Capacitance measuring circuit

• US 4,831,325 A
• Filed: 04/01/1987
• Issued: 05/16/1989
• Est. Priority Date: 04/01/1987
• Status: Expired due to Term

• Current Assignee: General Signal Corporation (SPX Corporation)
• Original Assignee: General Signal Corporation (SPX Corporation)
• Inventors: Watson, Charles W. Jr.
• Primary Examiner(s): Eisenzopf, Reinhard J.
• Assistant Examiner(s): Harvey, Jack B.
• Application Number:
  US07/032,664
• Time in Patent Office: 776 Days
• Field of Search: 324/60 CD, 324/61 R, 324/60 C, 307/246, 307/110, 73/336.2, 73/304 C, 73/718, 73/724, 364/551
• US Class Current: 324/678
• CPC Class Codes: G01N 27/225 by using hygroscopic materials
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24

Abstract

A variable capacitor, which may be a humidity sensitive capacitor, and a fixed reference capacitor are connected at a node. The node is clamped at a reference potential during a first phase of a two phase measuring cycle as the variable capacitor is charged to a fixed voltage and the fixed capacitor is charged to a feedback voltage. The node is unclamped during the second phase and the capacitors are connected in a series loop to allow a redistribution of the charge in the capacitors or force a reversal of that charge with a voltage source. The deviation of the node from its reference potential after charge redistribution occurs is used as input to a feedback circuit which integrates that deviation over a number of cycles until it provides a feedback voltage of magnitude sufficient to cause the node deviation to be reduced to zero. A second reference capacitor can be supplied to provide an offset. The capacitors are constructed by simultaneous deposition on a substrate of a first plate followed by a dielectric film and a second plate. The second plate of the variable capacitor is porous to admit water molecules and the second plate of the fixed capacitor is impervious to water. The simultaneous deposition provides similar characteristics for the capacitors.

Claims

1. A method for measuring the capacitance of a variable capacitor, comprising the steps of:

providing a fixed reference capacitor;

charging said variable capacitor to a fixed voltage during a first, setup phase of a repetitive two phase measuring cycle;

charging said reference capacitor to a variable output voltage during said setup phase;

connecting said capacitors in series in a closed loop during a second, sampling phase of said measuring cycle;

detecting the difference between the potential at a junction between said capacitors after a redistribution of charges in said sampling phase and a predetermined potential established as a balance value; and

iteratively varying said output voltage for the next setup phase in a direction to tend to reduce the difference detected in the next sampling phase so that when a difference of zero is detected the magnitude of the output voltage will be indicative of the capacitance of said variable capacitor.

2. A method for measuring the capacitance of a variable capacitor, comprising the steps of:

providing a fixed reference capacitor;

charging said variable capacitor to a fixed voltage during a first, setup phase of a repetitive two phase measuring cycle;

charging said reference capacitor to a variable output voltage during said setup phase;

connecting said capacitors in a series loop with a fixed sampling voltage source in a second, sampling phase of said measuring cycle;

detecting the difference between the potential at the junction between said capacitors after a redistribution of charges in said sampling phase and a predetermined potential established as a balance value; and

iteratively varying said output voltage for the next setup phase in a direction to tend to reduce the difference detected in the next sampling phase so that when a difference of zero is detected the magnitude of the output voltage will be indicative of the capacitance of said variable capacitor.

3. A method for measuring the capacitance of a variable capacitor, comprising the steps of:

connecting the variable capacitor in series with a reference capacitor to form a network having a single node at the point of connection of the two capacitors;

connecting across said network during a first phase of a repetitive two phase measuring cycle a feed back output voltage representative of the value of the capacitance being measuring to charge said capacitors in one sense;

clamping said node at a fixed potential during said first phase to establish a fixed voltage across said variable capacitor;

unclamping said node and connecting across said network during a second phase of a two phase measuring cycle a sampling voltage to charge said capacitors in an opposite direction;

integrating during the first of said phases the deviation of said node from said fixed voltage, as a result of switching to the previous one of said second phases, to produce as a result of said integration over a plurality of said measuring cycles said output voltage as a measure of the variable capacitance.

4. The method of claim 3 which includes the steps of

connecting a second reference capacitor to said node;

and

switching the other terminal of said second reference capacitor to put said reference capacitor in series with said variable capacitor across said sampling voltage when the series combination of the first of said reference capacitors and the variable capacitor are connected in series across said output voltage and to be in series with said variable capacitor across said sampling voltage in an opposite sense when the series combination of the first of said reference capacitors and the variable capacitor are connected across said sampling potential.

5. Apparatus for measuring the capacitance of a variable capacitor, comprising:

a reference capacitor having a fixed capacitance;

a sampling voltage source providing a fixed voltage;

a fixed voltage source providing another fixed voltage of smaller magnitude than said sampling voltage;

an output voltage source providing a variable output voltage representative of the measured capacitance;

means operable to connect said variable capacitor across said fixed voltage source to charge said variable capacitor to said other fixed voltage and to connect said reference capacitor across said output voltage source to charge said reference capacitor to the output voltage during a first, setup phase of a repetitive two phase measuring cycle;

means operable to connect said capacitors in a loop with said sampling voltage source in a second, sampling phase of said measuring cycle to allow a redistribution of charges between said capacitors;

means for detecting the difference between the voltage across said reference capacitor as a result of said redistribution of charges in said sampling phase and a predetermined value established as a balance value and for iteratively varying the output voltage produced by said output voltage source for the next setup phase in a direction and to an extent to tend to reduce said difference to zero so that when a difference of zero is reached the magnitude of the output voltage will be indicative of the capacitance of said variable capacitor.

6. A circuit for measuring the capacitance of a variable capacitor comprising:

an output voltage source;

a reference capacitor connected at a node to said variable capacitor to form a series network;

means operable to clamp said node at a fixed voltage during a first of the two phases of a measuring cycle;

a sampling voltage source;

means for connecting said output voltage source across said network during said first of said phases to charge said capacitors and for connecting said sampling voltage source across said network during the second of said phases to charge said capacitors in an opposite direction;

said output voltage source including an integrating amplifier having a high impedance input connected to said node and operable to produce an output voltage in one of said phases, said amplifier being so constructed that its output voltage is the integral of that change in the potential at said node which occurs upon switching from said one of said two phases to the other with the output voltage being in sense to tend to reduce said change in potential for a particular value of capacitance of said variable capacitor so that said output voltage will, after reducing said change to zero represent the capacitance of the variable capacitor.

7. A circuit as set forth in claim 6 in which

the means for connecting the sampling and output voltages across the network is an automatic switching means timed by a clock circuit so that the two phases of the measuring cycle are not overlapping.

8. A circuit for measuring the capacitance of a variable capacitor, which comprises:

a reference potential source;

first and second reference capacitors;

a two phase non-overlapping clock for providing signals for timing a setup phase and a sampling phase so that they do not overlap;

means for connecting said reference capacitors and said variable capacitor in a network having a single node at which all of said capacitors have one of their terminals connected;

an integrating amplifier circuit having a high impedance input connected to said node and an output which produces a potential proportional to the integral of the change in potential level on said input due to switching to said sampling phase from said setup phase, said amplifier including

means responsive to said clock signals for holding said input at a fixed level during said setup phase,

means responsive to said clock signals for sampling the potential level at said node during said sampling phase, and

means for integrating the change in potential level at said node upon switching to said sampling phase to change said amplifier output potential accordingly during the next setup phase; and

switching means operable to connect the output potential of said amplifier to the other terminal of said first reference capacitors, to connect said sampling voltage to the other terminal of said second reference capacitor, and to connect the variable capacitor to circuit common during said setup phase so as to charge said capacitors and, said switching means being operable during said sampling phase to connect so that said capacitors are charged in an opposite direction so that said node changes potential upon switching to said sampling phase, said output voltage being of sense such that the changes in potential of said node are decreased during successive cycles of said clock.

9. A circuit for measuring the capacitance of a variable capacitor, comprising:

a reference capacitor;

a two phase non-overlapping clock providing timing signals for timing two non-overlapping phases of a measuring cycle;

a measuring cycle having a high impedance input which is held at a fixed potential level during the first phase of a measuring cycle and which is allowed to float during the second phase of the measuring cycle so as to produce an output potential which changes directly as a function of the integral of the deviations of the potential level at said input from said fixed value during said second phase;

means connecting a first terminal of said variable capacitor to a first terminal of said reference capacitor and to the input of said measuring circuit;

means for switching the other terminal of said variable capacitor between a first potential during the first phase of a measurement cycle and a second potential during the second phase of said measuring cycle; and

means for switching the other terminal of said reference capacitor between the potential at the output of said measuring circuit during the first phase of the measuring cycle and said first potential during the second phase of the measuring cycle so that as the capacitance of said variable capacitor changes the output of said measuring circuit changes to tend to maintain the potential level at said input during said second phase at the fixed value at which it is maintained during the first phase, whereby the output potential of said measuring circuit is a function of the capacitance of said variable capacitor.

10. A method for measuring the capacitance of a variable capacitor, comprising the steps of:

providing at least one fixed reference capacitor;

connecting said reference capacitor in circuit with said variable capacitor so as to form, at a single common connection, a circuit node;

charging at least one of said capacitors to a fixed voltage during a first phase of a repetitive two phase measuring cycle while clamping said node at a fixed potential;

connecting said circuit in a closed loop so that said capacitors are in series during the second of said phases;

unclamping said node during said second phase to allow redistribution of the charges between said capacitors;

detecting the change of potential at said node due to the unclamping of said node during said second phase;

automatically modifying the charge in said circuit over a number of measuring cycles in response to the detected change of potential at said node, said modification being in direction and extent such that over said number of measuring cycles said detected change is reduced to zero; and

integrating the amount the charge is modified over said number of measuring cycles, whereby the integral of said modifications is an indication of the magnitude of the variable capacitance.

11. The method of claim 10 in which the step of modifying the charge is carried out by means of another capacitor connected to said node at one of its terminals and connected to a variable output voltage at its other terminal with the output voltage being modified in response to detected changes in the potential of said node due to its being unclamped so that the output voltage is indicative of said integral.

12. A method for measuring the capacitance of a variable capacitor, comprising the steps of:

providing at least one fixed reference capacitor of known capacitance;

connecting said reference capacitor in circuit with said variable capacitor so as to form, at a single common connection, a circuit node;

charging said capacitors to different voltages during a first phase of a repetitive two phase measuring cycle by placing in series circuit with the circuit combination of said capacitors a fixed voltage source, while clamping said node at a fixed potential;

reversing the polarity of said fixed voltage source in said series circuit during the second of said phases while unclamping said node to allow redistribution of the charges between said capacitors;

detecting the change of the potential at said node after the node is unclamped; and

automatically modifying the charge in said circuit in response to the detected change in potential at said node after the node is unclamped, said modification being in direction and extent such that over a number of measuring cycles said detected change is reduced to zero;

integrating the amount the charge is modified over said number of measuring cycles, whereby the integral of said modifications is an indication of the magnitude of the difference between the capacitances of the reference and the variable capacitors and hence the magnitude of the variable capacitance.

13. Apparatus for measuring the capacitance of a variable capacitor, comprising:

at least one fixed capacitor of known capacitance;

means for establishing a predetermined charge on at least oe of said capacitors;

means for connecting said capacitors in a series circuit to allow redistribution of the charges on said capacitors; and

means operable in response to the magnitude of said redistribution to modify the charges in said circuit to bring the magnitude of the redistribution to a predetermined value, whereby the amount of modification required is indicative of the capacitance of the variable capacitor.

14. Apparatus as set forth in claim 13 in which

the means operable in response to the magnitude of the redistribution of charge is responsive to the deviation from a predetermined value of the potential on a circuit node between said fixed and said variable capacitors.

15. Apparatus as set forth in claim 13 in which

said means operable in response to the magnitude of the redistribution of charge operates in one phase of a two phase repetitive measuring cycle to detect said redistribution of charge and in the other phase to change the charge in the circuit to bring the magnitude of the redistribution toward said predetermined value.

16. Apparatus for measuring the capacitance of a variable capacitor, comprising:

first and second reference capacitors, each having a fixed capacitance of known value;

circuit means connecting one side of each of said reference capacitors to one side of said variable capacitor to form at the connection a circuit node;

first and second fixed voltage sources providing first and second fixed voltages;

an output voltage source providing a variable output voltage;

switching means operable during a first phase of a two phase repetitive measuring cycle to connect the other side of said variable capacitor to said first fixed voltage source, to connect the other side of the second reference capacitor to said second fixed voltage source, and to clamp the potential of said node at a fixed value to establish initial charges on said capacitors;

said switching means also being operable during the second phase of said measuring cycle to unclamp said node, to connect the other side of said variable capacitor to said second fixed voltage source, and to connect the other side of the second reference capacitor to said first fixed voltage source to cause a rearrangement of the charges on said capacitors so that for different values of capacitance for said variable capacitor there will result correspondingly different values of change in the potential of said node as it goes from its clamped condition to its unclamped condition;

means connecting said output voltage to the other side of said first reference capacitor so that changes in said output voltage will function to change the magnitude of the change in potential of said node which occurs when it is unclamped; and

detecting means connected to said node and operable over successive measuring cycles in response to said change in potential of the node upon the clamping of said node to vary said output voltage in direction and extent tending to reduce said change in potential to zero, whereby said output voltage when said change in potential is zero will be a function of the magnitude of the capacitance of said variable capacitor.

17. Apparatus as set forth in claim 16 in which said first fixed voltage source provides a fixed voltage of zero volts.

18. Apparatus as set forth in claim 16 in which said variable output voltage is connected to the other side of said first reference capacitor only during said first phase of the measuring cycle with said other side of the first reference capacitor being connected to said first fixed voltage source during said second phase of the measuring cycle.

19. A circuit for measuring relative humidity which comprises:

an output voltage source;

a variable capacitor responsive to the humidity of the ambient atmosphere;

a reference capacitor of fixed value;

said variable capacitor and said reference capacitor being manufactured at the same time by a single process so that they will have identical characteristics, said capacitors being integrated onto the same substrate by the simultaneous construction on said substrate of

a first plate for each capacitor,

a dielectric film for each capacitor deposited to overlay said first plate, said dielectric film being of material which changes its dielectric constant with changes in the concentration of water molecules absorbed by the film, and

a second plate for each capacitor deposited to overlay the said dielectric opposite said first plate, said second plate for said variable capacitor being pourous so as to admit water molecules from the ambient atmosphere to the dielectric film, with said second plate for said reference capacitor being impervious to water molecules so that said reference capacitor will not be affected by said water molecules;

means connecting said reference capacitor to said variable capacitor at a node to form a series network;

means operable to clamp said node at a fixed voltage during a first of two phases of a two phase measuring cycle;

a sampling voltage source;

means for connecting said output voltage source across said network during said first of said phases to charge said capacitors and for connecting said sampling voltage source across said network during the second of said phases to charge said capacitors in an opposite directions; and

said output source including an integrating amplifier having a high impedance input connected to said node and operable to produce an output voltage in one of said phases, said amplifier being so constructed that its output voltage is the integral of that change in the potential at said node which occurs upon switching from said one of said two phases to the other with the output voltage being in sense to tend to reduce said change in potential for a particular value of capacitance of said variable capacitor so that said output voltage will, after reducing said change to zero represent the capacitance of the variable capacitor.

20. A circuit as set forth in claim 19 in which

said substrate is silicon and all of the circuit elements are constructed as integrated circuit elements on said substrate.

21. A method for measuring the capacitance of a variable capacitor, comprising the steps of:

providing at least one fixed reference capacitor;

connecting said reference capacitor in circuit with the variable capacitor so as to form, at a single common connection, a circuit node;

charging at least one of the capacitors to a fixed voltage during a first phase of a repetitive two phase measuring cycle while clamping said node at a fixed potential;

connecting said circuit in a passive closed loop so that the capacitors are in series during the second of said phases;

unclamping said node during said second phase to allow redistribution of the charges between said capacitors;

detecting the change of potential at said node due to the unclamping of said node during said second phase;

automatically modifying the charge in said circuit in response to the detected change of potential at said node, said modification being in direction and extent such that said detected change is reduced to zero over a number of measuring cycles; and

integrating the amount the charge is modified over said number of measuring cycles, whereby the integral of said modifications is an indication of the magnitude of the variable capacitance.

22. A circuit for measuring humidity, which comprises:

a variable capacitor having a dielectric which changes its dielectric constant with changes in humidity;

a source of sampling potential;

a reference capacitor whose dielectric does not change with changes in humidity;

means for connecting said reference capacitor and said variable capacitor in a network having a single node at which each of said capacitors have one of their terminals connected;

clamping means operable upon selection to clamp said node at a fixed voltage;

feedback means having an input connected to said node and operable to produce an output voltage which is proportional to the integral of the deviation of said node from said fixed voltage when said clamping means is deselected;

switching means whose operation is timed to establish two non-overlapping phases of a repetitive measuring cycle, said switching means being operable during a first of said phases to charge said capacitors to a setup condition by

connecting the output voltage of said feedback means across said network while said sampling potential is disconnected from said network, and

selecting said clamping means so as to hold said node at said fixed potential during said first phase;

said switching means being operable during a second sampling phase to

connect the sampling potential across said network in a polarity opposite to that in which said output voltage was connected during said first phase while said output voltage is disconnected from the network, and

deselect said clamping means so that the node is no longer held at said fixed voltage and so that the resulting change in charge on said capacitors which results from switching from said first to said second phase will cause said node to deviate from said fixed potential in a sense such that the action of said feedback means will cause said output potential to tend toward a value indicative of the value of the capacitance of said variable capacitor;

an indicator for indicating the humidity measured;

an A/D converter circuit having a predetermined number of counts defining the full scale range of said indicator and having differential reference voltage and measured voltage inputs;

a circuit for tailoring the inputs to said converter so that the full scale range and zero offset of the output voltage for the range of humidity being measured generates a full scale indication on said indicator when the output voltage is connected to the high measured input to said converter, said circuit including

an adjustable potentiometer connected across said sampling potential for providing from its adjustable contact the low measured input to said converter,

a voltage divider connected across said sampling voltage to provide at its potential tap a potential representing a factor times said sampling voltage for supplying that potential as an input to the high reference voltage input of said converter, and

another divider circuit connected across a variable portion of said sampling voltage as determined by said adjustable contact and whose tap is connected to one of said reference voltage inputs to said converter with the dividing factor of the divider circuit being proportional to the span of the capacitance range for the variable capacitor over the range to be measured divided by the capacitance at the low end of said range.

23. A circuit as set forth in claim 22 in which

the value of said fixed voltage is approximately half the value of said sampling voltage.

24. A circuit as set forth in claim 22 which includes

a second reference capacitor;

and

said switching means is operable during said first phase to connect the sampling voltage across the series combination of said second reference capacitor and said variable capacitor in sense to produce a flow of charge through said variable capacitor in the same direction as that produced by the connection of said output voltage across the series combination of said first reference capacitor and said variable capacitor, said switching means being operable during said second phase to reverse the connection of said sampling voltage across said series combination of said second reference capacitor and said variable capacitor.

25. A circuit as set forth in claims 23 or 24 in which all of the capacitors in said network are integrated onto a single monolithic substrate by a manufacturing process which produces all of the capacitors of the network at the same time and all of said capacitors have a dielectric which changes its dielectric constant with changes in humidity of the ambient atmosphere and said reference capacitors are protected from the ambient atmosphere so that the capacitance of said reference capacitors is constant.

26. A circuit for measuring the capacitance of a variable capacitor, which comprises:

a source of sampling potential;

a first reference capacitor;

a second reference capacitor;

means for connecting said reference capacitors and said variable capacitor in a network having a single node at which each of said capacitors have one of their terminals connected;

a clock circuit operable to time two non-overlapping phases of repetitive measuring cycles, the first of said phases being a set-up phase and the second phase being the sampling phase;

feedback means made up of

cascaded CMOS logic inverter stages whose input is connected to said node,

clamping means including means operable upon selection for shorting the outputs of said CMOS logic inverters to their respective inputs so as to clamp the input at said node to a fixed potential of value corresponding to the trigger voltage of said logic inverters;

an operational amplifier having a first integrating capacitor connected in its negative feedback path and operable to produce an output voltage, and

a second integrating capacitor connected to be charged up by the output of said logic inverters during said sampling phase and connected to provide the input of said operational amplifier during said setup phase so that said feedback means produces an output voltage which is proportional to the integral of the deviation of said node from said fixed potential when said clamping means is deselected;

switching means operated by said clock circuit during said set-up phase to charge said capacitors to a setup condition by

connecting the output voltage of said feedback means to the other terminal of said first reference capacitor,

connecting the sampling voltage across the series combination of said second reference capacitor and said variable capacitor in sense to produce a flow of charge through said variable capacitor in the same direction as that produced by the connection of the output voltage across said network,

selecting said clamping means so as to hold said node at said fixed potential during said set-up phase;

said switching means being operable during the sampling phase to

connect the sampling potential across said network in a polarity opposite to that in which said output voltage was connected during said set-up phase while said output voltage is disconnected from the network,

reverse the connection of said sampling voltage across said series combination of said second reference capacitor and said variable capacitor, and

deselect said clamping means so that the node is no longer held at said fixed potential and so that the change in charge on said capacitors which results from switching from said first to said second phase will cause said node to deviate from said fixed potential in a sense such that the action of said feedback means will cause said output potential to tend toward a value indicative of the value of the capacitance of said variable capacitor.