Method and apparatus for selecting and measuring a capacitance from a plurality of interconnected capacitances

A method for coupling signals to a plurality of capacitances, each capacitance having first and second terminals, each first and second terminal of each capacitance being coupled to one of the first or second terminals of another one of the plurality of capacitances at a node so that the plurality of capacitances are interconnected through a plurality of nodes, the method comprising the steps of;

• coupling a first multiplexer having an input terminal and a plurality of output terminals to the plurality of capacitances so that each output terminal is coupled to one of the plurality of nodes;
• enabling the communication of a signal from the input terminal of the first multiplexer to the node coupled to the first terminal of a selected one of the plurality of capacitances;
• coupling a second multiplexer having a plurality of input terminals and an output terminal to the plurality of capacitances so that each input terminal is coupled to one of the plurality of nodes;
• enabling the communication of current from the node coupled to the second terminal of the selected capacitance to the output terminal of the second multiplexer;
• and
• coupling all of the plurality of nodes to an electrical ground other than the node coupled to the first terminal of the selected capacitance and the node coupled to the second terminal of the selected capacitance.

A method for measuring the capacitance of a selected one of a plurality of capacitances, each capacitance having first and second terminals, each first and second terminal of each capacitance being coupled to one of the first or second terminals of another one of the plurality of capacitances at a node so that the plurality of capacitances are interconnected through a plurality of nodes, the method comprising the steps of;

• coupling a first multiplexer having an input terminal and a plurality of output terminals to the plurality of capacitances so that each output terminal is coupled to one of the plurality of nodes;
• providing a test signal to the input terminal of the first multiplexer, the test signal having a predefined frequency and predefined low and high voltage levels;
• enabling the communication of the test signal from the input terminal of the first multiplexer to the node coupled to the first terminal of the selected capacitance;
• coupling a second multiplexer having a plurality of input terminals and an output terminal to the plurality of capacitances so that each input terminal is coupled to one of the plurality of nodes;
• enabling the communication of current from the node coupled to the second terminal of the selected capacitance to the output terminal of the second multiplexer;
• coupling all of the plurality of nodes to an electrical ground other than the node coupled to the first terminal of the selected capacitance and the node coupled to the second terminal of the selected capacitance;
• measuring current flowing through the output terminal of the second multiplexer during one of a rise or fall time of the test signal;
• and
• calculating the value of the selected capacitance using the measured current, the voltage difference between the high and low voltage levels of the test signal, and the frequency of the test signal.