USB-Based Temperature Monitor Using the ADuC7061 and External RTD

　Circuit Description

　　The circuit shown in Figure 1 is powered entirely from the USB interface. The 5 V power from the USB interface is regulated to 2.5 V using the ADP3333 2.5 V low dropout linear regulator to provide the DVDD voltage to the ADuC7061. The AVDD power supply of the ADuC7061 is additionally filtered as shown. A filter is also placed at the input of the linear regulator to filter the USB power.

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

　　• 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 for this application.

　　• Programmable excitation current source to force a controlled current through the RTD: The dual-channel current source can be configured from 0 μA to 2 mA in 200 μA steps. In this example, it is set to 200 μA.

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

　　•ARM7TDMI® core: A powerful 16/32-bit ARM7 core with 32 kB of Flash and SRAM memory runs the user code that configures and controls the ADC, processes the ADC conversions via the RTD, and controls the communications over the UART/USB interface.

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

　　• Two external switches are used to force the device into flash boot mode: By holding S1 low and switching S2, the ADuC7061 enters boot mode instead of normal user mode. In boot mode, the internal flash can be reprogrammed through the UART interface.

　　The RTD used in this circuit is a 100 Ω platinum RTD, model Enercorp PCS 1.1503.1. It comes 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.

　　The USB cable itself should also use ferrite beads to enhance EMI/RFI protection in addition to the decoupling shown in Figure 1. The ferrite bead used in this circuit is Taiyo Yuden #BK2125HS102-T, which has an impedance of 1000 Ω at 100 MHz.

　　This circuit must be constructed on a multilayer circuit board with a large area ground plane. Proper layout, grounding, and decoupling techniques must be used to achieve optimum performance.