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  • Created: Jun 06, 2017

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This design is a circuit that measures temperature through a temperature sensor with the help of a programmable gain amplifier (PGA) interface to the microcontroller. Even though it is very easy to determine temperature without using a PGA, some problems can be removed by implementing temperature-sensing capability in multiplexed applications with a PGA. One benefit is the elimination of a second signal path to the microcontroller while maintaining the accuracy of your sensing system.

The design is composed of the TC1047A temperature sensor, MCP6S2G PGA, MCP3201 ADC, and a PIC microcontroller. The output range of the TC1047A, and, consequently the programming of VREF and gain of the MCP6S26, is dependent on your measurement needs. In a temperature measurement range of -30 to +125 °C, the TC1047A could have a known output of 0.2V, minimum, to 1.75V, maximum. This known output of the temperature sensor is used in the calculation to optimize the MCP6S26 PGA and set the PGA’s gain and voltage reference. The VREF for the PGA can be selected through the use of a digital potentiometer MCP41100. At the output of the PGA, an anti-aliasing filter is inserted. This is done prior to the A/D conversion in order to reduce noise. The anti-aliasing filter can be designed with a gain of one or higher, depending on the circuit requirements. Again, the MCP6022 operational amplifier is used to match the frequency response of the PGA. The anti-aliasing filter in this circuit is a Sallen-Key (non-inverting configuration) with a cut-off frequency of 10 Hz. This frequency is low enough to remove most of the noise in this, essentially, DC measurement. Finally, the signal at the output of the filter is connected to the input of a MCP3201 12-bit A/D converter. In this circuit, if noise is kept under control, it is possible to obtain 12-bit accuracy from the converter. Aside from the anti-aliasing filter, noise is kept under control by using appropriate bypass capacitors, short traces, linear supplies and a solid ground plane. The entire system is manipulated on the same SPI bus for the PGA, digital potentiometer and A/D converter with no digital feed through from the converter during conversion.

After the A/D converter, the signal goes to the PIC microcontroller for further processing. With the multiplexed PGA, a user can connect a number of additional temperature sensors in the system without adding more signal paths to the microcontroller depending on the target application. The circuit design is powered by a +5V voltage source.



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