Method, apparatus and system for level shifting of common mode voltage from input to output for capacitive feedback transimpedance amplifier

This application claims priority from U.S. Provisional Patent Application No. 61/949,391 filed on Mar. 7, 2014, which is hereby incorporated by reference in its entirety.

1. Field of the Invention

Embodiments of the present invention generally relate to a method, apparatus and system for touch screen processing. More specifically, to a level shifting of common mode voltage from input to output for capacitive feedback transimpedance amplifier

2. Description of the Related Art

In a touch screen's amplifier configuration, the input of amplifier is capacitive. However, the amplifier configuration is required to be held at DC voltage different than output of the amplifier. As a result, a lot of DC current is wasted, which is inefficient, or another analog stage of amplifier is required, which comes at a cost of adding another source of noise to the system and its associated complexity.

Therefore, there is a need to hold the DC voltage without the extra stage or wasted DC power.

Embodiments of the present invention relate to a circuit for a level shifting of common mode voltage. The circuit includes a first amplifier, wherein the input of the first amplifier is coupled to a voltage source and another input of the first amplifier is coupled 2.5v, feedback resistor, R fb , and feedback capacitor, C fb , connected coupled to the voltage source, wherein other side of feedback resistor is coupled between two resistors, R 2 and R 2 ′, and wherein the other side of the feedback capacitor is coupled between R 2 ′ and the output of the first amplifier, R 2 is connected to V bias from one side and R fb and R 2 ′ from the other, R 2 ′ is connected to R fb and R 2 from one side and C fb and output of the first amplifier from the other side, another resistor, R 1 , is connect to the output of the first amplifier, C fb and R 2 ′ from one side and R 1 ′, yet another resistor, and input of amp 2 from the other, a second amplifier, Amp 2 , is connected to the R 1 and R 1 ′ at one input and 1.5v at the other input, and wherein the output of the second amplifier is connected to R 1 ′, and R 1 ′ connects to one input of the second amplifier and the output of the second amplifier.

So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

FIG. 1 is an embodiment of a circuit schematic of the prior art;

FIG. 2 is an embodiment of a graph depicting performance of the prior art;

FIG. 3 is an embodiment of a circuit schematic for a level shifting of common mode voltage;

FIG. 4 is an embodiment of a graph depicting performance of the circuit schematic of FIG. 3 .

The proposed solution provides for a way of providing DC feedback which leads the amplifier to think it is regulating the same voltage both at the input and the output. The solution uses a common mode DC shift current inside the feedback loop, which maintains a different DC voltage at the output with respect to the input of the amplifier.

FIG. 1 is an embodiment of a circuit schematic of the prior art. The circuit comprises ample 1 , amp 2 , R fb , C fb , R 1 , R 1 ′ and switch S 1 . The amp 1 receives input from an input voltage source and 2.5 v. The amp 1 is connected in series with R fb and C fb , where one lead of R fb and C fb is coupled to the voltage source and the other is coupled to the output of amp 1 . V 1 is measures at the output of amp 1 to be 2.5v. R 1 is connect in series at the output of amp 1 . R 1 is coupled to the output of amp 1 , R 1 ′ on one side and R 1 ′ and S 1 on the other. S 1 is coupled to R 1 , R 1 ′ and amp 2 from one side and V bias one the other. R′ 1 is connected to the input of amp 2 , R 1 and V bias ; on the other side, R 1 ′ is connected to the output of amp 2 . Amp 2 has input voltage of 1.5 v on one lead and R 1 , R 1 ′ and S 1 on the other input lead. The output of amp 2 is v 2 . FIG. 2 is an embodiment of a graph depicting performance of the prior art. The performance of this circuit is shown in the graph of FIG. 2 and is defined as follows:

FIG. 3 is an embodiment of a circuit schematic for a level shifting of common mode voltage. The circuit comprises ample 1 , amp 2 , R fb , C fb , R 1 , R 1 ′, R 2 , and R 2 ′. One lead of amp 1 is coupled to the voltage source and the other to 2.5v. One lead of R fb and C fb are connected to the voltage source. The other side of R fb is connected between R 2 and R 2 ′. R 2 is connected to V bias from one side and R fb and R 2 ′ from the other. R 2 ′ is connected to R fb and R 2 from one side and C fb and output of amp 1 from the other side. R 1 is connect to the output of amp 1 , C fb and R 2 ′ from one side and R 1 ′ and input of amp 2 from the other. Amp 2 is connected to the R 1 and R 1 ′ at one input, 1.5v at the other input and R 1 ′ at its output. R 1 ′ connects to one input of amp 2 and the output of amp 2 . FIG. 4 is an embodiment of a graph depicting performance of the circuit schematic of FIG. 3 . The performance of this circuit is shown in the graph of FIG. 4 and is defined as follows:

The improved solution between FIG. 1 and FIG. 3 does not require an increase DC power or noise of system and is simple and simple to implement.

While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.