[How to] configure Charge Phase in Capacitive Sensing
One of the parameters that engineers have to know in order to properly configure the controller is called ‘Charge Phase’. Tuning the Charge Phase parameter can be done in different ways. In this post, we will show how simulation can help identify the ‘Charge Time’ values of the sensor that are used to properly tune the Charge Phase parameter value.
How a touch system works
Initially, it is important to take a look on how a touch system works. A touch system which in its basic form consists of:
The touch sensor with two sets of electrodes
The touch controller (IC)
In a touch sensor one set of electrodes may serve as Receivers and the electrodes in the other set may serve as Transmitters. The Traces connect the two sets of electrodes to the controller (IC). A voltage with a specific waveform (usually square wave pulse) is applied to the Transmitting electrodes from the IC and then the voltage response is measured through the Receiving electrodes by the Measurement Circuit inside the IC. This method is called Charge-Transfer modulation and it is one of the most used methods of measuring changes in the capacitance of a touch sensor.
To get into more detail, this method works by charging a large sampling capacitor (Cs) in several steps using the charge that is stored in the sensor capacitances as well. The sensor capacitors are smaller and will be charged much quicker than the sampling one. Sensor capacitors will, then, be discharged to the sampling capacitor. This process is performed many times until the sampling capacitor also reaches a charged threshold.
The time needed for the voltage response of the Receivers to reach the steady state – approximately 90% of Vout – is called ‘Charge Time’ (tc). The period that a charge is applied to the sensor is called ‘Charge Phase’ (Cph), followed by the ‘Transfer Phase’ (Tph) which is the period of the discharge to the large capacitor (Cs). Charge Time values are defined in the nodes of the Receiver and Transmitter electrodes of the sensor. For example, in a touch sensor that uses 20 Receivers and 30 Transmitters, the sensor will have 600 individual Charge Time values. By identifying the Charge Time values of the sensor, the engineer will be able to determine the proper Charge Phase input value in the IC firmware, as well as, tounderstand potential mistakes in the design of the touch sensor.
How to tune Charge Phase in touch sensing
In practice, engineers choose two ways of tuning the Charge Phases of a touch sensor:
The first way is the ‘firmware-tuning way’, in which the engineer configures the duration of the Charge Phases through firmware and then measures if the sensor(s) is fully charged and discharged. If the measured voltage reaches the desired voltage, then the charge transfer is ideal and it works.
If the measured voltage does not reach the desired voltage, the charge transfer is non-ideal and the Charge Transfer period needs to be adjusted, potentially together with other parameters as well. Although the above process is considered a trusted way for engineers to configure the controller, it creates a lengthy procedure with multiple lab measurements that might result in configuring the controller in a functional but sub-optimal way.
The second way is the ‘simulation way’, where the engineers identify the Charge Time values of the touch sensor by simulating the operational parameters of a touch system that determine its performance. Knowing the Charge Time values, it will allow the engineer to determine the Charge Phase parameter in controller’s firmware. Below we will explain the process foridentifying the Charge Time in a touch sensor using simulation.
You can read more about how to identify Charge Time in a touch sensor using simulationHERE.