Temperature monitor built using ADuC7061 and external RTD based on USB power supply

 　　As shown in the figure below, the circuit is completely powered by the USB interface . The ADP3333 2.5 V low dropout linear regulator can be used to regulate the 5 V power supply provided by the USB interface to 2.5 V, thereby providing the DVDD voltage to the ADuC7061. The AVDD power supply of the ADuC7061 is additionally filtered, as shown in the figure below. A filter is also placed at the input end of the linear regulator to filter the USB power supply.

　　The following features of the ADuC7061 are used in this application:

　　1. 24-bit Σ-Δ main ADC with built-in programmable gain amplifier (PGA): The gain of the PGA is set to 32 in the software of this application.

　　2. Programmable excitation current source to force controlled current through the RTD: Dual-channel current source configurable in 200 μA steps from 0 μA to 2 mA. This example is set to 200μA.

　　3. External reference voltage source for the ADC in the ADuC7061: For this application, we use a ratiometric setup with an external reference resistor (RREF) connected to the external VREF+ and VREF- pins. Alternatively, a 1.2 V internal reference can be provided in the ADuC7061.

　　4. ARM7TDMI? Core: The powerful 16/32-bit ARM7 core integrates 32 kB flash and SRAM memory to run user code, configure and control the ADC, handle ADC conversion through RTD, and control UART/USB interface communication .

　　5.UART: UART is used as a communication interface with the PC host.

　　6. Two external switches are used to force the device into flash boot mode: by holding S1 low and toggling S2 at the same time, the ADuC7061 will enter boot mode instead of normal user mode. In boot mode, the internal flash memory can be reprogrammed via the UART interface.

　　The RTD used in this circuit is a 100 Ω platinum RTD, model number Enercorp PCS 1.1503.1. It is in a 0805 surface mount package and has a temperature change rate of 0.385 Ω/°C.

　　Note that the reference resistor RREF should be a precision 5.62 kΩ (±0.1%) resistor.

The USB interface　　of the ADuC7061 is implemented through the FT232R UART to USB transceiver, which converts USB signals directly to UART.

　　In addition to the decoupling shown in the image below, the USB cable itself should use ferrite beads for enhanced EMI/RFI protection. The ferrite beads used in this circuit are Taiyo Yuden #BK2125HS102-T, which has an impedance of 1000 Ω at 100 MHz.

　　This circuit must be built on a multilayer circuit board with a large area ground plane. For optimal performance, proper layout, grounding, and decoupling techniques must be used.