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).


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.*


Figure 2.	*A 3.3V linear regulator, IC₂, powers the microcontroller.*


Figure 3.	*Buffer the microcontroller’s reference output with op amp IC_4A.*

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