Impedance measuring circuit

An apparatus for measuring a physical variable, the apparatus comprising:

• a non-linear sensor, wherein the impedance thereof varies over at least two orders of magnitude responsive to said measured variable;
• a measurement terminal to which one end of said sensor is connected;
• first and second supply terminals for receiving an operating voltage therebetween, the other end of said sensor being connected to said first supply terminal;
• a plurality of charging means connected to said measurement and second supply terminals for supplying respective charging currents through said sensor, each charging means having a respective time constant in conjunction with said sensor, each of said charging means including means for selective operation thereof;
• time interval means coupled to said sensor, responsive to first and second predetermined voltage levels at said measurement terminal for measuring the time interval between the occurrence of said first and second voltage levels, said time interval means having a time interval output which is proportional to said respective time constant over a range;
• control means responsive to said time interval output for selectively operating one or more of said plurality of charging means so as to select that available time constant which provides a measured time interval which is as long as possible yet within the range of said time interval means;
• and
• data converter means for correlating said measured time interval to said measured physical variable,
• wherein the measurement resolution of said measured variable over at least a portion of the lower order of magnitude of impedance range of said sensor is thereby improved.

In combination:

• first and second supply terminals for receiving an operating voltage therebetween;
• a measurement terminal;
• an impedance of a first type and a plurality of impedances of a second type, all having respective first ends and respective second ends, the first end of the impedance of the first type being connected at said measurement terminal, the second end of said impedance of said first type being connected to said first supply terminal, one of said first and second types of impedance being primarily resistive and the other primarily reactive in nature, each of said impedances of second type having a respective time constant in association with said impedance of first type;
• a plurality of switch means having first, second and third electrodes, the third electrode controlling conduction between the first and second electrodes, each of said plurality of switch means arranged to selectively connect a respective impedance of the second type between said measurement terminal and said second supply terminal whenever said respective switch means conducts between its respective first and second electrodes;
• time interval means coupled to said impedance of the first type responsive to first and second predetermined voltage levels at said measurement terminal for measuring the time interval between the occurrence of said first and second voltage levels, said time interval means having a time interval output which is proportional to said respective time constant over a range;
• and
• control means responsive to said time interval output, the control means being connected to the respective third electrodes of said plurality of switch means for selecting that available time constant or combination of time constants which provides a measured time interval which is as long as possible yet within the range of said time interval means, thereby fully utilizing the resolution capability of said time interval means.