Abstract
The capacitive touch screen measurement system mainly realizes the conversion of capacitance to digital signal, but the parasitic capacitance existing on the chip IO and PCB board will seriously affect the measurement accuracy of the small inductive capacitance in the capacitance measurement circuit. To solve this problem, a capacitance measurement circuit with adjustable compensation time based on current injection is proposed.The mismatch error in the circuit is analyzed in detail, and the measurement accuracy of capacitance is further improved. The dynamic range of capacitance measurement can be effectively improved by appropriately changing the parameter value of the relevant quantity in the circuit. The circuit design is based on the SMIC 0.18 μm standard CMOS process, and the overall layout area is about 0.21 mm2. The circuit is simulated using Spectre tool, and the results show that the capacitance measurement circuit inductive capacitance of 0 ~ 1 pF in the temperature range of -40 ℃ to 125 ℃, the conversion time is about 50 μs, and the resolution can reach 1 fF, the signal to noise ratio of the converted 12 bit digital signal is 70.1 dB, the effective number of bits is 11.56 bits, the power supply voltage is 1.8 V, and the total power consumption is 0.19 mW.
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