CN0221 Datasheet, PDF (2/5 Pages) Analog Devices – Devices Connected

CN-0221
CIRCUIT DESCRIPTION
The following features of the ADuCM360/ADuCM361 are used
in this application:
 A 24-bit Σ-Δ ADC with a PGA set for a gain of 32 in the
software for the thermocouple and RTD. The ADC1 was
switched continuously between sampling the thermocouple
and the RTD voltages.
 Programmable excitation current sources for forcing a
controlled current through the RTD. The dual current
sources are configurable in from 0 μA to 2 mA. For this
example, a 200 μA setting was used to minimize the error
introduced by the RTD self-heating.
 An internal 1.2 V reference for the ADC in the ADuCM360/
ADuCM361. It measures the thermocouple voltage; the
internal voltage reference was used due to its precision.
 An external voltage reference for the ADC in the
ADuCM360/ADuCM361. It measures the RTD resistance;
a ratiometric setup was used where an external reference
resistor (RREF) was connected across the external VREF+
and VREF− pins.
 A bias voltage generator (VBIAS). The VBIAS function was
used to set the thermocouple common-mode voltage to
AVDD/2.
 The ARM Cortex-M3 core. The powerful 32-bit ARM core
with integrated 128 kB flash and 8 kB SRAM memory runs
the user code that configures and controls the ADC, processes
the ADC conversions from the RTD, and controls the
communications over the UART/USB interface.
 The UART was used as the communication interface to the
host PC.
 Two external switches are used to force the part into its
flash boot mode. By holding SD low and toggling the RESET
button, the ADuCM360/ADuCM361 enters boot mode
instead of normal user mode. In boot mode, the internal
flash can be reprogrammed through the UART interface.
Both the thermocouple and the RTD generate very small signals;
therefore, a PGA is required to amplify those signals.
The thermocouple used in 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 was used for cold junction 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.
Circuit Note
Note that the reference resistor, RREF, should be a precision
5.6 kΩ (±0.1%).
The USB interface to the ADuCM360/ADuCM361 is
implemented with an FT232R UART to USB transceiver, which
converts USB signals directly to the UART.
In addition to the decoupling shown Figure 1, the USB cable itself
must have a ferrite bead for added EMI/RFI protection. The ferrite
beads used in the circuit were Taiyo Yuden, #BK2125HS102-T,
which have an impedance of 1000 Ω at 100 MHz.
Construct the circuit on a multilayer printed circuit board (PCB)
with a large area ground plane. Use proper layout, grounding,
and decoupling techniques to 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 ADuCM360TCZ Evaluation Board layout).
The PCB used for evaluating this circuit is shown in Figure 2.
Figure 2. EVAL-ADuCM360TCZ Board Used for this Circuit
Rev. C | Page 2 of 5