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4 mA-to-20 mA Loop-Powered Temperature Monitor Using the ADuC7060/ADuC7061 Precision Analog Microcontroller (CN0145)
This circuit provides a simple highly integrated temperature monitor solution that can interface to a 4 mA-to-20 mA host controller. This circuit provides a very low cost solution to temperature monitoring because so much of the circuit functionality is integrated on the ADuC7060/ ADuC7061 precision analog microcontroller, including dual 24-bit Σ-Δ ADCs, the ARM7 processor core, and the DAC/PWM features for controlling the 4 mA-to-20 mA feedback circuit.
The performance of the ADCs and other analog circuits within the ADuC7060/ADuC7061 is superior to competitive microcontrollers that have integrated analog circuits. The circuit presented here represents the most cost-effective, lowest power, lowest board area solution compared to using discrete ADCs and a separate microcontroller. Because of its high level of integration and low power, the ADuC7060/ADuC7061 can operate directly from the loop power in the 4 mA-to-20 mA application. With the ARM7 core running at 640 kHz, the primary ADC active and measuring the external RTD tempera-ture sensor, and the PWM controlling the 4 mA-to-20 mA feedback circuit, the entire circuit typically consumes 3.15 mA. Further details on the power consumption are provided in the Circuit Description section.
Figure 1. ADuC7061 Controlling 4 mA-to-20 mA Loop-Based Temperature Monitor Circuit. (Simplified Schematic: All Connections and Decoupling Not Shown)
During the intervals between temperature measurements, the ADC and the RTD excitation current sources can be turned off to further reduce power.
The 100 Ω Pt RTD is an Enercorp #PCS11503.1. The complete circuit meets a better than ±1°C specification. The temperature range of this RTD is −50°C to +130°C. The RTD is available in a 1206 size SMD package and has a temperature coefficient of 0.385 Ω/°C.
The primary ADC within the ADuC7060/ADuC7061 has a peak-to-peak noise-free code resolution of greater than 18 bits. The PWM based DAC output provides 12 effective bits of resolution. The performance of the overall circuit is discussed in the Circuit Description section.
Due to the small form factor of the ADuC7061, a 5 mm × 5 mm 32-LFCSP package, the entire circuit will fit on an extremely small PCB thus further reducing cost.
The focus here is primarily the 4 mA-to-20 mA interface aspect of the circuit. For details on the RTD sensor interface to the ADC and on linearization techniques of the RTD measurements, see Application Note AN-0970 and Circuit Note CN-0075.
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