Capacitive Touch Sensor Charge Time
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This document describes the steps to calculate the charge time of a capacitive touch sensor. It involves simulating the sensor design in SENSE to extract resistance (R) and capacitance (C) values. An RC equivalent circuit is created for a single cell and the whole sensor. SPICE analysis is then used to simulate the circuit with a controller and measure the voltage output at different nodes to determine the charge time.
Capacitive Touch Sensor Charge Time
- 2. This is a typical capacitive touch sensor design. It consists of the Transmitting (Tx) and Receiving (Rx) electrodes which in this case are located in two different layers (Diamond Double-Layer design) and their traces which connect them to the controller (IC). During operation, a voltage with a specific waveform (usually pulse) is repeatedly applied to the Transmitting electrodes and then measured through the Receiving electrodes by a high resolution measurement system inside the IC. Capacitive Touch Sensor
- 3. How to calculate the charge time of a touch sensor In order to calculate the charge time of a touch sensor, the following steps should be followed: 1. Simulation in SENSE - Extract R and C values a. Cell definition b. Convert R and C values c. Create RC-equivalent of one cell and the whole sensor 2. Run SPICE analysis (circuit simulation) a. Add controller equivalent circuit b. Define voltage source c. Define points of voltage output 3. Get charge time
- 4. Simulation in SENSE - Extract R and C values In order to calculate the Resistances and the Capacitances needed for the RC- equivalent, we perform a simulation of our 3x3 Diamond Double Layer design in Fieldscale SENSE 3.3.0
- 5. The RC-equivalent circuit of the whole touch sensor is needed to provide accurate results for the charge time. The analysis starts from a simple cell (1 Transmitting and 1 Receiving electrode). The RC-equivalent of this cell is shown here: Cell Definition ● Rx/2 is the half resistance of X electrode ● Ry/2 is the half resistance of Y electrode ● C_mutual is the mutual capacitance between X and Y electrode ● Cx_self is the self capacitance of X electrode ● Cy_self is the self capacitance of Y electrode
- 6. In order to complete the RC-equivalent of the whole sensor, we need to add the controller functionality into the circuit. The controller can be represented by the following simplified circuitry: During operation, the controller applies a pulse voltage of 1V to each transmitting electrode. We measure the output voltage at the receiving electrodes. Taking into consideration that the pattern is symmetric, we can replicate the RC equivalent of one cell and create the equivalent of a whole touch sensor along with the effect of the controller’s circuit and functionality. For example a design with 20x14 electrodes, which corresponds to a 6.0 inch screen is shown below: Create RC-equivalent of the whole sensor
- 7. RC-equivalent of the whole sensor
- 8. In order to obtain stable results, it is important that the voltage pulse applied in the Transmit electrode is allowed to settle properly and hence transfer all the charge into the electrodes’ capacitances. The time (tc ) that the voltage output (VOUT ) requires to reach a specific threshold (VTH ) is called charge time. Figure source Definition of Charge Time
- 9. In touch sensing applications the capacitance of the electrodes (both self and mutual) is changing when a touch event occurs. Taking this principle into consideration, the controller is able to determine if a finger is “touching” the electrode or not. Figure source Charge time during touch event
- 10. Based on the previous approach and choosing a voltage threshold of 50 mV, a typical behavior of the voltage output with and without a touch event in our design is shown below: Charge time Calculation
- 11. The charge times of different nodes across the complete touch sensor were measured. Nodes Selected
- 12. Results
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- 15. APPENDIX
- 16. There are two different ways to define the self-capacitance of a conductor: 1. Self-capacitance is the capacitance of the conductor with reference to the infinite ground (Cself ) 2. Self-capacitance is the addition of the capacitance of the conductor with reference to the infinite ground (Cself ) and the sum of all the mutual capacitances between the conductor and the rest of the conductors (ΣCmutual ). SENSE calculates the self-capacitance of the electrodes according to the second principle: Cself,SENSE =Cself +ΣCmutual So based on that, if C2 is the self capacitance result of Y-electrode and C3 is the self capacitance result of X-electrode of our 3x3 cell pattern: Cy_self = (C2-ΣCmutual )/3 Cx_self = (C3-ΣCmutual )/3 Conversion of Capacitance results
- 17. SENSE also computes the Resistance of the whole electrode in 3X3 cell, so: Ry/2 = Resistanceelectrode 1 /6 Rx/2 = Resistanceelectrode 2 /6 Conversion of Resistance results