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Design high-resolution sensor-to-USB interface

25 Mar 2013  | Zoltan Gingl

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The analogue-input architecture allows you to directly connect many kinds of sensors. For example, you can connect a thermistor or a photoresistor between the ground and the input pins and switch on the pullup resistor to form a voltage divider; the on-chip ADC can directly digitise this voltage divider's output (figure 5). This approach also features ratiometric operation, meaning that the ADC uses the same reference as the driving voltage of the voltage divider. Current-output sensors can also be connected as you would connect photodiodes—directly between the ground and the input pins. Switch the pulldown resistor so that the photocurrent develops a voltage.

The high-resolution ADC and PGA allow direct connection of thermocouples (figure 6). You achieve the required bias point by switching on both the pullup and the pulldown resistors on one channel. You can use directly connected bridge-type sensors, such as load cells and pressure sensors, by switching off all of the internal resistors. In these cases, you should operate the ADC in differential mode. Leaving all of the switches open also suits use in potentiometer inputs or IC sensors, such as the SS49E Hall-effect magnetic-field sensor.

When using directly connected sensors, you should consider source impedance, signal range, filtering, and noise pickup (references 2 and 3). You might need to add external buffer amplifiers or a more precise voltage reference. The availability of a reference voltage and 3.3V power on the analogue ports makes this setup possible. You can also use the DAC outputs in connector J1 to trim a value or to provide an arbitrary voltage to the sensors. Note that J1 also has five digital-I/O pins (figure 1).

The design fits into a 2.36×1.38-in. enclosure (figure 7). The underside of the PCB holds several passive components (figure 8). For more details, check reference 4.

Kopasz, Katalin; Peter Makra; and Zoltan Gingl, "Edaq530: a transparent, open-end and open-source measurement solution in natural science education," European Journal of Physics, Volume 32, February 2011, pg 491.

1. C8051F35x Delta-Sigma ADC User's Guide, Silicon Laboratories, 2005.

2. Kester, Walt; James Bryant; and Joe Buxton, "High resolution signal conditioning ADCs," Analog Devices.

3. Gingl, Zoltan, "EDAQ24 24-bit microcontroller sensor-to-USB interface and data logger," 2011.

About the author
Zoltan Gingl is from the University of Szeged in Hungary.

To download the PDF version of this article, click here.

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