JP2004222018A - Switched capacitor amplifier circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a switched capacitor amplifier circuit capable of solving the problem that, when, in a conventional switched capacitor amplifier circuit, an input voltage itself has an offset DN pressure, the offset voltage is amplified and of reducing influences of the offset voltage. <P>SOLUTION: It is possible to cancel the offset voltage included in an input signal by constituting the circuit such that, in a sampling phase, a capacity connected to an input terminal is connected to a reference voltage in a reset phase and a difference between the reference voltage and a standard voltage given to a switch is controlled. Thus, it is possible to amplify just a signal element by canceling the offset voltage which the input voltage has. Also, when the offset voltage, which the input voltage has, has a temperature characteristic, it is possible to cancel the offset voltage including the temperature characteristic. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a switched capacitor amplifier circuit for canceling an offset voltage.
[0002]
[Prior art]
A conventional offset-cancelled switched-capacitor amplifying circuit is configured to store an offset voltage of an operational amplifier in a capacitor so that an offset voltage is not generated in an output (for example, see Patent Document 1).
[0003]
[Patent Document 1]
U.S. Pat. No. 4,543,534 (section 1-3, FIG. 1)
FIG. 4 shows an example of a circuit configuration of a conventional offset cancellation type switched capacitor amplifier circuit.
[0004]
In the reset phase φ1, the switch circuits 123, 124, 125, 128, 129, 132 are closed. At this time, the capacitors 101, 102, 103 and 104 are discharged through the switch circuits 123, 124, 125 and 129. After a certain time, the switch circuits 123, 124, 125, 128, 129, 132 open and the reset phase Φ1 ends.
[0005]
Next, the process proceeds to the sampling phase Φ2. The switch circuits 121, 122, 126, 130, 128, 132 are closed. The voltage of the input terminal 141 is stored in the capacitor 101 and the voltage of the input terminal 142 is stored in the capacitor 102 as electric charge. The charge of the capacitor 103 changes by an amount equal to the change in the charge of the capacitor 101. At the same time, the charge of the capacitor 104 changes by an amount equal to the change in the charge of the capacitor 102. As a result, the voltage of the output terminal 151 changes. The voltage at the output terminal 151 is given by equation (1).
[0006]
Vout = − (C1 / C2) * (Vin1-Vin2) (1)
Is the input offset voltage of the operational amplifier reset phase? 1 and stored in the capacitors 101 and 102. The change in the potential between both ends of the capacitor 101 during the sampling phase Φ2 is the difference between the voltage at the input terminal 141 and the reference voltage supplied to the switch 123. Similarly, the change in the potential between both ends of the capacitor 102 during the sampling phase Φ2 is the difference between the voltage at the input terminal 142 and the reference voltage supplied to the switch 124. Therefore, a change in the voltage stored between both ends of the capacitors 101 and 102 becomes a difference between the input voltage and the reference voltage, and does not include the offset voltage. Therefore, the offset voltage of the operational amplifier is not amplified and is canceled.
[0007]
[Problems to be solved by the invention]
However, in the conventional switched capacitor amplifier circuit, although the offset voltage of the operational amplifier can be canceled, when the input voltage itself has the offset voltage, there is a disadvantage that the offset voltage is amplified and output.
[0008]
[Means for Solving the Problems]
In order to solve the above problem, the present invention uses a reference voltage in addition to the reference voltage, and controls the difference between the reference voltage and the reference voltage to cancel the offset voltage of the input voltage. In the switched capacitor amplifier circuit configured as described above, since the offset voltage of the input voltage is canceled, no error occurs in the output offset.
[0009]
Even if two reference voltages are provided and the difference between the reference voltages is equal to the offset voltage of the input voltage, no offset error occurs in the output. By exchanging the nodes for providing these two reference voltages with each other, it is possible to correspond both when the polarity of the offset voltage is positive and when the polarity of the offset voltage is negative.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
The switched capacitor amplifier circuit according to the present invention includes a first input terminal to which a first input signal is input, a second input terminal to which a second input signal is input, and an output of the first input terminal. A first capacitor to which a signal based on the output of the first capacitor is input, a second capacitor to which a signal based on the output of the second input terminal is input, a signal based on the output of the first capacitor, and the second output And an operational amplifier that compares the signals based on the signals and outputs the signals. Further, a first reference voltage terminal to which a first reference voltage for supplying charges to the first and second capacitors is applied, and a second reference voltage for supplying charges to the first and second capacitors are applied. And a second reference voltage terminal. Then, the first or second reference voltage is changed so that the difference between the voltage values of the first reference voltage and the second reference voltage matches the offset voltage between the first input terminal and the second input terminal. It is characterized in that one or both are adjusted. Thereby, the offset voltage of the input voltage can be canceled, and only the signal component can be amplified.
[0011]
Further, in the switched capacitor amplifier circuit according to the present invention, when the first reference voltage has a temperature characteristic and the offset voltage between the first input terminal and the second input terminal has a temperature characteristic, the first reference voltage has a temperature characteristic. The first reference voltage sets the temperature characteristic such that the difference between the reference voltage of the second reference voltage and the voltage value of the second reference voltage matches the offset voltage between the first input terminal and the second input terminal. Features. Thus, when the offset voltage of the input voltage has a temperature characteristic, the offset voltage including the temperature characteristic can be canceled.
[0012]
【Example】
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is an example of a configuration diagram of a switched capacitor amplifier circuit according to the present invention. In the reset phase φ1, the switch circuit 123 is closed, the capacitor 101 is connected to the reference voltage 111, the switch circuit 124 is closed, and the capacitor 102 is connected to the reference voltage 112. At the same time, the switch circuits 125 and 129 are closed and the capacitors 103 and 104 are discharged. After a certain period of time, the switch circuits 123, 124, 125, and 129 open, and the reset phase Φ1 ends. During the reset phase Φ1, the charge stored in the capacitor 101 is q = C1 * VREF1. The charge stored in the capacitor 102 is q = C1 * VREF2.
[0013]
Next, the process proceeds to the sampling phase Φ2. The switch circuits 121, 122, 126, 130, 128, 132 are closed. The voltage of the input terminal 141 is stored in the capacitor 101 and the voltage of the input terminal 142 is stored in the capacitor 102 as electric charge. The charge of the capacitor 103 changes by an amount equal to the change in the charge of the capacitor 101. At the same time, the charge of the capacitor 104 changes by an amount equal to the change in the charge of the capacitor 102.
[0014]
As a result, the voltage of the output terminal 151 changes. In the sampling phase Φ2, the electric charge stored in the capacitance 101 is q = C1 * Vin1. The electric charge stored in the capacitance 102 is q = C1 * Vin2.
[0015]
Therefore, the change in the charge amount of the capacitor 101 after the change from the reset phase Φ1 to the sampling phase Φ2 is Δq = C1 * (Vin1−VREF1), and the change in the charge amount of the capacitor 102 is Δq = C1 * ( Vin2-VREF2).
[0016]
When the voltage Vin1 of the input terminal 141 is composed of the signal voltage Vinp and the offset voltage Vos, and the voltage Vin2 of the input terminal 142 is composed of only the signal voltage Vinn, the voltage of the output terminal 151 becomes
Vout =-(C1 / C2) * {(Vin1-Vin2)-(VREF1-VREF2)}
= − (C1 / C2) * {(Vinp + Vos−Vinn) − (VREF1-VREF2)}
= − (C1 / C2) * {(Vinp−Vinn) + (Vos− (VREF1−VREF2))}
Given by
[0017]
By adjusting VREF1 and VREF2 so that Vos = VREF1-VREF2,
Vout =-(C1 / C2) * (Vinp-Vinn), and the offset voltage Vos of the input signal Vin1 can be canceled.
[0018]
As described above, in the circuit system of the present invention, it is possible to amplify only the signal component by canceling the offset voltage of the input voltage.
[0019]
In this example, two reference voltages 111 and 112 are used, but one of the two reference voltages is replaced with a reference voltage as shown in FIGS. It is clear that the offset voltage of the input voltage can be canceled using the difference between the input voltages. For example, in the example of FIG. 3, the voltage of the output terminal 151 is
Vout = − (C1 / C2) * {(Vinp−Vinn) + (Vos−VREF)}
[0020]
By adjusting the reference voltage 111 to be equal to Vos, Vout = (C1 / C2) * (Vinp−Vinn), and the offset voltage Vos of the input signal Vin1 can be canceled.
[0021]
The reference voltage mentioned here is usually called analog ground, and is also connected to other switches. The output voltage swings around this reference voltage. In the above-described example, the calculation is performed with the reference voltage set to 0 V to simplify the description.
[0022]
The advantages of having two reference voltages as in FIG. 1 are as follows. For example, the reference voltage 111 is a reference voltage having no temperature characteristics, and the reference voltage 112 is assumed to have a temperature characteristic. In this way, when the offset voltage of the input voltage has a temperature characteristic, the temperature characteristic of the offset voltage of the input voltage is canceled by using the reference voltage 112 having the temperature characteristic, and the input voltage is changed by using the reference voltage 111. The absolute value of the offset voltage generated by applying the voltage offset voltage and the reference voltage 112 can be matched.
[0023]
As shown in FIG. 7, for two input voltages, a node that applies two reference voltages 111 and 112 is switched by a switch, so that the temperature correction of the polarity opposite to the polarity of the temperature characteristic of the reference voltage 112 can be performed. Will be possible. For example, assume that the reference voltage 112 in FIG. 7 has a negative temperature characteristic as shown in FIG. When the offset voltage of the input voltage has a positive temperature characteristic, the positive temperature characteristic of the offset voltage of the input voltage is canceled using the clock 1a as shown in FIG. Conversely, when the offset voltage of the input voltage has a negative temperature characteristic, the negative temperature characteristic of the offset voltage of the input voltage is canceled using the clock 1b as shown in FIG.
[0024]
The circuit of FIG. 7 can be used in a two-stage amplifier circuit configuration as shown in FIG. At this time, the temperature characteristic of the offset voltage of the input voltage is multiplied by the gain of the first-stage amplifier circuit. When the temperature characteristics of the offset voltage of the input voltage are smaller than the temperature characteristics of the reference voltage, the temperature characteristics can be easily matched by adopting such a two-stage amplification configuration.
[0025]
The switch used in the description of the present invention is constituted by an N-type MOSFET, a P-type MOSFET, or a CMOS transmission gate in which these two are connected in parallel.
[0026]
The circuit of FIG. 1 can be implemented in a fully differential circuit having two inputs and two outputs as shown in FIG. It is obvious that not only the circuit of FIG. 1 but also the circuit used in the description of the present invention can be implemented in a fully differential circuit having two inputs and two outputs similarly to the circuit of FIG. By using a fully differential circuit configuration, an effect of reducing common-mode noise can be obtained.
[0027]
The circuit in FIG. 6 is an example in which the switched capacitor amplifier circuit has a two-stage configuration and offset cancellation is performed in the second stage.
[0028]
The circuit shown in this embodiment is an example of a switched-capacitor amplifier circuit, and can be implemented in other types of switched-capacitor amplifier circuits.
[0029]
If the value of the offset voltage included in the input voltage is known in advance and the value is constant, the reference voltage may be a fixed voltage.If the value of the offset voltage included in the input voltage is unknown, the reference voltage is set to the input voltage. By using a variable voltage that can be adjusted in accordance with the offset voltage, the offset voltage can be adjusted while observing the output voltage.
[0030]
The following method is considered as one example of creating this adjustable variable voltage. A desired voltage can be obtained by connecting switches in parallel to some of the resistors constituting the resistance ladder obtained by dividing the power supply voltage between the power supply voltages and opening and closing the switches based on data written in the EEPROM.
[0031]
An electronic device having the above-described switched-capacitor amplifier circuit can operate accurately because signal noise is reduced.
[0032]
【The invention's effect】
By canceling the offset voltage of the input voltage, only the signal component can be amplified. When the offset voltage of the input voltage has a temperature characteristic, the offset voltage including the temperature characteristic can be canceled.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of a switched capacitor amplifier circuit of the present invention.
FIG. 2 is a configuration diagram of a switched capacitor amplifier circuit of the present invention.
FIG. 3 is a configuration diagram of a switched capacitor amplifier circuit of the present invention.
FIG. 4 is a configuration diagram of a conventional switched capacitor amplifier circuit.
FIG. 5 is a configuration diagram of a switched capacitor amplifier circuit of the present invention.
FIG. 6 is a configuration diagram of a switched capacitor amplifier circuit of the present invention.
FIG. 7 is a configuration diagram of a switched capacitor amplifier circuit of the present invention.
FIG. 8 is an example of the temperature dependence of a reference voltage.
FIG. 9 shows a clock waveform used in the switched capacitor amplifier circuit of the present invention.
FIG. 10 is a configuration diagram of a switched capacitor amplifier circuit of the present invention.
[Explanation of symbols]
100 operational amplifiers 101, 102, 103, 104, 105, 106 capacitances 111, 112 reference voltages 123, 124 switch circuits 141, 142 input terminals 151, 152 output terminals

Claims (3)

A first input terminal to which a first input signal is input;
A second input terminal to which a second input signal is input;
A first capacitor to which a signal based on an output of the first input terminal is input;
A second capacitor to which a signal based on the output of the second input terminal is input;
An operational amplifier that compares a signal based on the output of the first capacitor with a signal based on the second output and outputs a signal;
A first reference voltage terminal to which a first reference voltage for supplying charges to the first and second capacitors is applied;
A second reference voltage terminal to which a second reference voltage for supplying a charge to the first and second capacitors is applied;
The first or second reference voltage such that a difference between the voltage values of the first reference voltage and the second reference voltage matches an offset voltage between the first input terminal and the second input terminal. A switched capacitor amplifier circuit that adjusts one or both of the following. The first reference voltage has a temperature characteristic;
When an offset voltage between the first input terminal and the second input terminal has a temperature characteristic, a difference between a voltage value of the first reference voltage and a voltage value of the second reference voltage is equal to the first input terminal. 2. The switched-capacitor amplifier circuit according to claim 1, wherein the first reference voltage sets a temperature characteristic so as to coincide with an offset voltage between the first reference voltage and the second input terminal. An electronic apparatus comprising the switched capacitor amplifier circuit according to claim 1.

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