Sunday, October 2, 2016
Precision Capacitive Touch Proximity Sensor Circuit Part 2
Precision Capacitive Touch Proximity Sensor Circuit Part 2
The previous post explained the datasheet of the precision capacitive touch/proximity sensor IC PCF8883, here we learn a typical circuit configuration using the same IC which can be applied in all products requiring precision remote touch stimulated operations.
The proposed capacitive touch and proximity sensor may be diversely used in many different applications as indicated in the following data:
A typical application configuration using the IC can be witnessed below:
The + input supply is attached with the VDD. A smoothing capacitor may be preferably connected across and VDD and ground and also across VDDUNTREGD and ground for more reliable working of the chip.
The capacitance value of COLIN as produced on pin CLIN fixes the sampling rate effectively. Increasing sampling rate may enable enhance reaction time on the sensing input with a proportionate increase in the current consumption
The sensing capacitive touch plate could be in the form of a miniature metal foil or plate shielded and isolated with a non conductive layer.
This sensing area could be either terminated over a longer distances via a coaxial cable CCABLE whose other ends may be linked with the IN of the IC, or the plate could be simply directly connected with the INpinout of the IC depending on the application needs.
The IC is equipped with an internal low pass filter circuitry which helps to suppress all forms of RF interferences that may try to make way in to the IC through the IN pin of the IC.
Additionally as indicated in the diagram one may also add an external configuration using RF and CF to further enhance the RF suppression and reinforce RF immunity for the circuit.
In order to achieve an optimal performance from the circuit, its recommended that the sum of the capacitance values of CSENSE + CCABLE + Cp should be within a given appropriate range, a good level could be around 30pF.
This helps the control loop to work in a better way with the static capacitance over CSENSE for equalizing the rather slower interactions on the sensing capacitive plate.
For achieving an increased levels of capacitive inputs it may be recommended to include a supplementary resistor Rc as indicated in the diagram which helps to control the discharge time as per the internal timing requirement specs.
The cross sectional area of the attached sensing plate or a sensing foil becomes directly proportional to the sensitivity of the circuit, in conjunction with the value of the capacitor Ccpc, reducing Ccpc value can greatly affect the sensitivity of the sensing plate. Therefore for achieving an effective amount of sensitivity, Ccpc could be increased optimally and accordingly.
The pinout marked CPC is internally attributed with a high impedance and therefore could be susceptible to leakage currents.
Make sure that Ccpc is chosen with a high quality PPC of MKT type of capacitor or X7R type for obtaining optimal performance from the design.
In case the system is intended to be operated with a restricted input capacitance of upto 35pF and at freezing temperatures -20 degrees C, then it may be advisable to bring down the supply voltage to the IC to around 2.8V. This in turn brings down the operating range of Vlicpc voltage whose specification lies between 0.6V to VDD - 0.3V.
Moreover, lowering the operating range of Vucpc could result in lowering the input capacitance range of the circuit proportionately.
Also, one may notice that as Vucpc value increases with decreasing temperatures as demonstrated in the diagrams, which tells us why appropriately lowering the supply voltage helps in decreasing temperatures.
Table 6 and Table7 indicates the recommended range of the components values which may be appropriately chosen as per the desired application specifications with reference to the above instructions.
Reference: PCF8883 data sheet
The proposed capacitive touch and proximity sensor may be diversely used in many different applications as indicated in the following data:
A typical application configuration using the IC can be witnessed below:
The + input supply is attached with the VDD. A smoothing capacitor may be preferably connected across and VDD and ground and also across VDDUNTREGD and ground for more reliable working of the chip.
The capacitance value of COLIN as produced on pin CLIN fixes the sampling rate effectively. Increasing sampling rate may enable enhance reaction time on the sensing input with a proportionate increase in the current consumption
The sensing capacitive touch plate could be in the form of a miniature metal foil or plate shielded and isolated with a non conductive layer.
This sensing area could be either terminated over a longer distances via a coaxial cable CCABLE whose other ends may be linked with the IN of the IC, or the plate could be simply directly connected with the INpinout of the IC depending on the application needs.
The IC is equipped with an internal low pass filter circuitry which helps to suppress all forms of RF interferences that may try to make way in to the IC through the IN pin of the IC.
Additionally as indicated in the diagram one may also add an external configuration using RF and CF to further enhance the RF suppression and reinforce RF immunity for the circuit.
In order to achieve an optimal performance from the circuit, its recommended that the sum of the capacitance values of CSENSE + CCABLE + Cp should be within a given appropriate range, a good level could be around 30pF.
This helps the control loop to work in a better way with the static capacitance over CSENSE for equalizing the rather slower interactions on the sensing capacitive plate.
For achieving an increased levels of capacitive inputs it may be recommended to include a supplementary resistor Rc as indicated in the diagram which helps to control the discharge time as per the internal timing requirement specs.
The cross sectional area of the attached sensing plate or a sensing foil becomes directly proportional to the sensitivity of the circuit, in conjunction with the value of the capacitor Ccpc, reducing Ccpc value can greatly affect the sensitivity of the sensing plate. Therefore for achieving an effective amount of sensitivity, Ccpc could be increased optimally and accordingly.
The pinout marked CPC is internally attributed with a high impedance and therefore could be susceptible to leakage currents.
Make sure that Ccpc is chosen with a high quality PPC of MKT type of capacitor or X7R type for obtaining optimal performance from the design.
In case the system is intended to be operated with a restricted input capacitance of upto 35pF and at freezing temperatures -20 degrees C, then it may be advisable to bring down the supply voltage to the IC to around 2.8V. This in turn brings down the operating range of Vlicpc voltage whose specification lies between 0.6V to VDD - 0.3V.
Moreover, lowering the operating range of Vucpc could result in lowering the input capacitance range of the circuit proportionately.
Also, one may notice that as Vucpc value increases with decreasing temperatures as demonstrated in the diagrams, which tells us why appropriately lowering the supply voltage helps in decreasing temperatures.
Table 6 and Table7 indicates the recommended range of the components values which may be appropriately chosen as per the desired application specifications with reference to the above instructions.
Reference: PCF8883 data sheet
Available link for download
