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Characterisation of a 24-bit capacitance-to-digital converter (AD7746)

Jürgen JOSEF

This thesis is concerned with possibilities to measure the cutting radiuses of knife bands during the production process. The major factors, which restrained the number of the detection methods, was the fast band velocity and the high degree of pollution. Due to this fact optical methods, for example, were excluded.

Therefore the main focus was put on the capacitive methode where the requirements of the application form the challange (especially capacities in the range of femto-to-atto farad). At this time nobody at the institute had experience to design high precision capacitance measurement circuits and there was not enough time to develop a new one. So the decision was made to use an already existing solution - an SoC.

The AD7746, a 24-bit capacitance-to-digital converter from the company ANALOG DEVICES. This device meets the demands on resolution and accuracy. Furthermore a relatively fast update rate may be accomplished as compared to similar products. Additionally the price and the availability of an evaluation board were considered. The relatively slow update rate of the chip was still sufficient for the measurements in the laboratory, since in the first place only the feasibility on the basis of a prototyp had to be shown.

Target of this project was to test the performance of the chip and determine the actual resolution and accuracy of the device. Furthermore dynamic characteristics and the influence of factors like temperature were observed. Special attention has been paid to user-friendliness, platform independence and easy adaption to other projects during the development of ther peripheral modules and the design of the user interface.

Figure 1: A representative result for a dynamic collection of data. In this case, a 30 Hz excitation source was measured, transfered with 90 Hz to the PC and analyzed by using a FFT. The variable capacity was generated by a modified microphone capsule.

Keywords: 24-bit capacitance-to-digital converter, AD7746, AVR, I2C

July 30, 2012