There are several capacitive touch controller groups that utilize different acquisition and measurement techniques. Most famous techniques are Charge Transfer, E-field Sensing, Relaxation Oscillator, Capacitance-to-Digital Conversion (CDC), Dual-Ramp and Sigma-Delta Modulator. In most cases, the controller obtains a sample from the touch sensor, which is translated into raw data, called Counts. Counts usually have a direct relation to the Capacitance of the touch sensor.

1. fieldscale.com
Converting Capacitance
Into Controller Counts

2. There are several capacitive touch controller groups that utilize different
acquisition and measurement techniques.
Most famous techniques are Charge Transfer, E-field Sensing, Relaxation
Oscillator, Capacitance-to-Digital Conversion (CDC), Dual-Ramp and
Sigma-Delta Modulator.
In most cases, the controller obtains a sample from the touch sensor, which is
translated into raw data, called Counts.
Counts usually have a direct relation to the Capacitance of the touch sensor.
Counts
Capacitance
Controller
Introduction to Controller Counts

3. Using SENSE we can simulate a touch button design and extract the results
for the self-capacitance Cx of the button inside the red box both with and
without a touch.
SENSE simulation for Capacitance extraction

4. Vs Cx Cr
Vr
S1 S2
Charge transfer is an effective way to measure a change in capacitance
based upon a fixed capacitance. By means of simple analogy, charge and
capacitance are represented by a liquid and a container, as shown below.
The smaller container is the variable capacitance Cx while the larger
container is the fixed capacitance Cr.
unknown
capacitor Cx
large capacitor Cr
threshold
Capacitance measurement using charge transfer method

5. In most capacitive touch systems, the interest is not in the
absolute capacitance but in the change in capacitance.
That is when a touch or other interaction occurs, the
capacitance of the smaller container changes and
consequently the number of times it takes to charge and
empty the smaller capacitance into the larger changes.
It is this change that is used to determine if a touch
occurred.
Capacitance measurement using charge transfer method

6. Vs Cx Cr
Vr
S1 S2
● S1 closed
● S2 open
Vs Cx Cr
Vr
S1 S2
● S1 open
● S2 close
Cx can be determined by counting the number N
of charge transfer cycles needed to charge Cr
to a given threshold voltage Vr.
Charge Cx Transfer to Cr
Charge transfer circle
threshold Vr
Capacitance measurement using charge transfer method

7. So as an example for a self capacitance measurement, let’s assume that we
have two sensors. Sensor 1: Cx=5pF and Sensor 2: Cx=1pF. Let’s also assume
that Cr = 2nF, Vs = 1.5V, Vr = 0.5V and the touch adds an extra Ct=0.1pF.
Touch state Unknown
capacitance Cx
Counts
N
Sensor 1
No touch 5pF 163
Touch 5pF + 0.1pF 160
Sensor 2
No touch 1pF 812
Touch 1pF + 0.1pF 738
ΔNcounts
= -3
ΔNcounts
= -74
That’s good so far! But which sensor has better functionality?
Capacitance measurement using charge transfer method

8. Conversion time: More counts are equal to greater conversion time so sensor 1
has approximately 8 times quicker response than sensor 2.
Sensitivity: A touch event is detectable when delta-count is bigger (in absolute
terms) than a threshold. Thus, sensor 2 is more sensitive than sensor 1.
● So, the knowledge of a sensor’s capacitance with and without the effect
of a touch can lead to a sensor design that would achieve the desirable
performance.
● Some capacitive touch controllers often give us an option for a
calibration to some point with a signal conditioning stage (e.g. MSP430
from Texas Instruments).
● The combination of a successful sensor design with the proper
calibration parameters of controller is the key for a robust system
performance.
Signal conditioning: Sensor’s performance

9. Charge transfer
engine
Gain Stage Offset stage
The gain stage provides the ability to scale the effective size of the unknown
external capacitor relative to the internal capacitor. This serves two main
purposes:
● First, it allows the controller to handle a wide range of capacitances in a
reasonable conversion time.
● Second, it allows the designer of the system to dial in a desired
measurement resolution.
Signal conditioning: Gain and Offset stage

10. Charge transfer
engine
Gain Stage Offset stage
The offset stage provides a mechanism to remove a set amount of charge
during each charge transfer. This charge is typically associated with the
parasitic capacitance of the sensor. Parasitic capacitance can be thought of as
an unwanted DC offset in the measurement.
● This provides an increase in sensitivity to touch or proximity.
Signal conditioning: Gain and Offset stage

11. MSP430 microcontroller from Texas Instruments uses charge transfer
method with signal conditioning stages for capacitive touch applications.
The input parameters that a user can choose to calibrate (gain and offset) its
own sensor are the Conversion Gain and Conversion Count.
Conversion Gain parameter defines the counts you want to achieve with
the gain stage and before the offset subtraction.
Conversion Count parameter defines the counts you want to achieve
overall, after the signal conditioning.
Signal conditioning: TI MSP430 controller - Calibration
technique

12. Calibration
Conversion Gain
Conversion Count
Gain
Offset
So, if you want:
● a faster response for your system, decrease the Conversion Count.
● to increase resolution, increase the Conversion Count.
● to increase sensitivity, increase the Conversion Count or decrease
the Conversion Gain.
Signal conditioning: TI MSP430 controller - Calibration
technique

13. ● Knowing the self or mutual capacitance of the sensor is crucial for the
system performance.
● Using SENSE, you can simulate a sensor design and extract the results
for the capacitance of a sensor with and without a touch.
● With capacitance to counts conversion, you can observe and adjust the
real-time performance of a sensor.
● Conversion from capacitance to counts depends on the specifics of
each controller.
The experienced team of Fieldscale will guide you to an end-to-end
simulation, from the capacitance extraction to the count conversion.
Conclusions

14. Want to know more
about Controllers for
Capacitive Touch
Sensors?
Download the
“Introduction to
Controllers Guide”
for Free.
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