Common voltage generating circuit and switched capacitor circuit using the same

Abstract

Depending on the power supply voltage of the analog circuit, the input voltage of the operational amplifier circuit may exceed the operating range of the input differential pair. In particular, in the case of designing in a semiconductor process with a low power supply voltage, there is a problem that the operating range of the input voltage of the operational amplifier becomes narrow and the input voltage of the operational amplifier exceeds the operating range.SOLUTION: A common voltage generation circuit according to the present disclosure is a common voltage generation circuit for a switched capacitor circuit, and dynamically changes and outputs an analog common voltage (VSG) in accordance with at least a common component (VIN - C) of an input voltage.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The disclosed technology relates to a common voltage generating circuit and a switched capacitor circuit using the same. [Background technology]

[0002] Switched capacitor circuits are often used for analog signal processing in ICs. Switched capacitor circuits substitute the function of resistive elements by charging and discharging electric charges to capacitive elements, and are effective in circuits that require relative accuracy between capacitance and resistance, such as frequency filter circuits and integrating circuits. Examples of applications of switched capacitor circuits as frequency filter circuits or integrator circuits include ΔΣ DACs and ΔΣ ADCs. The switched capacitor circuit is disclosed in, for example, Non-Patent Document 1 below. In a ΔΣ DAC, a switched capacitor circuit is used to perform D / A conversion of a digitally modulated signal and as a low-pass filter to remove quantization noise. A ΔΣ ADC is roughly divided into an integrator and a comparator, and a switched capacitor circuit including an operational amplifier is used in the integrator section.

[0003] Analog circuits inside ICs are often designed as fully differential. Designing a switched-capacitor circuit as fully differential has the advantage of being able to remove noise components that are common-phase on the ± side, such as errors caused by charge injection during switching. For example, an example of a ΔΣ DAC using a differential input operational amplifier is disclosed in the following non-patent document 2. On the other hand, a problem with fully differential circuits is that when a fully differential operational amplifier circuit is used, the input common voltage of the operational amplifier circuit may fluctuate depending on conditions such as the input voltage to the circuit. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Akira Matsuzawa, "First Analog Electronic Circuits: Practical Circuits", Kodansha, pp. 28-32 [Non-patent document 2] Shanthi Pavan, Richard Schreier, Gabor C. Temes, Takao Waho, Akira Yasuda, "Introduction to Delta-Sigma Analog-to-Digital Converters," Maruzen Publishing, p. 483, Figure 13.29 Summary of the Invention [Problem to be solved by the invention]

[0005] The input stage of an operational amplifier circuit designed using a CMOS process is generally composed of an input differential pair of Nch or Pch MOSTr. Here, for the operational amplifier circuit to operate normally, the gate-source voltage of the input differential pair must be greater than the threshold value of the MOSTr. If the input common voltage of an operational amplifier is close to the ground potential or the power supply voltage, the circuit designed as an Nch or Pch type may not be able to amplify properly and may not function as a circuit.

[0006] An example of a case in which the input common voltage of an operational amplifier circuit in a fully differential circuit fluctuates is when integrating the input voltage of an ADC. For example, when a single-ended signal referenced to ground (0 V) is input to an ADC designed as a fully differential type, the input common voltage will fluctuate from 0 to half the worst-case reference voltage. Depending on the power supply voltage of the analog circuit, the input voltage of the operational amplifier circuit may exceed the operating range of the input differential pair. In particular, when designing using a semiconductor process with a low power supply voltage, the operating range of the input voltage of the operational amplifier becomes narrower, posing the problem of the input voltage exceeding the operating range of the operational amplifier. [Means for solving the problem]

[0007] The common voltage generating circuit according to the present disclosure is a common voltage generating circuit for a switched capacitor circuit, and generates an analog common voltage (V SG ) at least the common component of the input voltage (V IN_C ) and dynamically change and output it. [Effects of the Invention]

[0008] Because of these technical features, the common voltage generation circuit of the disclosed technology has the effect of preventing the input voltage of the operational amplifier circuit from exceeding the operating range of the input differential pair, regardless of the power supply voltage of the analog circuit. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a circuit diagram showing a ΔΣ ADC using a differential input switched capacitor circuit. [Figure 2] FIG. 2 is a circuit diagram showing a common voltage generating circuit (continuous time type) according to the first embodiment. [Figure 3] FIG. 3 is a circuit diagram showing a common voltage generating circuit (switched capacitor circuit type) according to the second embodiment. [Figure 4] FIG. 4 is a timing chart showing the timing of φ1 and φ2. DETAILED DESCRIPTION OF THE INVENTION

[0010] 1 is a circuit diagram showing a ΔΣ ADC using a differential input switched capacitor circuit (also called a "fully differential switched capacitor circuit"). In the circuit shown in FIG. 1, the front stage is an integrator and the rear stage is a comparator. In Figure 1, V IN+ and V IN- indicates a differential input terminal of an input signal or its voltage. In this specification, the subscript "+" indicates a positive input, and the subscript "-" indicates a negative input. In Figure 1, V REF+ and V REF-indicates a differential input terminal of a reference signal or its voltage (hereinafter referred to as "reference voltage"). In Figure 1, V SG is the analog common voltage. V SG The subscript SG is derived from the initials of Signal Ground. In Fig. 1, the switches labeled "φ1" are switches that are turned on in the φ1 phase, and the switches labeled "φ2" are switches that are turned on in the φ2 phase. Details regarding the switches will become clear from the explanation below. Fig. 4 is a timing chart showing the timing of φ1 and φ2. In FIG. 1, V indicates the output terminal of the comparator or its voltage.

[0011] Figure 1 shows the following mathematical formula: TIFF2026005249000002.tif7166 Here, z in equation (1) is the operator in the z transform used in digital control. -1 means "shift one sample period into the past." The meaning of formula (1) is that "when it is in the phase of φ2 and the comparator output voltage (V) one sample period ago is High, it is a switch that turns ON." In other words, when "in the phase of φ2 and the comparator output voltage (V) one sample period ago is high," the upper left of Figure 1 is V REF+ The switch connected to the terminal of is V REF- The switches connected to the terminals are closed and turned ON.

[0012] Figure 1 shows the following mathematical formula: TIFF2026005249000003.tif14166Equation (2) means that "this is a switch that turns ON when it is in the φ2 phase and the comparator output voltage (V) one sample period ago is LOW." In other words, when "in the phase of φ2 and the comparator output voltage (V) one sample period ago is LOW," the upper left of Figure 1 is V REF- The switch connected to the terminal of is V REF+ The switches connected to the terminals are closed and turned ON.

[0013] The differential input switched capacitor circuit shown in Figure 1 has an input voltage (V IN+ , V IN- ) and analog common voltage (V SG ) is connected, and the input signal is held as a charge in the input capacitance (C1). In the differential input switched capacitor circuit shown in Figure 1, during the φ2 phase, the signal input side (left side in Figure 1) of the input capacitance (C1) is connected to the reference voltage (V REF+ , V REF- ), and the other terminal (on the right in Figure 1) is connected to an integrator containing an operational amplifier. As a result of this action, the input voltage (V IN+ , V IN- ) and reference voltage (V REF+ , V REF- ) is charged to the integrator capacitance (C2).

[0014] In the differential input switched capacitor circuit shown in FIG. 1, the discrete transfer function of the front-stage integrator can be determined as follows: First, since the law of conservation of charge holds when the operating phase is φ1 and φ2, the following relationship holds: TIFF2026005249000004.tif17166Here, V, which appears in formula (3), CM represents the input common voltage of the op amp, and V O+ and V O- represents the op-amp output.

[0015] In this specification, the term "common" appears frequently, and it means a common signal component that is input to both the positive and negative sides of a differential amplifier. Expressed more intuitively, the common voltage can be said to be the average voltage of the positive and negative voltages. Therefore, the common components of the input voltage, output voltage, and reference voltage are given by the following equations, respectively: (Common component of input voltage) TIFF2026005249000005.tif12166 (common component of output voltage) TIFF2026005249000006.tif11166 (common component of reference voltage) TIFF2026005249000007.tif11166

[0016] By applying the common components defined in equations (4) to (6) to the two equations shown in equation (3), the relational equation for the common components can be expressed as follows: TIFF2026005249000008.tif10166Equation (7) is the input common voltage (V CM ) gives the following formula: TIFF2026005249000009.tif21166

[0017] Here, the output common voltage of the op amp (V O_C In order to keep the common-mode feedback constant, feedback control (hereafter referred to as "common-mode feedback") is applied. The output common voltage (V O_C ) can be considered constant, so the following relationship can be assumed: TIFF2026005249000010.tif7166If the relationship shown in equation (9) holds, equation (8) can be rewritten as follows. TIFF2026005249000011.tif17166

[0018] As can be seen from equation (10), in a differential input switched capacitor circuit, the input common voltage (V CM ) is the analog common voltage (V SG ) and the common component of the reference voltage (V REF_C ) and the common component of the input voltage (V IN_C ) and fluctuates depending on .

[0019] Embodiment 1 The common voltage generating circuit according to the present disclosure generates the common component (V IN_C ) and the common component of the reference voltage (V REF_C ) are assumed to be sufficiently slow relative to the operation of the differential input switched capacitor circuit. Then, (1-z -1 ) is small enough to be ignored. Equation (10) can be replaced by the following approximation: TIFF2026005249000012.tif8166

[0020] The common voltage generating circuit according to the present disclosure generates an analog common voltage (V SG ) to actively manipulate the input common voltage (V CM ) is the power supply voltage (hereinafter referred to as "V DD It is stabilized at half the value of the reference voltage (represented by "). Specifically, the common voltage generating circuit according to the present disclosure generates an analog common voltage (V SG ) actively manipulated. TIFF2026005249000013.tif12166Equation (11) and equation (12) determine the input common voltage (V CM ) stabilizes at the following value: TIFF2026005249000014.tif12166

[0021] 2 is a circuit diagram showing a common voltage generating circuit (continuous time type) according to the first embodiment. The common voltage generating circuit (continuous time type) shown in FIG. 2 is a subtraction circuit using an operational amplifier, and therefore the output voltage (V SG ) is given by the following formula: TIFF2026005249000015.tif23166 In this way, the common voltage generation circuit (continuous time type) according to the first embodiment generates an analog common voltage (V SG ) and actively manipulates the input common voltage (V CM ) to the power supply voltage (V DD ) can be stabilized at half the value.

[0022] Embodiment 2 The common voltage generating circuit according to the first embodiment solves the problems of the present invention by using a continuous-time circuit. The common voltage generating circuit according to the disclosed technique is not limited to the continuous-time type. The common voltage generating circuit according to the second embodiment solves the problem of the present invention by using a switched capacitor circuit.

[0023] FIG. 3 is a circuit diagram showing a common voltage generating circuit (switched capacitor circuit type) according to the second embodiment. FIG. 3 is an imperfect integrator circuit using a switched capacitor circuit. The output voltage (V SG ) can be expressed as follows by using the z-transform: First, the following relationship holds due to the law of conservation of charge in the φ1 phase and the φ2 phase. TIFF2026005249000016.tif13166Here, γ appearing in equation (15) is a coefficient.

[0024] From equation (15), the operational amplifier output (V O ) can be derived as follows: TIFF2026005249000017.tif24166

[0025] In the case of the common voltage generating circuit (switched capacitor circuit type) according to the second embodiment, the output voltage (V SG ) becomes: TIFF2026005249000018.tif23166Here, the common component of the input voltage (V IN_C ) and the common component of the reference voltage (V REF_C ) are assumed to be sufficiently slow relative to the operation of the differential input switched capacitor circuit. Then, (1-z -1 ) is small enough to be ignored. Equation (17) can be replaced by the following approximation: TIFF2026005249000019.tif12166 In this way, the common voltage generating circuit (switched capacitor circuit type) according to the second embodiment also generates an analog common voltage (V SG ) and actively manipulates the input common voltage (V CM ) to the power supply voltage (V DD ) can be stabilized at half the value.

[0026] Both the common voltage generation circuit according to the first embodiment and the common voltage generation circuit according to the second embodiment are intended for the ΔΣ ADC using the differential input switched capacitor circuit shown in FIG. The optimum analog common voltage will vary depending on the configuration of the switched capacitor circuit other than that shown in Figure 1. As such, when designing a common voltage generation circuit, it is important to consider how the common voltage of the input signal and the common component of the reference voltage each contribute.

[0027] One aspect of the common voltage generation circuit according to the disclosed technology is a common voltage generation circuit for a switched capacitor circuit, which generates an analog common voltage (V SG ) at least the common component of the input voltage (V IN_C ) and dynamically change and output it. Because of these technical features, the common voltage generation circuit of the disclosed technology has the effect of preventing the input voltage of the operational amplifier circuit from exceeding the operating range of the input differential pair, regardless of the power supply voltage of the analog circuit.

[0028] One aspect of the common voltage generation circuit according to the disclosed technology is a common voltage generation circuit for a fully differential switched capacitor circuit, in which an analog common voltage (V SG ) to the common voltage (V IN_C ) dynamically changes depending on the input common voltage (V CM ) is controlled to a constant value. Because of these technical features, the common voltage generation circuit of the disclosed technology has the effect of preventing the input voltage of the operational amplifier circuit from exceeding the operating range of the input differential pair, regardless of the power supply voltage of the analog circuit.

[0029] One aspect of the common voltage generation circuit according to the disclosed technology is a common voltage generation circuit for a fully differential switched capacitor circuit, in which an analog common voltage (V SG ) to the common voltage (V IN_C ) dynamically changes depending on the input common voltage (V CM ) is controlled to a constant value, and further, the fully differential switched capacitor circuit is an integrator. Because of these technical features, the common voltage generation circuit of the disclosed technology has the effect of preventing the input voltage of the operational amplifier circuit from exceeding the operating range of the input differential pair, regardless of the power supply voltage of the analog circuit, which is the integrator.

[0030] One aspect of the common voltage generation circuit according to the disclosed technology is a common voltage generation circuit for a fully differential switched capacitor circuit, in which an analog common voltage (V SG) to the common voltage (V IN_C ) dynamically changes depending on the input common voltage (V CM ) is controlled to a constant value, and further, the fully differential switched capacitor circuit is a filter (for example, a frequency filter). Because of these technical features, the common voltage generation circuit according to the disclosed technology has the effect of preventing the input voltage of the operational amplifier circuit from exceeding the operating range of the input differential pair, regardless of the power supply voltage of the analog circuit, which is a filter.

[0031] One aspect of the common voltage generation circuit according to the disclosed technology is a common voltage generation circuit for a fully differential switched capacitor circuit, in which an analog common voltage (V SG ) to the common voltage (V IN_C ) dynamically changes depending on the input common voltage (V CM ) is controlled to a constant value, and furthermore, the fully differential switched capacitor circuit is a ΔΣ ADC. Because of these technical features, the common voltage generation circuit of the disclosed technology has the effect of preventing the input voltage of the operational amplifier circuit from exceeding the operating range of the input differential pair, regardless of the power supply voltage of the analog circuit, which is a ΔΣ ADC.

[0032] The switched capacitor circuit according to the present disclosure has an analog common voltage (V SG ) at least the common component of the input voltage (V IN_C ) and includes a common voltage generation circuit that dynamically changes and outputs the common voltage. Because of these technical features, the switched-capacitor circuit according to the disclosed technology has the effect of preventing the input voltage of the operational amplifier circuit from exceeding the operating range of the input differential pair, regardless of the power supply voltage of the analog circuit. [Industrial Applicability]

[0033] The disclosed technology can be applied to, for example, a ΔΣ ADC, a ΔΣ DAC, and a ΔΣ modulator, and has industrial applicability.

Claims

1. A common voltage generating circuit for a fully differential switched capacitor circuit, comprising: Analog common voltage (V ＳＧ ) at least the common component of the input voltage (V ＩＮ＿Ｃ ) and dynamically change and output it. Common voltage generation circuit.

2. A common voltage generating circuit for a fully differential switched capacitor circuit, comprising: Analog common voltage (V ＳＧ ) to the common voltage (V ＩＮ＿Ｃ ) and dynamically change and output it. Op-amp input common voltage (V ＣＭ ) is controlled to a constant value. Common voltage generation circuit.

3. the fully differential switched capacitor circuit is an integrator.

3. The common voltage generating circuit according to claim 2.

4. the fully differential switched capacitor circuit is a frequency filter; 3. The common voltage generating circuit according to claim 2.

5. A common voltage generating circuit for a fully differential switched capacitor circuit, comprising: Analog common voltage (V ＳＧ ) to the common voltage (V ＩＮ＿Ｃ ) and the common voltage of the differential input reference voltage (V REF＿Ｃ ) and dynamically change and output it. Op-amp input common voltage (V ＣＭ ) is controlled to a constant value. Common voltage generation circuit.

6. the fully differential switched capacitor circuit is an ADC; 6. The common voltage generating circuit according to claim 2 or 5.

7. A common voltage generating circuit according to any one of claims 1 to 5 is used. Switched capacitor circuits.

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