JP2003283266A - Offset canceller - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an offset canceller composed of a filter whose cutoff frequency is variable, and having a volume capable of being mounted on the same integrated circuit. <P>SOLUTION: The offset canceller is a feedback control circuit composed of a first amplifier A1, a low-pass filter LPF capable of adjusting its cutoff frequency by a control signal, and a second amplifier A2. The low-pass filter LPF extracts a signal band near DC level in an offset component generated in the first amplifier A1, and adjusts the cutoff frequency for detecting the offset component near the DC level by the control signal. Capacitance C equivalent to that of a conventional RC filter is constituted by capacitance Cm capable of being integrated on the same circuit, and a third amplifier A3. Though the capacitance Cm itself to be integrated is small, the capacitance C similar to that of the conventional RC filter is realized by connecting the capacitance Cm parallel to the third amplifier A3. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an offset canceller circuit that corrects an offset component by detecting an offset component generated in an amplifier and feeding back a detection error to the target amplifier.

[0002]

2. Description of the Related Art FIG. 5 is a basic block diagram of a conventional offset canceller circuit. The offset canceller circuit shown in FIG. 5 is a feedback control circuit, which supplies the output of the first amplifier A1 as the input of the second amplifier A2 and the output of the second amplifier A2 as the input of the first amplifier A1. We are supplying. In FIG. 5, the first amplifier A1
The input conversion offset component for the input signal of
Shown as fset. This offset component is the second
In the amplifier A2, the offset is canceled by amplifying based on the reference voltage Vref and supplying the amplified voltage to the first amplifier A1.

FIG. 6 is a block diagram of a conventional offset canceller circuit when a differential signal is used. It is a feedback control circuit similar to FIG. In FIG.
The low-pass filter circuit LPF is provided in front of the second amplifier A2.
have. As a result, unnecessary frequency components can be removed and effective band frequencies can be obtained.

An effective band frequency is taken out by the low pass filter circuit LPF from the positive and negative differential signals output from the first amplifier A1. The DC offset component is detected by inputting the extracted band frequency to the second amplifier A2. This DC offset component is applied to the first amplifier A1.
To cancel the offset.

As described above, in the conventional offset canceller circuit, in order to detect the offset component from the first amplifier A1, the low pass filter circuit LPF for selecting the cutoff frequency effective for the frequency near the direct current is constructed. There was a need.

[0006]

For example, a low pass filter circuit is composed of a resistor R and a capacitor C as shown in FIG. In particular, when configuring such a low-pass filter circuit, the capacitance C has a large value for detecting a signal in the vicinity of the DC band, and integration on the same integrated circuit becomes difficult. Therefore, the capacitor C becomes an external component outside the integrated circuit, which poses an integration problem such as an increase in the number of components.

Further, in the conventional offset canceller circuit shown in FIG. 6, since the low-pass filter circuit LPF is connected to each of the two output signal lines of the first amplifier A1, two capacitors C are required. is there. On the other hand, it is conceivable to use a transconductance circuit as a circuit configuration that makes the capacitance C one.

FIG. 7 is a block diagram of an offset canceller circuit when a transconductance circuit is used as a filter. The differential output signal (voltage component) of the first amplifier A1 is supplied to the transconductance circuit gm. The differential output signal (current component) amplified by the transconductance circuit gm is output, the voltage is converted by the capacitance C that short-circuits the two pairs of output signal lines, and the voltage is supplied to the first amplifier A1. As described above, in the offset canceller circuit shown in FIG. 7, the number of capacitors C forming the filter is reduced to one.

However, even when the transconductance circuit and the capacitor C are used as the filter in this way, the capacitor C requires a large value and becomes an external component outside the integrated circuit. Furthermore, since two pairs of output signal lines and the capacitor C, which is an external component, must be connected, two pins are required as external terminals. Especially, in a circuit including a plurality of offset canceller circuits,
This leads to a double increase in the number of pins in proportion to the number of offset canceller circuits.

An object of the present invention is to provide an offset canceller circuit composed of a filter having a variable cutoff frequency, which uses a capacitor having a size mountable on the same integrated circuit.

[0011]

An offset canceller circuit according to the present invention is a low pass filter for extracting a signal in a desired signal band from a first amplifier for amplifying an input signal and an output signal of the first amplifier. Circuit, a DC offset component is detected from an output signal of the low pass filter circuit and a reference signal, and the DC offset component is detected by the first
The second low-pass filter circuit has a cut-off frequency variable according to a control signal.

In addition, the offset canceller circuit according to the present invention has the first and second input signals for amplifying the first and second input signals.
Amplifier, a low-pass filter circuit for extracting a signal in a desired signal band from the first and second output signals of the first amplifier, and first and second outputs of the low-pass filter circuit. A second amplifier for detecting first and second offset components from the signal and supplying the first and second offset components to the first amplifier,
The low pass filter circuit is characterized in that the cutoff frequency is variable according to a control signal.

In particular, the low-pass filter circuit has one end connected to the first amplifier, an input terminal connected to the other end of the resistor, and an output terminal connected to the second amplifier. A third amplifier, and a capacitor connected between the input terminal and the output terminal of the third amplifier, wherein the third amplifier has a variable amplification degree according to a control signal. .

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION According to the present invention, the offset canceller circuit itself for correcting the offset generated in the first amplifier does not have a capacitance as an external component. Further, in order to correct the offset, the cutoff frequency of the low pass filter circuit can be adjusted with respect to the signal band. Hereinafter, embodiments of the present invention will be described with reference to the drawings. (First Embodiment) FIG. 1 is a block diagram of an offset canceller circuit according to the first embodiment. The offset canceller circuit in the first embodiment is a feedback control circuit including a first amplifier A1, a low-pass filter circuit LPF whose cutoff frequency can be adjusted by a control signal, and a second amplifier A2. is there.

The low-pass filter circuit LPF extracts the signal band near the direct current of the offset component generated in the first amplifier A1. Further, the control signal adjusts a desired frequency signal band for detecting an offset component near DC, that is, a cutoff frequency.

The second amplifier A2 detects an offset component from the signal taken out by the low pass filter circuit LPF based on the reference signal Vref. Then, the detected offset component is supplied to the first amplifier A1. Then, in the first amplifier A1, the offset generated in the first amplifier A1 is canceled based on the offset component detected by the second amplifier A2.

FIG. 2 is a specific circuit diagram of the low pass filter circuit according to the first embodiment. The low-pass filter circuit LPF in the first embodiment has a conventional R
The capacitance C in the C filter is composed of a capacitance Cm and a third amplifier A3 that can be integrated on the same circuit. As shown in FIG. 2, the capacitance Cm is arranged in parallel with the third amplifier A3 having the amplification degree of -A times. As a result, the following capacitance Cin can be obtained. Cin = (1 + A) Cm Even if the capacitance Cm itself is a small value that can be integrated, it is possible to increase the capacitance seen from the input side of the third amplifier A3 by connecting it in parallel to the third amplifier A3. it can.
Then, a capacitance having a size corresponding to the capacitance C in the conventional RC filter is realized.

Further, the cutoff frequency of the third amplifier A3, which has the amplification factor of −A times shown in FIG. 2, is adjusted by a control signal from the outside. The control signal allows the cutoff frequency to be adjusted to the frequency signal band.

As described above, in the present invention, the capacitance Cm forming the low pass filter circuit LPF has a size that can be sufficiently formed on the same integrated circuit as the other constituent circuits. Therefore, it is not necessary to externally attach a large value of capacitance in order to realize a low cutoff frequency near the DC band as in the conventional case. Therefore, in the offset canceller circuit of this embodiment, necessary circuits can be formed on the same integrated circuit.

Further, the third amplifier A3 constituting the low pass filter circuit LPF of the present invention can adjust the cutoff frequency in accordance with a desired signal band by the control signal, and therefore the first amplifier A1. It is possible to take out the signal band near the direct current of the offset component generated in. As a result, the output signal with good accuracy can be obtained by the first amplifier A1. (Second Embodiment) FIG. 3 is a block diagram of an offset canceller according to the second embodiment. The second embodiment is an offset canceller circuit that uses a differential signal as an input. The positive and negative differential output signals from the first amplifier A1 are supplied to the second amplifier A2 via the low pass filter circuit LPF to which the mirror capacitance shown in FIG. 2 is applied. Then, in the second amplifier A2, the DC offset component is detected, and the first amplifier A2 is detected.
A feedback control circuit is configured by supplying the feedback control circuit 1 to 1.

In the offset canceller circuit of FIG. 3, positive and negative differential signals are supplied to the first amplifier A1.
The positive differential signal is supplied to the first input signal line, and the negative differential signal is supplied to the second input signal line. Then, the supplied differential signal is amplified in the first amplifier A1,
Is output. The subtracted signal (negative output signal) is output to the first output signal line, and the added signal (positive output signal) is output to the second output signal line.

The first and second output signal lines are connected to the input terminal of the second amplifier A2 via the low pass filter circuit LPF shown in FIG. The low pass filter circuit LPF adjusts the cutoff frequency for a desired signal band by a control signal. With respect to each output signal from the first amplifier A1, a signal in a desired signal band is detected by the adjusted low pass filter circuit LPF. The resistor R1 and the capacitor C that constitute the low-pass filter circuit LPF connected to each of the first and second output signal lines.
m and the third amplifier A3 are both of the same performance.

The DC offset component generated in the first amplifier A1 is detected in the second amplifier A2. Second
The output signal supplied to the amplifier A2 is subtracted or added and amplified. The subtracted signal is supplied to the second input signal line, the added signal is supplied to the first input signal line, and is supplied to the first amplifier A1.

In this way, the DC offset component is detected from the output signal of the first amplifier A1, and the first amplifier A1 is detected again.
By feeding the signal to 1, the feedback control circuit is configured to cancel the offset component.

As described above, in the present invention, the capacitance Cm forming the low pass filter circuit LPF has a size that can be sufficiently formed on the same integrated circuit as other constituent circuits. Therefore, it is not necessary to externally attach a large value of capacitance in order to realize a low cutoff frequency near the DC band as in the conventional case. Therefore, in the offset canceller circuit of this embodiment, necessary circuits can be formed on the same integrated circuit.

Further, the cutoff frequency of the third amplifier A3 constituting the low pass filter circuit LPF of the present invention can be adjusted in accordance with a desired signal band by the control signal, so that the first amplifier A1 is used. It is possible to take out the signal band near the direct current of the offset component generated in. As a result, the output signal with good accuracy can be obtained by the first amplifier A1. (Third Embodiment) FIG. 4 is a block diagram of an offset canceller circuit according to the third embodiment. The first amplifier A1 in the first embodiment is a transimpedance circuit having a resistance load. The second amplifier A2 is a transconductance circuit.

As shown in FIG. 4, the offset canceller circuit according to the third embodiment has a transimpedance circuit Z, a low pass filter circuit LPF whose cutoff frequency can be adjusted by a control signal, and a transconductance circuit gm. It consists of

The transimpedance circuit Z is an amplifier which receives a current and outputs a voltage. A resistor R2 is connected between the input terminal and the output terminal of each pair of signal lines. Since the resistor R2 is connected,
The voltage on the output terminal side is determined.

Positive and negative differential signals are supplied to the transimpedance circuit Z. The positive differential signal is supplied to the first input signal line, and the negative differential signal is supplied to the second input signal line. Then, in the transimpedance circuit Z, the supplied differential signal is amplified and output. The subtracted signal (negative output signal) is output to the first output signal line, and the added signal (positive output signal) is output to the second output signal line.

The first and second output signal lines are connected to the input terminal of the transconductance circuit gm via the low pass filter circuit LPF. The low pass filter circuit LPF is the same as that shown in the first embodiment. Resistor R1, capacitance C that can be integrated on the same circuit
m and a third amplifier A3, the cutoff frequency for the desired signal band is adjusted by the control signal. The resistor R1, the capacitor Cm, and the third amplifier A3, which form the low-pass filter circuit LPF connected to the first and second output signal lines, have the same performance.

With respect to each output signal from the transimpedance circuit Z, a signal in a desired signal band is detected by a low pass filter circuit LPF whose cutoff frequency is adjusted by a control signal.

The transconductance circuit gm is an amplifier which receives a voltage and outputs a current. In the transconductance circuit gm, the DC offset component generated in the transimpedance circuit Z is detected. The differential output signal of the voltage component supplied to the transconductance circuit gm is subtracted or added, amplified and output as a current component. The subtracted signal is supplied to the second input signal line, the added signal is supplied to the first input signal line, and is supplied to the transimpedance circuit Z.

In this way, the DC offset component is detected from the output signal of the transimpedance circuit Z and is supplied to the transimpedance circuit Z again to form a feedback control circuit, which cancels the offset component.

As described above, in the present embodiment, the capacitance Cm forming the low pass filter circuit LPF has a size that can be sufficiently formed on the same integrated circuit as the other constituent circuits. Therefore, it is not necessary to externally attach a large value of capacitance in order to realize a low cutoff frequency near the DC band as in the conventional case. Therefore, in the offset canceller circuit of this embodiment, necessary circuits can be formed on the same integrated circuit.

Further, the third amplifier A3 constituting the low-pass filter circuit LPF in the present invention can adjust the cutoff frequency according to the desired signal band by the control signal, so that in the transimpedance circuit Z. The signal band near the direct current of the generated offset component can be extracted. As a result, the transimpedance circuit Z can obtain an accurate output signal.

Of course, various modifications can be made without departing from the spirit of the invention.

[0037]

The offset canceller circuit of the present invention does not need to be an external component because the capacity of the low-pass filter circuit LPF can be formed on the same integrated circuit as other constituent circuits. , The number of pins on the chip can be reduced.

Further, by making the gain (amplification degree) of the amplifier constituting the low pass filter circuit LPF variable,
The cutoff frequency of the low pass filter circuit LPF can be adjusted according to a desired signal band. As a result, it is possible to generate an accurate output signal in the offset canceller circuit.

[Brief description of drawings]

FIG. 1 is a block diagram of an offset canceller circuit according to a first embodiment.

FIG. 2 is a specific circuit diagram of a low pass filter circuit according to the first embodiment.

FIG. 3 is a block diagram of an offset canceller circuit according to a second embodiment.

FIG. 4 is a block diagram of an offset canceller circuit according to a third embodiment.

FIG. 5 is a block diagram of a conventional offset canceller circuit.

FIG. 6 is a block diagram of a conventional offset canceller circuit.

FIG. 7 is a block diagram of a conventional offset canceller circuit.

[Explanation of symbols]

A1 ... First amplifier A2 ... Second amplifier A3 ... Third amplifier LPF ... Low pass filter circuit R, R1, R2 ... Resistance Cm, C ... Capacity gm ... Transconductance circuit Z ... Transimpedance circuit

Continued front page    (72) Inventor Akira Tanabe             1st Komukai Toshiba-cho, Sachi-ku, Kawasaki-shi, Kanagawa             Ceremony Company Toshiba Microelectronics Sen             Inside F-term (reference) 5J090 AA01 CA13 CA91 CA92 CA98                       DN01 FA17 HA25 HA29 KA00                       KA42 MA11 NN12 TA01                 5J091 AA01 CA13 CA91 CA92 CA98                       FA17 HA25 HA29 KA00 KA42                       MA11 TA01                 5J098 AA11 AA14 AB01 AB31 AC02                       AC12 AD16 AD26 CA02                 5J500 AA01 AC13 AC91 AC92 AC98                       AF17 AH25 AH29 AK00 AK42                       AM11 AT01 ND01 NN12

Claims (6)

[Claims] 1. A first amplifier that amplifies an input signal, a low-pass filter circuit that extracts a signal in a desired signal band from an output signal of the first amplifier, and an output signal of the low-pass filter circuit. And a second amplifier that detects a DC offset component from a reference signal and supplies the DC offset component to the first amplifier, and the cutoff frequency of the low pass filter circuit is variable according to a control signal. An offset canceller circuit characterized in that 2. The low pass filter circuit has a resistor having one end connected to the first amplifier, an input terminal connected to the other end of the resistor, and an output terminal connected to the second amplifier. A third amplifier, and a capacitor connected between an input terminal and an output terminal of the third amplifier, wherein the third amplifier has a variable amplification degree according to a control signal. The offset canceller circuit according to claim 1. 3. A first for amplifying first and second input signals.
Amplifier, a low-pass filter circuit for extracting a signal in a desired signal band from each of the first and second output signals of the first amplifier, and first and second outputs of the low-pass filter circuit. A second amplifier that detects first and second offset components from the signal and supplies the first and second offset components to the first amplifier, wherein the low-pass filter circuit comprises: An offset canceller circuit characterized in that the cutoff frequency is variable by a control signal. 4. The offset canceller circuit according to claim 3, wherein the first amplifier is a transimpedance circuit, and the second amplifier is a transconductance circuit. 5. The low-pass filter circuit has a resistor having one end connected to the first amplifier, an input terminal connected to the other end of the resistor, and an output terminal connected to the second amplifier. A third amplifier, and a capacitor connected between an input terminal and an output terminal of the third amplifier, wherein the third amplifier has a variable amplification degree according to a control signal. An offset canceller circuit according to claim 3 or 4. 6. The offset canceller circuit according to claim 2, wherein the capacitor is formed on the same integrated circuit as another circuit that constitutes the offset canceller circuit.