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10-10-2012

Fabricate a high-resolution sensor-to-USB interface. Part 1

A precision analog acquisition system fits in your pocket.

Zoltan Gingl, University of Szeged, Szeged

The circuit in this Design Idea combines a mixed-signal microcontroller, a USB UART (universal asynchronous receiver/transmitter), and a novel adaptable analog sensor-input circuit. It allows you to connect many types of sensors to the design’s two analog-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 IDEs (integrated development environments), 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 24-bit 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 analog-input ports. You also send the microcontroller’s reference output to one of the analog-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).

Fabricate a high-resolution sensor-to-USB interface
Figure 1.
Microcontroller IC1 has an input multiplexer allowing differential or single-ended mode and two DAC outputs, and it can provide five unassigned digital-i/O pins.

Fabricate a high-resolution sensor-to-USB interface
Figure 2.
A 3.3V linear regulator, IC2, powers the microcontroller.

Fabricate a high-resolution sensor-to-USB interface
Figure 3.
Buffer the microcontroller’s reference output with op amp IC4A.

In the second part we consider the analog ports and their configuration for the main types of sensors.

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