ADuC7061 and external RTD to build a USB-based temperature monitor

Circuit Description

　　The circuit shown in Figure 1 is completely powered by the USB interface. Using 2.5 V low dropout linear regulator ADP3333 can be USB interface provides 5 V power supply adjusted to 2.5 V, and then to provide ADuC7061 DVDD voltage.ADuC7061 the AVDD power supply through additional filtering, as shown. Input terminal linear regulator also place a filter on the USB power supply filtering.

　　This application uses the following characteristics ADuC7061 of:

　　• Built-in programmable gain amplifier (PGA) of 24 Σ-Δ type main ADC: PGA gain setting of 32 in this application software.

　　• Programmable excitation current source, used to force the controlled current through the RTD: Dual-channel current source may be within the range of 0 μA to 2 mA to 200 μA step configuration. The case is set to 200 μA.

　　External reference voltage source • ADuC7061 in ADC: For this application, we use the ratio of type setting, an external reference resistor (RREF) connected to the external VREF + and VREF- pins. Alternatively, you can provide 1.2 V internal reference voltage source in the ADuC7061.

　　• ARM7TDMI® kernel: 16/32 powerful ARM7 core with integrated 32 kB Flash memory and SRAM memory, used to run user code, you can configure and control the ADC, the ADC conversion process by RTD, and control UART / USB communication interface.

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

　　• Two external switches are used to force the device into flash boot mode: allows S1 is low, while the switch S2, ADuC7061 will enter boot mode, instead of the normal user mode. In boot mode, the internal flash memory can be reprogrammed via the UART interface.

　　This circuit is used for RTD 100 Ω platinum RTD, model Enercorp PCS 1.1503.1. It uses a 0805 surface mount package, the temperature change rate of 0.385 Ω / ° C.

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

　　ADuC7061 USB interface to USB transceiver is achieved by FT232R UART, USB signal will be directly converted to UART.

　　In addition to decoupling the outside as shown in Fig, USB cable itself but also to enhance the use of a ferrite bead EMI / RFI protection. The circuit used ferrite beads for Taiyo Yuden # BK2125HS102-T, its impedance at 100 MHz to 1000 Ω.

　　The circuit must be constructed on a multilayer circuit board with a large area ground plane. To achieve optimal performance, you must use the appropriate layout, grounding, and decoupling techniques