How to calculate the charge time of a capacitive touch sensor

1. fieldscale.com

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

13. Experience SENSE yourself.
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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