CN-0300 Datasheet, PDF (5/7 Pages) Analog Devices – Complete Closed-Loop Precision Analog Microcontroller Thermocouple

Circuit Note
VLOOP+
DAC
NPN
BC548
RLOOP
47Ω
VLOOP–
ADC0
AIN9
AIN8
(BUFFERED V_REF)
100kΩ
100kΩ
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).
CN-0300
circuit performance when the initial calibration is performed
and when using the closed-loop control of the VDAC output
resultsin temperature valuesof0.5°C being reportedbythe DAC
output circuit. Nonlinearity errorsfrom the DAC andthe external
transistor circuit are adjustedout thanks to the 24-bit ADC.
Because temperature is a slow changing input parameter,this
closed scheme is ideal for this application. Figure 11 shows the
ideal DAC outputin blue andthe real DAC output with no closed-
loop control (ADC0 is not used to compensate the DAC output).
The error can be >10°C without closed-loop control.
25
20
15
ACTUAL CURRENT
10
5
IDEAL CURRENT
0
–200 –150 –100 –50 0 50 100 150 200 250 350
TEMPERATURE (°C)
Figure 11. Temperature in °C vs. Current Out in mA (Blue = Ideal Value, Open
Loop Operation: DAC Output Uncompensated)
Figure 10. UART Strings Used for Debug
COMMON VARIATIONS
For a standardUART-to-RS-232interface,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 asa Type J. 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
temperature sensor canbe used. Forexample, the ADT7410can
connect to the ADuCM360 via the I²C interface.
For more details on cold junction compensation, referto Sensor
Signal Conditioning, Analog Devices, Chapter 7, “Temperature
Sensors.”
If isolation between the USB connector and this circuitis required,
the ADuM3160/ADuM4160 isolation devicesmustbe added.
CIRCUIT EVALUATION AND TEST
Current Output Measurements
The DAC and external-voltage-to-current-convertor circuit
performance testswere all completed together.
A currentmeter was placed in serieswiththe VLOOP+ connection,
as shown in Figure 1. The meter used wasa HP 34401A. The
Figure 12 shows the same information when the closed-loop
control is used as is recommended. The error is tiny, less than
0.5°C from the ideal value.
25
20
15
ACTUAL CURRENT
IDEAL CURRENT
10
5
0
–200
–100
0
100
200
300
400
TEMPERATURE (°C)
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 basic test setup is shown in Figure 13. The thermocouple is
connected to J2.
Two methodswere used to evaluate the performance of the circuit.
Initially, the circuit was tested with the thermocouple attached
to the board,andit wasusedto measure the temperature of an ice
bucket. Then, it was used to measure the temperature of boiling
water.
Rev. A | Page 5 of 7