Capacitance sensor linearity improvement method

Abstract

The invention discloses a capacitance sensor linearity improvement method. According to the method, a fourth voltage buffer M4 is added, the fourth voltage buffer M4, a first voltage buffer M1, a third voltage buffer M3 and a second voltage buffer M2 are sequentially connected in series between a high-level Vdd and an earth terminal Vss, the fourth voltage buffer M4 tracks an input signal througha level shifter, and therefore no matter how the input signal of an input end IN changes, the VDS of the first voltage buffer M1 can maintain unchanged or have a little change; output impedance of thefirst voltage buffer M1 is effectively increased; meanwhile, a current matched with an output load current is led into a connection point between the third voltage buffer M3 and the second voltage buffer M2; and the current flowing through the first voltage buffer M1 maintains Ibias without the influence of an output current source. Through the capacitance sensor linearity improvement method, thecurrent flowing through the first voltage buffer M1 maintains Ibias without the influence of the output current source.

Description

technical field [0001] The invention relates to a method for increasing the linearity of a capacitive sensor. Background technique [0002] Currently, in capacitive sensors, the input capacitor is converted into a voltage signal at the front end and buffered for input to subsequent circuits such as PGA (Programmable Gain Amplifier) ​​and ADC (Analog to Digital Converter). In many applications, the linearity of the data signal is very important. For example, when the thickness of the smartphone display increases, in addition to maximizing the sensitivity, the linearity requirements of the capacitive sensor also need to be met as much as possible to adapt to the subsequent digital processing; the higher the linearity at that time, the false acceptance rate (FAR) of fingerprint acceptance will be lower. The lower it is, the lower the False Recognition Rate (FRR). [0003] The voltage buffer plays an important role in improving the linearity, in the case of the fingerprint cap...

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Problems solved by technology

[0005] On the one hand, limited by the output impedance of the voltage buffer-M1, the source-following function of the voltage buffer-M1 at the output end will be distorted, especially under the short-channel effect, the distortion will be more serious; on the other hand, the load The current iL changes the voltage buffer-M1, which will cause signal distortion and reduce the linearity of the buffer

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