Applicant hereby claims priority under 35 U.S.C. xc2xa7119 to Japanese Application No. 2002-002964, filed Jan. 11, 2002, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an analogous IC for use in a digitizer, which is an input device of a computer, and in particular relates to a reference circuit having a variable-impedance function for reducing power consumption.
2. Background of the Invention
A conventional digitizer includes a reference circuit. Various analogous IC reference circuits for digitizers are known in the art. In an analogous IC for a digitizer driven by a single power supply, a common (COM) end of a sensor coil is raised to 0.5 voltage at the common collector (Vcc) using the reference circuit to deliver an alternating current during transmission. The reference circuit for raising the COM end to 0.5 Vcc has low output impedance for increasing the resistance against noise when a response signal receiver receives a signal from the sensor coil.
A high-frequency signal transmitter transmits a high-frequency signal to the sensor coil. The circuit includes a bypass capacitor, which receives a high-frequency current delivered from a driver circuit of the high-frequency signal transmitter. Since the output impedance of the reference circuit is relatively small, a current flows into the reference circuit from the sensor coil, thereby increasing current consumption.
In a conventional system, in order to reduce noise produced when the receiver receives a signal from the sensor coil, the reference coil is designed to have low output impedance. When the high-frequency signal transmitter transmits a signal to the sensor coil, the reference circuit absorbs as far as the electric charge stored in the bypass capacitor, thereby wasting the transmitting current.
A variable-impedance reference circuit switches output impedance of a reference circuit between the transmitting operation and the receiving operation of a high-frequency signal. During the transmitting operation, the output impedance of the reference circuit is switched to high impedance so as not to absorb the current delivered from a driver of a high-frequency signal transmitter. During the receiving operation, the output impedance of the reference circuit is switched to low impedance so as not to reduce the resistance against noise.
A variable-impedance reference circuit according to the present invention comprises a reference circuit for supplying a reference electric potential to a sensor coil, wherein the variable-impedance reference circuit switches an output impedance of the reference circuit so that the output impedance is large when transmitting a high-frequency signal to the sensor coil, and the output impedance is small when receiving a response signal from the sensor coil, thereby reducing current consumption. Preferably, the reference circuit comprises a loop gain adjusting circuit having a switch for switching a gain, so that the magnitude of the output impedance of the reference circuit is switched by opening and closing the switch to switch the gain.
The output impedance of the reference circuit according to the present invention is switchable. Therefore, when a signal from a sensor coil is received, the output impedance of the reference circuit is decreased to reduce noise. When a signal is transmitted to the sensor coil, the output impedance of the reference circuit is increased to prevent the charge of a bypath capacitor from being absorbed. Thus, when receiving a signal from the sensor coil, noise is reduced; when transmitting a signal to the sensor coil, current loss is reduced.