CN-0300 Datasheet, PDF (2/7 Pages) Analog Devices – Complete Closed-Loop Precision Analog Microcontroller Thermocouple

CN-0300
CIRCUIT DESCRIPTION
The following features of the ADuCM360 are used in this
application:
• The 12-bit DAC output with its flexible on-chip output
bufferis usedto control anexternal NPN transistor,BC548.
By controlling the VBE voltage of this transistor, the current
passing through a 47 Ω load resistor can be set to the
desired value. When NPN mode is selected,the buffered
on chip 1.2 V reference voltage is present on AIN8.
• The DAC is 12-bit monotonic; however, the accuracy of
the DAC output is typically around 3 LSBs. In addition, the
bi-polar transistor introduces linearity errors. To improve
the accuracy of the DAC output andto eliminate offset and
gain end-point errors, ADC0 measures, on AIN9, a
feedback voltage reflecting the voltage across the load
resistor (RLOAD). Based on this ADC0 reading, the DAC
output is corrected by the source code. This provides
±0.5°C accuracy on the 4 mA to 20 mA output.
• The 24-bit Σ-Δ ADC with a PGA set for a gain of 32 in the
software forthe thermocouple andthe RTD. ADC1 switches
continuously between sampling the thermocouple and the
RTD voltages.
• Programmable excitation current sources force a controlled
current through the RTD. The dual current sourcesare
configurable in steps from 0 µA to 2 mA. For this example,
a 200 µA setting is used to minimize the error introduced
by the RTD self-heating.
• An internal 1.2 V reference is providedfor the ADC in the
ADuCM360. When measuring the thermocouple voltage,
the internal voltage reference is used due to its precision.
• An external voltage reference forthe ADC in the ADuCM360.
When measuring the RTD resistance, a ratiometric setup
was used where an external reference resistor (RREF) was
connected acrossthe external VREF+ and VREF− pins.The
on-chip reference input bufferis enabled because the reference
source in thiscircuitis high impedance.The on-chipreference
buffermeansno external buffer is requiredto minimize input
leakage effects.
• A bias voltage generator(VBIAS). The VBIAS function is
used to set the thermocouple common-mode voltage to
AVDD_Reg/2 (900 mV). Again, this removes the need for
external resistors to set the thermocouple common-mode
voltage.
• The ARM Cortex-M3 core. The powerful 32-bit ARM core
with integrated 126 kB flash and 8 kB SRAM memory runs
the usercode thatconfiguresandcontrols the ADCsand
converts the ADC conversions from the thermocouple and
RTD inputs to a final temperature value. It also controls the
DAC output and continuously monitors this DAC output
using the closed-loop feedback from the voltage level on
AIN9. For extra debug purposes, it also controls the
communications over the UART/USB interface.
Circuit Note
• The UART is usedasthe communication interface to the host
PC. This is usedto program the on-chipflash.It is also used as
a debug port and for calibrating the DAC and ADC.
• Two external switches are used to force the part into itsflash
boot mode.By holding SD lowandtogglingthe RESET button,
the ADuCM360 entersboot mode insteadof normal user
mode.In bootmode,the internal flash can be reprogrammed
through the UART interface.
• The J1 connector, an 8-pin dual-in-line connector, connectsto
the Analog DevicesJ-Link OB emulator that is providedwith
the CN0300support hardware. This allows programming
and debugging of this application board. See Figure 3.
Both the thermocouple and the RTD generate very small signals;
therefore, a programmable gain amplifier (PGA) is requiredto
amplify those signals.
The thermocouple usedin this application is a Type T (copper-
constantan) that has a temperature range of −200°C to +350°C.
Its sensitivity is approximately 40 µV/°C, which means that the
ADC in bipolar mode, with a PGA gain of 32, can cover the
entire temperature range of the thermocouple.
The RTD wasusedfor coldjunction compensation. The particular
one used in this circuit was a platinum 100 Ω RTD, Enercorp
PCS 1.1503.1. It is available in a 0805, surface-mount package.
This RTD has a temperature variation of 0.385 Ω/°C.
Note that the reference resistor, RREF, must be a precision 5.6 kΩ
(±0.1%).
Construct the circuit ona multilayer printed circuitboard(PCB)
with a large area groundplane. Use properlayout, grounding,
and decouplingtechniquesto achieve optimum performance (see
Tutorial MT-031, Grounding Data Converters and Solving the
Mystery of "AGND" and "DGND," Tutorial MT-101, Decoupling
Techniques, and the ADuCM360TCZEvaluation Board layout).
The PCB used for evaluating this circuit is shown in Figure 2.
Figure 2. EVAL-CN0300-EB1Z Board Used for this Circuit
Rev. A | Page 2 of 7