3.2 Circuits Implementation and Simulated Results
3.2.3 Switch Design
In the proposed circuit, many switches are used to control the circuit operation.
The detail design is considered in this subchapter. Charge injection and clock feedthrough effects are discussed.
Charge Injection [31]
The conduction of MOS is based on the existence of a channel. When the gate of MOS is biased at an appropriate voltage, for NMOS, many electrons (or holes for PMOS) are attracted to the oxide-silicon surface and the channel is formed to conduct the current from the source to drain. The effect of charge injection is shown in Fig. 3.6.
Assuming Vin≈Vout, the total charge Qch in the inversion layer can be expressed as:
(
DD in th)
ch ox
Q =WLC V −V −V , (3.5)
where W represents the width of MOS devise, L denotes the effective channel length, Cox is the gate oxide capacitance per unit area, and VDD is the voltage level when clock is ‘1’. When the MOS switch is turned off, half of the charge will inject to both the source and drain terminal contribute to an error voltage equals:
( )
where the CH represents the output capacitance.
However, in real circuit, this charge which injects to the source and drain terminal is not exact half of the channel charge. It depends on the impedance of both sides. If the impedance of one side is approximately infinite, total channel charge will
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flow to the other side.
Figure 3.6 Charge injection effect when a switch turns off.
Clock Feedthrough [31]
In additional to the error caused from charge injection, the MOS switch couples the gate clock through the gate-drain and gate-source overlap capacitance and further contribute to an another type error term. As shown in Fig. 3.7, this error can be
where Cov is the overlap capacitance per unit length.
This kind of error can be viewed as a constant offset if Cov is constant. Because it is independent of input level, post-calibration can be applied to cancel this offset perfectly. Besides, clock feedthrough is a trade-off between speed and precision. The response time of touch panel is in millisecond order which is a slow process. Hence, the clock feedthrough effect is not obvious in circuit of touch panel.
Figure 3.7 Clock feedthrough effect when a switch turns off.
Based on the formula mentioned above, both errors are proportional to the length and width of MOS switch. Therefore, in the proposed circuit, the switch is designed with the minimum width 3µm and minimum length 3µm. Although the smaller size of MOS devices will result in slower operation speed for circuit, the demand for touch panel response time is between tens Hz which is not a quite high value.
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Figure 3.8 Circuit configuration of A/D converter.
Fig. 3.8 shows the configuration of ADC suitable for LTPS technology [29], [30].
Again, the switch capacitor technique is applied to cancel the influence of threshold voltage variation of TFT device. All switches are controlled by the clock signals CLK5 or CLK6. The circuit operation has two steps, (1) storing the logic threshold voltage Vth,log on capacitor and (2) compensating Vth,log and comparing Vo with the reference voltage. At first, CLK6 is set to high and the difference between logic threshold voltage Vth,log of inverter and Vref is stored on the capacitor C2. In the comparison period, CLK6 is switched to low and CLK5 is set to high. Due to charge conservation, the input voltage of inverter becomes (Vo+Vth,log-Vref). Two inverter stages as buffer are added to guarantee full-swing of the output voltage.
Furthermore, this circuit also has immunity from threshold voltage variation since the Vth,log is cancelled by storing itself on C2. Four-bit resolution is achieved by using four same ADC structure with different reference voltages Vref1~Vref4 as shown in Fig. 3.9. Fig. 3.10 shows the simulated result of the proposed circuit under the non-touch event with the digital output code of ‘1111’. Fig. 3.11 shows the
simulated results of the proposed readout circuit under different Ct. The digital code of ADC presents ‘1110,’ ‘1100,’ ‘1000,’ and ‘0000’ under Ct = 1pF, 2pF, 3pF, and
>3pF, respectively. Depending on the digital bits of Vout, different touch position between two sensor lines can be judged by interpolation method and the overall resolution for touch panel can be enhanced.
Figure 3.9 The 4 bits on-panel readout circuit of capacitive sensor suitable for LTPS process.
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Figure 3.10 The simulated result of the proposed circuit under the non-touch event with the digital output code of ‘1111’.
The number of sensor lines on panel is limited. If the readout circuit can only distinguish whether the panel is touched or not, when the area between two sensors lines is touched, this kind of circuit cannot judge the correct position but choose one sensor line as the touched side. If the readout circuit can distinguish the different capacitance value due to different touch area, the interpolation method can be utilized to identify the more accurate position without more sensor lines and further to enhance the resolution for touch panel applications. The method for extracting touch position has been shown in Fig. 3.12. As shown in Fig. 3.12, when the touch position is between two sensor lines, the approximate touch position can be calculated by the equation:
the induced capacitance between touch object and sensor line, and WY is the distance between two sensor lines. Since Ca and Cb can be judged by digital codes, more output bits from ADC can gain the higher resolution for touch panel applications.
(a) (b)
(c) (d)
Figure 3.11 The simulated results of the proposed readout circuit with (a) Ct = 1pF (digital output code: ‘1110’), (b) Ct = 2pF (digital output code: ‘1100’), (c) Ct = 3pF (digital output code: ‘1000’), and (d) Ct > 3pF (digital output code: ‘0000’).
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Figure 3.12 The diagram of panel touched by finger.
3.3 Summary
An on-panel readout circuit for capacitive sensor has been designed and simulated. Using the proposed threshold voltage compensation technique, the output current can be reduced from 3120% to 29.3%. With 4 bits ADC, 4 different capacitances can be sensed. The interpolation method can be utilized to enhance the resolution for touch panel applications.