Coded data generation or conversion – Analog to or from digital conversion – Analog to digital conversion
Reexamination Certificate
2002-05-09
2003-06-24
Jeanglaude, Jean Bruner (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Analog to digital conversion
C341S142000
Reexamination Certificate
active
06583746
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an A/D converter that matches the common voltage of an analog input signal to the common voltage of reference voltages.
2. Description of Related Art
FIG. 5
is a circuit diagram showing a conventional A/D converter. In this figure, the reference numeral
1
designates an input buffer for temporarily holding an analog input signal; and
2
designates an A/D core for generating a plurality of reference voltages, and for converting the analog input signal supplied via the input buffer
1
into a digital output signal on the basis of the reference voltages.
Next, the operation of the conventional A/D converter will be described.
In
FIG. 5
, the input buffer
1
temporarily holds the analog input signal. The A/D core
2
generates the reference voltages corresponding to the number of bits of the digital conversion. For example, for the 2-bit digital conversion, it generates 2
2
=4 types of voltages such as 1, 2, 4 and 8 V, and converts the analog input signal supplied via the input buffer
1
to the digital output signal on the basis of the reference voltages.
Here, the analog input signal usually includes a DC (direct current) component, which is called a common voltage of the analog input signal.
An intermediate voltage between the plurality of reference voltages generated by the A/D core
2
is called a common voltage of the reference voltages. For example, as for the A/D core for the 2-bit digital conversion, the average of the intermediate reference voltages (2 and 4 V) of the four types of the reference voltages (1, 2, 4 and 8 V), that is, 3 V, is called the common voltage of the reference voltages.
To achieve precise digital conversion of the analog input signal, it is necessary for the A/D converter to match the common voltage of the analog input signal to the common voltage of the reference voltages.
The conventional A/D converter as shown in
FIG. 5
comprises a common mode feedback circuit in the input buffer
1
in order to match the common voltage of the analog input signal to the common voltage of the reference voltages.
The common mode feedback circuit comprises a voltage generator and an operational amplifier. The voltage generator generates a voltage nearly equal to the common voltage of the reference voltages. The operational amplifier compares the voltage generated by the voltage generator with the output voltage of the input buffer
1
, which is fed back. Thus, the common mode feedback circuit controls in such a fashion that the output voltage of the input buffer
1
agrees with the voltage generated by the voltage generator, that is, the common voltage of the analog input signal supplied to the A/D core
2
agrees with the common voltage of the reference voltages generated by the A/D core
2
.
With the foregoing configuration that comprises the common mode feedback circuit in the input buffer
1
, the conventional A/D converter matches the common voltage of the analog input signal supplied to the A/D core
2
to the common voltage of the reference voltages generated in the A/D core
2
, thereby enabling the analog input signal to undergo precise digital conversion.
High-speed A/D converters required recently, however, must have fast input buffers. Therefore, it is very difficult to fabricate complicated conventional input buffers including the common mode feedback circuit with the operational amplifier for handling the analog input signal.
SUMMARY OF THE INVENTION
The present invention is implemented to solve the foregoing problem. It is therefore an object of the present invention to provide a high-speed A/D converter with a simple configuration.
According to a first aspect of the present invention, there is provided an A/D converter comprising: a capacitor for removing a DC component of an analog input signal; a first DC voltage generator for generating a DC voltage controlled in response to a feedback control voltage, and for applying the DC voltage to the analog input signal passing through the capacitor; a first input buffer for temporarily holding the analog input signal passing through the first DC voltage generator; an A/D core that generates a plurality of reference voltages and an intermediate voltage of the reference voltages, and that converts the analog input signal passing through the first input buffer into a digital output signal on the basis of the reference voltages; a second DC voltage generator for generating, in response to the feedback control voltage, a same DC voltage as the DC voltage the first DC voltage generator generates; a second input buffer consisting of a replica of the first input buffer, for temporarily holding the DC voltage the second DC voltage generator generates; and an operational amplifier for supplying the feedback control voltage to the first DC voltage generator and the second DC voltage generator to match the DC voltage passing through the second input buffer to the intermediate voltage of the reference voltages the A/D core generates.
Here, the first DC voltage generator may comprise a first conductivity type transistor and a resistor, which are connected in series between a power supply and a ground, and the first input buffer may be connected to a connecting point of the first conductivity type transistor and the resistor; the second DC voltage generator may comprise a first conductivity type transistor and a resistor, which are connected in series between the power supply and the ground, and the second input buffer may be connected to a connecting point of the first conductivity type transistor and the resistor; and the first conductivity type transistors may have their gates supplied with the feedback control voltage.
The first DC voltage generator may comprise a resistor and a second conductivity type transistor, which are connected in series between a power supply and a ground, and the first input buffer may be connected to a connecting point of the resistor and the second conductivity type transistor; the second DC voltage generator may comprise a resistor and a second conductivity type transistor, which are connected in series between the power supply and the ground, and the second input buffer may be connected to a connecting point of the resistor and the second conductivity type transistor; and the second conductivity type transistors may have their gates supplied with the feedback control voltage.
The second input buffer may consist of a downsized replica of the first input buffer.
The A/D converter may further comprise a low-pass filter connected to an output terminal of the operational amplifier, for filtering the feedback control voltage.
According to a second aspect of the present invention, there is provided an A/D converter comprising: a first capacitor for removing a DC component of a first analog input signal; a first DC voltage generator for generating a DC voltage controlled in response to a feedback control voltage, and for applying the DC voltage to the first analog input signal passing through the first capacitor; a first input buffer for temporarily holding the first analog input signal passing through the first DC voltage generator; a second capacitor for removing a DC component of a second analog input signal; a second DC voltage generator for generating a DC voltage controlled in response to a feedback control voltage, and for applying the DC voltage to the second analog input signal passing through the second capacitor; a second input buffer for temporarily holding the second analog input signal passing through the second DC voltage generator; an A/D core that generates a plurality of reference voltages and an intermediate voltage of the reference voltages, and that converts a differential input between the first analog input signal passing through the first input buffer and the second analog input signal passing through the second input buffer into a digital output signal on the basis of the reference voltages; a third DC voltage generator for generating, in response to the feedback control
Burns Doane , Swecker, Mathis LLP
Jeanglaude Jean Bruner
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