CN-0221

Circuit Note

Rev. 0 | Page 2 of 5

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

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 126 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.

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