The touch control technology used in touch panels can be classified into resistive type, capacitive type, surface acoustic wave type, and optics type, based on their sensing principles.
FIGS. 1 and 2 are a block diagram and a circuit diagram of a control circuit of a conventional capacitive touch panel, respectively. The control circuit 10 is electrically connected to the capacitive touch panel 12, and comprises a pulse generator 20, a multiplexer 22, a multiplexer 24, an integrator 26, a sampling and holding circuit 28, and an analog to digital converter (hereinafter called the ADC) 32.
The pulse generated by the pulse generator 20 functions as a driving which is transmitted through the multiplexer 22 to the X-axial driving wires and the Y-axial driving wires of the capacitive touch panel 12, and the value of capacitive change sensed by the Y-axial and X-axial sensing wires of the capacitive touch panel 12 functions as a touch control sensing signal, which is transmitted through the multiplexer 24 to the integrator 26.
In FIG. 2, the touch control sensing signal of the capacitive touch panel 12 transmitted through the multiplexer 24 to the integrator 26 is a pulse signal, and the integrator 26 integrates several pulse signals into a ladder-shaped wave signal and transmits it to a sampling and holding circuit 28.
Then, in FIG. 1, the sampling and holding circuit 28 samples the ladder-shaped wave signal. integrated by the integrator 26, and keeps the sampled signal at a direct current (DC) voltage level. The ADC 32 will convert the aforementioned sampled analog signal into a digital signal and transmit the digital signal to a microprocessor (not shown) to conduct a signal processing.
The aforementioned control circuit 10 used in the conventional capacitive touch panel 12, at least, has the following disadvantages:
There is noise signal generated in the circuit environment, and the noise signal will also be integrated by the integrator 26 as a portion of the signal, resulting in a low signal to noise ratio (SNR) of the whole system.
It takes a period of time for the integrator 26 to transfer and accumulate the charges on the capacitive touch panel 12 to a voltage that can be processed by a rear-end circuit. Such a long integration time will affect the frame rate of the capacitive touch panel 12. This problem may be compensated by way of a specific hardware (for example, the sampling and holding circuit 28); however, the cost will increase.
The use of the integrator 26, sampling and holding circuit 28, and digital to analog converter 32, will occupy a larger area on the chip.