Instruments4Chem

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A Propeller-Based, AD7746 Capacitance Probe


The Analog Devices AD7746 capacitance-to-digital converter has many interesting potential applications in position sensing, level sensing, flowmeters, humidity sensing and impurity detection. A paper has also appeared describing its use for non-contact conductance measurements in ion chromatography : http://www.sciencedirect.com/science/article/pii/S003991400800252X.

The chip features two capacitance channels, with a full scale measurement range of +- 4 pF, an accuracy of 4 fF and an update rate for conversions of 10-90 Hz.

In addition, the AD7746 can be used for temperature measurements as well as for voltage measurements using its on-board ADC; this delivers a resolution of 17-21 bits depending on the conversion time.

Here, in order to evaluate the AD7746, a Sparkfun breakout board has been interfaced to a P8X32 on a Propeller activity board. Just two I/O pins are required to set up and take data from the AD7746 via an I2C interface - P0 for SCLK and P1 for SDA. In addition, a serial LCD can be connected to P2 for display of the capacitance readings and the on-board FT232 USB-to-serial converter provides a link to a host PC running a custom LabVIEWTM vi.

In conjunction with code running on the activity board, this vi enables a user to log capacitance readings in real-time, displaying the results in a chart window. In addition the vi gives the user full control over the AD7746's register set. These registers allow for channel selection, single ended/differential mode, control of the excitation voltage level and frequency, as well as the conversion time between readings.

Capacitance Probe - LabVIEWTM Front Panel

The image below shows the front panel of the capacitance probe while conducting a simple experiment to investigate taking readings from the AD7746. Controls immediately to the left of the main display area give the user control over the AD7746 internal registers.

Solvent Evaporation Experiment


In this experiment one of the AD7746's two capacitance channels is being monitored. As seen in the image opposite, there are two pads on the printed circuit board - a central circular island of copper approximately 7.5 mm in diameter and a second, 4 mm wide annular region of copper around this with a gap of ~ 0.8 mm. This provides a test capacitance of just over 3 pF.

At the commencement of the experiment a single sheet of tissue paper is placed over the test capacitance. A single drop of isopropanol is then spotted onto the tissue paper and this spreads out to form a circular patch immediately above the capacitor.

As seen in the LabVIEWTM screen above, one observes a rapid rise in capacitance in response to the isopropanol addition, but as the solvent evaporates, the capacitance falls, eventually settling back to a value close to 3.385 pF after about 4 minutes. A second drop of isopropanol is spotted on at point ~6000 and the whole process repeats over.

The total change in capacitance in response to the isopropanol addition/evaporation process is < 50 fF, while the noise in the capacitance readings here is just a few hundred attofarads. The latter figure could be further improved upon by signal averaging.

On account of their different dielectric constants and evaporation rates, different solvents would produce different response curves, suggesting that instrumentation of this type might be used to distinguish different liquids.