MEASURING CIRCUIT AND MEASURING METHOD FOR A CAPACITIVE TOUCH-SENSITIVE PANEL

• US 20160018867A1
• Filed: 06/30/2015
• Published: 01/21/2016
• Est. Priority Date: 07/18/2014
• Status: Active Grant

• Current Assignee: Semtech Corporation
• Original Assignee: Semtech Corporation
• Inventors: NYS, Olivier, MONNEY, Pascal, HOFFMANN, Mathieu
• Granted Patent: US 9,727,118 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24
• CPC Class Codes: G06F 1/3231 Monitoring the presence, absence or movement of users
• CPC Class Codes: G06F 1/324 by lowering clock frequency
• CPC Class Codes: G06F 3/04166 Details of scanning methods, e.g. sampling time, grouping of sub areas or time sharing with display driving Synchronisation with the driving of the display or the backlighting unit to avoid interferences generated internally G06F3/04184
• CPC Class Codes: G06F 3/044 by capacitive means

Abstract

A measuring circuit connectable to a capacitive touch-sensitive panel, the panel including a plurality of sense electrodes and optionally a common guard electrode, adapted to measure variations in the instantaneous electric capacity of the sense electrodes in response to proximity to conductive bodies, wherein the sense electrodes are biased at a fixed voltage relative to the common guard electrode, the measuring circuit comprising: a power management integrated circuit comprising a voltage source generating a modulation voltage that is available at a guard terminal of the power management integrated circuit that is in electric connection with the guard electrode; one or more slave integrated circuits, each connected to a plurality of sense electrodes and comprising a Capacity-to-Digital converter or a plurality of Capacity-to-Digital converters that are operatively arranged for generating digital measure codes representing the instantaneous electric capacity of sense electrodes; a means for varying the frequency of the modulation voltage.

Claims

1. A measuring circuit connectable to a capacitive touch-sensitive panel, the panel including a plurality of sense electrodes, the circuit being adapted to measure variations in the instantaneous electric capacity of the sense electrodes in response to proximity to conductive bodies, wherein the sense electrodes are biased at a fixed voltage relative to a common floating reference potential, the measuring circuit comprising:

a power management circuit comprising a voltage source generating a modulation voltage that determines said floating reference potential, one or more readout units, each connected to a plurality of sense electrodes and operatively arranged for generating signals representing the instantaneous electric capacity of sense electrodes;

a means for varying the frequency of the modulated voltage source;

synchronous detection means for generating said signals synchronously with the modulation voltage.

2. The measuring circuit of claim 1, wherein the floating reference potential is available at a guard terminal of the power management integrated circuit that is electrically connectable to a common guard electrode of the touch-sensitive panel.

3. The measuring circuit of claim 1 wherein said means for carrying out synchronous detection is configured to sample the output signal at a frequency which is synchronized to the frequency of the modulating signal;

and

to average those samples over an averaging interval, wherein the averaging interval is a time interval over which a predefined number of samples have to be taken.

4. The measuring circuit of claim 3 wherein the means for varying the frequency of the modulation voltage is configured to vary the frequency of the modulation voltage ‘

N’

times within the averaging interval, wherein ‘

N’

is an integer value greater than ‘

2’

so that the modulation voltage has at least two different modulation frequencies within the averaging interval.

5. The measuring circuit of claim 4 wherein the ‘

N’

frequencies to which frequency of the modulation voltage is varied, are all within a predefined frequency range.

6. The measuring circuit of claim 5 wherein the predefined frequency range is a range of frequencies for the modulation voltage in which the time for synchronous detection and parasitic time constants are optimized and/or the time for synchronous detection and current consumption of the measuring circuit are optimized.

7. The measuring circuit of claim 3 wherein the means for varying the frequency of the modulation voltage is configured to vary the frequency of the modulation voltage ‘

N’

times within the averaging interval, wherein ‘

N’

is an integer value greater than ‘

2’

, and wherein the difference between two consecutive different frequencies of the modulation voltage is such that peaks of the transfer functions corresponding to each frequency do not overlap.

8. The measuring circuit of claim 3 wherein the means for varying the frequency of the modulation voltage is configured to vary the frequency of the modulation voltage in at least one of the following manners within the averaging interval:

continuously;

randomly;

pseudo randomly;

according to a predefined algorithm; and

/or according to a periodic function.

9. The measuring circuit of claim 1 wherein the means for varying the frequency of the modulation voltage comprises a means for changing the division ratio of a clock divider and/or a means for adjusting the frequency of an oscillator.

10. A method for measuring variations in the instantaneous electric capacity of the sense electrodes in a touch-sensitive panel, in response to proximity to conductive bodies, wherein the sense electrodes are biased at a floating reference voltage, the panel including a plurality of sense electrodes and a common guard electrode, the method comprising the steps of generating a modulation voltage that determines said floating reference voltage with a voltage source comprised in a power management circuit, detecting the instantaneous electric capacity of sense electrodes synchronously with the modulation voltage, generating signals representing the instantaneous electric capacity of sense electrodes;

varying the frequency of the modulated voltage.

11. The method of claim 10 wherein the step of carrying out synchronous detection comprises sampling the output signal at a frequency which is synchronized to the frequency of the output signal;

and averaging those samples over an averaging interval, wherein the averaging interval is a time interval over which a predefined number of samples have to be taken.

12. The method of claim 11 wherein the step of varying the frequency of the modulation voltage comprises varying the frequency of the modulation voltage ‘

N’

times within the averaging interval, wherein ‘

N’

is an integer value greater than ‘

2’

so that the modulation voltage has at least two different modulation frequencies between successive synchronous detections.

13. The method of claim 12 wherein the ‘

N’

frequencies to which frequency of the modulation voltage is varied, are all within a predefined frequency range.

14. The method of claim 13 further comprising the steps of determining frequencies for the modulation voltage at which the time for synchronous detection and parasitic time constants are optimized, and/or determining frequencies for the modulation voltage at which the time for synchronous detection and current consumption of the measuring circuit are optimized, and defining said predefined frequency range using said determined frequencies.

15. The method of claim 12 wherein the step of varying the frequency of the modulation voltage comprises, varying the frequency of the modulation voltage to provide a difference between two consecutive different frequencies of the modulation voltage which ensures that peaks of the transfer functions corresponding to each frequency do not overlap.

16. The method of claim 15 wherein the varying the frequency of the modulation voltage comprises varying the frequency of the modulation voltage so that the difference between two consecutive different frequencies of the modulation voltage is greater than the broadest width of the peaks of the transfer functions corresponding to each frequency, such that peaks of the transfer functions corresponding to each frequency do not overlap.

17. The method of claim 11 wherein the step of varying the frequency of the modulation voltage comprises varying the frequency of the modulation voltage in at least one of the following manners within the averaging interval:

continuously;

randomly;

pseudo randomly;

according to a predefined algorithm; and

/or according to a periodic function.

18. The method of claim 10 wherein the step of varying the frequency of the modulation voltage comprises the step of changing the division ratio of a clock divider and/or adjusting the frequency of an oscillator.