTUAL CURRENT

IDEAL CURRENT

TEMPERATURE (°C)