Mutual capacitance measurement

Abstract

A circuit portion for indicating a mutual capacitance between a first and second node is provided. The circuit portion comprises a switchable constant current source arrangement configured to supply a first current to the first node in a first direction or a second current to the first node in a second, opposite direction; a variable voltage source configured to output a voltage to the second node so as to hold the first node at a reference voltage; and a comparator arrangement configured to switch between said first and second directions of the constant current source when the voltage output by the variable voltage source reaches a lower threshold voltage or an upper threshold voltage and to output a signal in synchrony with said constant current direction switching. The signal is indicative of the mutual capacitance between the first and second nodes.

Claims

1. A circuit portion for indicating a mutual capacitance between a first and second node, the circuit portion comprising: a switchable constant current source arrangement configured to supply a first current to the first node in a first direction or a second current to the first node in a second, opposite direction; a variable voltage source configured to output a voltage to the second node so as to hold the first node at a constant reference voltage by a negative feedback loop, such that the voltage output to the second node changes whilst the first node is supplied with the first current or the second current in such a way as to hold the first node at the constant reference voltage whilst it is supplied with the first current or the second current; and a comparator arrangement configured to switch between said first and second directions of the constant current source when the voltage output to the second node by the variable voltage source reaches a lower threshold voltage or an upper threshold voltage and to output a signal in synchrony with said constant current direction switching; wherein the signal is indicative of the mutual capacitance between the first and second nodes.

2. The circuit portion as claimed in claim 1, further comprising a frequency measurement section which is arranged to measure a frequency of the signal output from the comparator arrangement and to use the measured frequency to determine said mutual capacitance.

3. The circuit portion as claimed in claim 1, wherein the first and second currents supplied by the constant current source arrangement have the same magnitude.

4. The circuit portion as claimed in claim 1, wherein the variable voltage source comprises an operational amplifier.

5. The circuit portion as claimed in claim 4, wherein the operational amplifier is configured in a negative feedback configuration, with the mutual capacitance connected between the output and an inverting input of the operational amplifier.

6. The circuit portion as claimed in claim 1, wherein the comparator arrangement comprises a comparator with first and second inputs and a digital output.

7. The circuit portion as claimed in claim 6, wherein the first input of the comparator is connected to the output of the variable voltage source and the second input is connected to a switchable reference voltage source, which is switchable between providing the lower threshold voltage and the upper threshold voltage.

8. The circuit portion as claimed in claim 7, wherein the comparator arrangement is further configured to switch the switchable reference voltage between the first reference voltage and the second reference voltage when the voltage output by the variable voltage source reaches the lower threshold voltage or the upper threshold voltage.

9. The circuit portion as claimed in claim 1, wherein the signal output by the comparator arrangement comprises a square wave.

10. A touch interface comprising first and second conductive elements and at least one circuit portion as claimed in claim 1, wherein the at least one circuit portion is arranged to measure the mutual capacitance between the first and second conductive elements so as to determine the presence of a touch on the touch interface.

11. The touch interface as claimed in claim 10, comprising two or more circuit portions as claimed in claim 1.

12. A method of indicating a mutual capacitance between a first and second node comprising: providing a first current to the first node in a first direction providing a variable voltage to the second node so as to hold the first node at a constant reference voltage by a negative feedback loop; comparing the variable voltage provided to the second node to a lower threshold voltage; when the variable voltage reaches the lower threshold voltage, stopping the first current and providing a second current to the first node in a second direction opposite to the first direction; comparing the variable voltage provided to the second node to an upper threshold voltage; and when the variable voltage reaches the upper threshold voltage, stopping the second current and providing the first current to the first node in the first direction; wherein a frequency at which the direction of the current is switched is indicative of the mutual capacitance between the first and second nodes; wherein the voltage output to the second node changes whilst the first node is supplied with the first current or the second current in such a way as to hold the first node at the constant reference voltage whilst it is supplied with the first current or the second current.

13. The method as claimed in claim 12, further comprising measuring the frequency at which the direction of the current is switched to determine the mutual capacitance between the first and second nodes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) One or more non-limiting examples of the present invention will now be described with reference to the accompanying Figures, in which:

(2) FIG. 1 is a schematic diagram of a circuit portion according to one embodiment of the present invention;

(3) FIGS. 2, 3 and 4 are timing diagrams illustrating the operation of the circuit portion of FIG. 1; and

(4) FIG. 5 is a schematic diagram of a touch screen sensor array for which the circuit portion of the present disclosure may be used.

(5) FIG. 1 is a circuit diagram showing a mutual capacitance measurement circuit 2 according to an embodiment of the present invention. The circuit 2 is configured to measure the mutual capacitance 4 between a first node 6 and a second node 8.

(6) The circuit 2 comprises a current supply section 10, which can be configured to supply a constant current to the first node 6 in either a first direction or a second, opposite, direction. The current supply section 10 comprises a first constant current source 12 arranged to supply a current/flowing in the first direction, and a second constant current source 14 arranged to supply the same current I, but flowing in the opposite direction. The first constant current source 12 is connected to the first node 6 via a first current supply switch 16 and the second constant current source 14 is connected to the first node 6 via a second current supply switch 18. The state (i.e. open or closed) of the first and second current supply switches 16, 18 is controlled by an external signal (indicated by the arrows pointing to the switches 16, 18 in FIG. 1). In the particular example shown here, a high external signal causes a switch 16, 18 to be open (i.e. non-conducting) and a low external signal causes a switch 16, 18 to be closed (i.e. conducting), but of course these might be reversed if consequential changes are made to the rest of the circuit.

(7) The circuit 2 further comprises an operational amplifier 20. An inverting input 22 of the operational amplifier 20 is connected to the first node 6, and a non-inverting input 24 of the operational amplifier 20 is connected to a reference voltage source 26 which provides a constant voltage V.sub.ref. An output 28 of the operational amplifier 20 is connected to the second node 8, providing a negative feedback loop via the mutual capacitance 4. The current supply section 10, the operational amplifier 20 and the first and second nodes 6, 8 are thus configured as an integrator, wherein the voltage at the output 28 of the operational amplifier 20 is proportional to the time for which a current is supplied by the current supply section 10.

(8) The constant of proportionality (i.e. the rate at which the voltage at the output 28 changes) is dependent upon the magnitude and direction of the current supplied by the current supply section 10 and the mutual capacitance 4 between the first node 6 and the second node 8.

(9) As mentioned above, the voltage provided by the reference voltage source 26 is fixed at V.sub.ref. The magnitude of the current I supplied by the current supply section 10 is also fixed. The rate at and direction in which the voltage of the output 28 changes is thus a direct indication of the mutual capacitance 4 between the first node 6 and the second node 8.

(10) The circuit 2 further comprises a comparator arrangement 29 which comprises a comparator 30. The output 28 of the operational amplifier 20 is connected to a positive input 32 of the comparator 30, and a switchable reference voltage source 34 is connected to a negative input 36 of the comparator 30. The comparator 30 further comprises a digital output 38, which is high when the voltage at the positive input 32 is greater than the voltage at the negative input 36 (i.e. when the voltage on the output 28 of the operational amplifier 20 is greater than the reference voltage provided by the switchable reference voltage source 34). Otherwise, the digital output 38 is low.

(11) The digital output 38 controls the state of the first and second current supply switches 16, 18. The output 38 is connected to the first supply switch 16 via an inverter 40, and to the second supply switch 18 directly. Thus, when the output 38 is high (the case shown in FIG. 1), a high signal is sent to the second current supply switch 18 and a low signal is sent to the first supply switch 16. This opens the second current supply switch 18 and closes the first supply switch 16, causing current to be supplied to the first node 6 in the first direction. When the output 38 is low, the opposite happens (i.e. current is supplied in the second direction).

(12) The digital output 38 also controls the switchable reference voltage source 34. When the digital output 38 is high, it causes the switchable reference voltage source 34 to output a lower threshold voltage V.sub.th_1 and when the digital output 38 is low, it causes the switchable reference voltage source 34 to output an upper threshold voltage V.sub.th_2.

(13) The operation of the circuit 2 will now be described with reference to the timing diagrams shown in FIGS. 2, 3 and 4.

(14) FIG. 2 shows the mutual capacitance between the first and second nodes 6, 8. The output 28 of the operational amplifier 20 is shown as a solid line in FIG. 3 and the voltage provided by the switchable reference voltage source 34 (either V.sub.th_1 or V.sub.th_2) is indicated on FIG. 3 with a dashed line. FIG. 4 shows the digital output 38.

(15) At a first time 202, the first constant current source 12 of the current supply section 10 supplies a current I to the first node in the first direction and charge builds up at the first and second nodes 6, 8. However, because of the negative feedback loop, the first node 6 is held at the voltage V.sub.ref of the voltage reference source 26 by the operational amplifier 20 and the output 28 of the operational amplifier 20 (and thus the voltage at the second node 8) decreases linearly, holding the first node 6 at the reference voltage V.sub.ref. The switchable reference voltage source 34 provides the lower threshold voltage V.sub.th_1. The voltage on the output 28 is greater than the upper threshold voltage V.sub.th_2 and the digital output 38 is therefore high.

(16) At a second, later time 204, the output 28 reaches the lower threshold voltage V.sub.th_1. The voltage on the output 28 is instantaneously less than the lower threshold voltage V.sub.th_1 and the digital output 38 goes low. This, in turn, causes the switchable reference voltage source 34 to provide the upper threshold voltage V.sub.th_2, the second current supply switch 18 to close and the first supply switch 16 to open, causing current to be supplied by the second constant current source 14 to the first node 6 in the second direction. The voltage on the output 28 subsequently, therefore, begins to increase.

(17) At a later time 206 the output 28 reaches the upper threshold voltage V.sub.th_2. The voltage on the output 28 is instantaneously greater than the upper threshold voltage V.sub.th_2 and the digital output 38 goes high. This causes the switchable reference voltage source 34 to once again provide the lower threshold voltage V.sub.th_1, and current to be supplied to the first node 6 in the first direction. The voltage on the output 28 subsequently, therefore, begins to decrease again.

(18) The output 28 continues to oscillate between the lower and upper thresholds V.sub.th_1, V.sub.th_2, producing the triangular wave seen in FIG. 3 and the square wave on the digital output 38. Because, as mentioned above, the rate at which the voltage at the output 28 changes is directly proportional to the mutual capacitance 4, the frequency of the square wave on the digital output 38 (and of the triangular wave on the output 28) can be used to determine the mutual capacitance 4.

(19) Before a time 208, the mutual capacitance 4 has a first value 252, as seen in FIG. 2. At the time 208 the mutual capacitance 4 decreases to a second value 254 (for example, due to the presence of a user's finger close to the first and second nodes 6, 8).

(20) Due to the decreased mutual capacitance 4, the rate at which the voltage at the output 28 changes increases, and the frequency of the oscillations of both the output 28 and the digital output 38 increases. This frequency change may be measured by additional circuitry (e.g. to detect the presence of the finger).

(21) FIG. 5 shows a touch interface 500 comprising first and second conductive grids 502, 504 and a control module 506. The first and second conductive grids 502, 504 each comprise a series of conductive channels which form the rows and columns respectively of a touch detection array 508. The touch detection array 508 is covered with a non-conductive layer (e.g. glass). The first and second conductive grids 502, 504 are not in electrical contact such that there exists a mutual capacitance between each pair of conductive channels.

(22) The control module 506 measures sequentially the mutual capacitance between each pair of channels (i.e. at each intersection of a row and a column) using a circuit portion as described herein. When a user touches the non-conductive layer (e.g. at position 510), the mutual capacitance of the intersection(s) nearest the touch location is decreased. The control module 506 detects this change in mutual capacitance and thus determines the position 510 of the touch.

(23) While the example described above comprises an entire touch detection array 508, the method described herein can of course be used to provide capacitance measurements for much simpler interfaces, e.g. capacitive buttons (just a single point).

(24) The particular embodiment described above is merely exemplary and many possible variants and modifications are envisaged within the scope of the invention as defined in the claims.