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Design high-resolution sensor-to-USB interface

25 Mar 2013  | Zoltan Gingl

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The circuit in this article integrates a mixed-signal microcontroller, a USB universal asynchronous receiver/transmitter (UART), and a novel adaptable analogue sensor-input circuit. It allows you to connect many types of sensors to the design's two analogue-input channels, control the device, and read measurement data on a USB host. The USB connection powers the circuit. You can control the device from your computer with simple commands; even terminal software can make the measurements. The 8051 core allows for easy programming with freely available tools, such as integrated development environments (IDEs), debuggers, and C compilers.

The design is based on a $8 microcontroller that features an 8051 architecture, as well as a PGA (programmable-gain amplifier) and a 24bit sigma-delta ADC (figures 1, 2, and 3). Microcontroller IC1 has an input multiplexer allowing differential or single-ended mode. It also has two DAC outputs and can provide five unassigned digital-I/O pins (figure 1). One output pin drives D1 under program control. The remaining digital pins are used to configure the two analogue-input ports. You also send the microcontroller's reference output to one of the analogue-input ports. Four remaining digital pins interface with the USB's UART chip (reference 1).

A 3.3V linear regulator, IC2, powers the microcontroller (figure 2). You power USB chip IC1 directly from the USB port through a ferrite bead and a filter network. This popular and reliable USB UART chip lets you communicate with a computer using any operating system. Op amp IC4 buffers the microcontroller's reference output (figure 3).

Two configurable analogue ports allow you to connect many sensor types using two three-input connectors, each of which has a ground pin (figure 4). One ground pin provides 3.3V power, and the other outputs the buffered reference voltage—nominally, 2.5V. Wire the central pins of the two connectors to the microcontroller's analogue-input multiplexer. In this way, you can either measure two single-ended voltages or use these two connectors as differential inputs. Both inputs have individually switched pullup and pulldown resistors, R10, R11, R14, and R15.

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