This invention relates to self capacitance sensing and, more particularly, to an analog front end that uses current conveyors to enable high frequency self capacitance sensing.
A touch screen is a device that can detect an object in contact with or in proximity to a display area. The display area can be covered with a touch-sensitive matrix that can detect a user's touch by way of a finger or stylus, for example. Touch screens are used in various applications such as smartphones, tablets, smartwatches, wearables, and other mobile devices. A touch screen may enable various types of user input, such as touch selection of items on the screen or alphanumeric input via a displayed virtual keypad. Touch screens can measure various parameters of the user's touch, such as the location, duration, etc.
One type of touch screen is a capacitive touch screen. A capacitive touch screen may include a matrix of conductive lines and conductive columns overlaid on the display area. The conductive lines and the conductive columns do not contact each other. The capacitive touch screen may be used for self capacitance sensing.
In self capacitance sensing, the capacitance between a conductive element of the capacitive touch matrix and a reference voltage, such as ground, is sensed. A change in the sensed capacitance may indicate that an object, such as a finger, is touching the screen or is in proximity to the screen near the conductive element being sensed. The scanning of the capacitive touch matrix involves alternate sensing of the conductive lines and the conductive columns.
Existing analog self capacitance sensing front ends are limited to low frequency applications where the self capacitance of the lines and columns of the matrix is high. This limitation to low frequency applications in turn limits external noise rejection, as the noise harmonics may have a higher power at lower frequencies. Moreover, during a given scan duration, low frequency scanning results in fewer samples, which is not advantageous for averaging out intrinsic noise in the touch screen device.
Therefore, further development in the area of analog front ends for self capacitance sensing is needed.
This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject.
Disclosed herein is a touch screen controller (TSC). The TSC includes a plurality of current conveyors, with each current conveyor having first and second inputs and first and second outputs. The first input of each current conveyor is coupled to a different one of a plurality of self capacitances from a plurality of sense lines. A driver is coupled to the second input of each current conveyor. The driver is configured to periodically drive the second input of each current conveyor between a first voltage and a second voltage less than the first voltage. Each of the plurality of current conveyors is configured to force its first input to a same voltage as its second input, and to replicate a current flowing into its first input at its first and second outputs, such that: when the driver drives the second input of each current conveyor to the first voltage, a different first current flows from the first input of each current conveyor into its associated self capacitance, charging the self capacitance by a known amount, and when the driver drives the second input of each current conveyor to the second voltage, a different second current flows from each self capacitance to the first input of its associated current conveyor, and that second current flows from the first and second outputs of that current conveyor as a sense current for the sense line associated with that current conveyor.
Also disclosed herein is a touch screen controller including a first current conveyor having first and second inputs and first and second outputs, the first input of the first current conveyor being coupled to a first self capacitance sense line. A second current conveyor has first and second inputs and first and second outputs, the first input of the second current conveyor being coupled to a second self capacitance sense line. A driver is coupled to the second input of the first current conveyor and the second input of the second current conveyor, with the driver is configured to periodically drive the second inputs of the first and second current conveyors between high and low voltages. The first current conveyor is configured to force its first input to a same voltage as its second input, and to replicate a current flowing into its first input at its first and second outputs, such that: when the driver drives the second input of the first current conveyor to the high voltage, a first current flows from the first input of the first current conveyor into the first self capacitance sense line; and when the driver drives the second input of the first current conveyor to the low voltage, a second current flows from the first self capacitance sense line into the first input of the first current conveyor, and the first current conveyor replicates the second current to its first and second outputs as a sense current for a sense line associated with the first self capacitance sense line. The second current conveyor is configured to force its first input to a same voltage as its second input, and to replicate a current flowing into its first input at its first and second outputs, such that: when the driver drives the second input of the second current conveyor to the high voltage, a first current flows from the first input of the second current conveyor into the first self capacitance sense line; and when the driver drives the second input of the second current conveyor to the low voltage, a second current flows from the first self capacitance sense line into the first input of the second current conveyor, and the second current conveyor replicates the second current to its first and second outputs as a sense current for a sense line associated with the second self capacitance sense line.
Further disclosed herein is an electronic device having a plurality of current conveyors, each current conveyor having first and second inputs and first and second outputs. The first input of each current conveyor is couplable to a different one of a plurality of self capacitances from a plurality of sense lines. A driver is coupled to the second input of each current conveyor, wherein the driver is configured to periodically drive the second input of each current conveyor between a first voltage and a second voltage less than the first voltage. The electronic device also includes a plurality of current to voltage converters, each current to voltage converter having first and second inputs coupled to the first or second outputs of different current conveyors of the plurality of current conveyors, and outputting a sense voltage.
Also disclosed herein is an electronic device with a first current conveyor having first and second inputs and first and second outputs, the first input of the first current conveyor being coupled to a first self capacitance. A second current conveyor has first and second inputs and first and second outputs, the first input of the second current conveyor being coupled to a second self capacitance. A driver is coupled to the second input of the first current conveyor and the second input of the second current conveyor, with the driver being configured to periodically drive the second inputs of the first and second current conveyors between high and low voltages. A first current to voltage converter has a first input coupled to the second output of the first current conveyor, a second input coupled to the first output of the second current conveyor, and is configured to output a first sense voltage. A second current to voltage converter has a first input coupled to the second output of the second current conveyor, a second input to be coupled to an additional current conveyor, and is configured to output a second sense voltage.
Another aspect is directed to a touch screen controller including a current conveyor having first and second inputs and first and second outputs, the first input of the current conveyor being coupled to a self capacitance sense line. The touch screen controller also includes a driver coupled to the second input of the current conveyor, with the driver being configured to periodically drive the second input of the current conveyor between high and low voltages. The current conveyor is configured to force its first input to a same voltage as its second input, and to replicate a current flowing into its first input at its first and second outputs, such that: when the driver drives the second input of the current conveyor to the high voltage, a first current flows from the first input of the current conveyor into the self capacitance sense line; and when the driver drives the second input of the current conveyor to the low voltage, a second current flows from the self capacitance sense line into the first input of the current conveyor, and the current conveyor replicates the second current to its first and second outputs as a sense current for the self capacitance sense line.
Also disclosed is a touch screen controller including a first current conveyor configured to charge a first self capacitance line, discharge the first self capacitance line, sense a first current resulting from discharge of the first self capacitance line, and replicate the sensed first current to first and second outputs as a first sense current for the first self capacitance line. A second current conveyor is configured to charge a second self capacitance line, discharge the second self capacitance line, sense a second current resulting from discharge of the second self capacitance line, and replicate the sensed second current to first and second outputs as a second sense current for the second self capacitance line. A differential integrator is coupled to receive the first and second sense currents and to generate a sense voltage as a function of a difference between the first and second sense currents.
The present description is made with reference to the accompanying drawings, in which example embodiments are shown. However, many different embodiments may be used, and thus the description should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete. Like numbers refer to like elements throughout.
An analog front end 100 for self capacitance sensing is now described with additional reference to
The first self capacitance Cp(N) is coupled to the “x” input of current conveyor 102a, and the second self capacitance Cp(N+1) is coupled to the “x” input of current conveyor 102b. A driver 108 serves to drive the “y” inputs of current conveyors 102a and 102b, as will be explained below.
Current conveyor 102a has a first output Z1 that is for use in circuitry associated with a N−1′th channel of the analog front end, and a second output Z2 that is coupled to the non-inverting input of differential amplifier 106a. Current conveyor 102b has a first output Z1 that is coupled to the inverting input of differential amplifier 106a, and a second output Z2 that is coupled to the non-inverting input of differential amplifier 106b.
First integration capacitor 140 is coupled between the non-inverting input and inverting output of differential amplifier 106a, and second integration capacitor 142 is coupled between the inverting input and non-inverting output of differential amplifier 106a. Similarly, third integration capacitor 144 is coupled between the non-inverting input and inverting output of differential amplifier 106b, and fourth integration capacitor 146 is coupled between the inverting input and non-inverting output of differential amplifier 106b. The capacitors 140, 142, 144, 146 have the same values in some applications.
Amplifier 104a forms a common mode feedback circuit and has its non-inverting input and its inverting output coupled to the non-inverting input of amplifier 106a, and has its inverting input and its non-inverting output coupled to the inverting input of amplifier 106a. Amplifier 104b also forms a common mode feedback circuit and has its non-inverting input and its inverting output coupled to the non-inverting input of amplifier 106b, and has its inverting input and its non-inverting output coupled to the inverting input of amplifier 106b.
The outputs of amplifier 106a are coupled to provide output to analog to digital converter (ADC) 120. The outputs of amplifier 106b are coupled to provide output to ADC 121. ADCs 120 and 121 are coupled to provide output to digital processing block 122, which provides a control signal to driver 108.
The current conveyors 102a, 102b function as current conveyors as known to those of skill in the art. Details on the internal structure and the operation of current conveyors may be found in The Current Conveyor—A New Circuit Building Block, by Sedra and Smith, Proceedings of the IEEE, August 1968, pages 1368-1369, the contents of which are hereby incorporated by reference in their entirety for all purposes. It is to be understood that the current conveyors 102a, 102b function as the current conveyors described in this incorporated reference, but with current mirroring circuitry on the output such that each current conveyor 102a, 102b has two outputs Z1, Z2 that provide substantially similar or substantially identical outputs.
Generally speaking, current conveyors function as follows. The voltage at the “x” input follows the input at the “y” input, such that a voltage applied to the “y” input is forced at the “x” input; and a current flowing into the “x” input is cloned, potentially in high impedance form, to the “Z1” and “Z2” outputs.
With that understanding, operation of the analog front end 100 is now described. The driver 108 drives the “y” inputs of the current conveyors 102a, 102b between high and low voltages with a periodic signal, shown in
When the “y” inputs of the current conveyors 102a, 102b are driven low, due to the self capacitances Cp(N) and Cp(N+1) being coupled between the “x” inputs of the current conveyors 102a, 102b and a reference voltage (that is greater than the low voltage from the driver 108), current flows from the self capacitances Cp(N) and Cp(N+1) into the “x” inputs. These currents are labeled as Ix(N) and Ix(N+1) in
The current conveyors 102a, 102b function to replicate the currents flowing into the “x” inputs onto their Z1 and Z2 outputs as Iz(N) for current conveyor 102a and Iz(N+1) for current conveyor 102b. Thus, currents Iz(N) and Iz(N+1) are sense currents having values which are a function of the self capacitances Cp(N) and Cp(N+1), which themselves represent touch data.
The amplifiers 104a, 104b serve to reject the common mode currents from the inputs of the differential amplifiers 106a, 106b. Differential amplifiers 106a, 106b are fully differential, having differential inputs and differential outputs, and are arranged as differential integrators. Thus, the differential amplifiers 106a, 106b function to convert to voltages and amplify the difference between the currents received at their inputs, producing differential sense voltages representing touch data at their outputs. These differential sense voltages are converted to the digital domain by analog to digital converters 120 and 121, and may then be further processed by digital processing block 122. The digital processing block 122 also happens to function to control the driver.
As will be shown mathematically, the output of the differential amplifiers 106a, 106b is independent of transients, and is dependent on the difference between the values of the self capacitances.
Mathematically represented, channels (N) and (N+1):
where T is the time taken for 1 sample, and Vacc+ and Vacc− are the differential outputs of the differential amplifiers 106a, 106b.
The relation of the currents Ix(N) and Ix(N+1) into the “x” inputs of the current conveyors 102a, 102b to Cp(N) and Cp(N+1) is:
∫0T[IX(N+1)−IX(N)]·dt=[Cp(N+1)−Cp(N)]·(Vhigh−Vlow)
Therefore:
Thus, the output of the differential amplifiers 106a, 106b, as stated, is independent of the transients.
It should be understood that the analog front end 100 may contain any number of current conveyors to service any number of self capacitances. Where the Z1 output of current conveyor 102a states “to channel (N−1)”, it is meant that that Z1 output will be coupled to the inverting input of the differential amplifier for channel (N−1). Similarly, where inverting input of differential amplifier 106b received input “from channel (N+2)”, it is meant that it is receiving the Z1 input from the current conveyor of channel (N+2). Thus, the analog front end 100 may service any number of columns C1-Cn and lines L1-Ln.
Many modifications and other embodiments will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that various modifications and embodiments are intended to be included within the scope of the appended claims.