Cascade continuous time Sigma-Delta modulator resistant to out-of-band overshoot

Abstract

The invention relates to the technical field of integrated circuits, and discloses an out-of-band overshoot-resistant cascade continuous time Sigma-Delta modulator, which comprises a first-stage modulator, a second-stage modulator, a third-stage modulator, a fourth-stage modulator and a fifth-stage modulator, wherein the first-stage modulator comprises a first-stage integrator and a first-stage quantizer; comprising a second-stage integrator and a second-stage quantizer; the first summator is used for adding the output of the first-stage quantizer and the output of the second-stage quantizer in a digital domain and outputting an output signal of the cascade continuous time Sigma-Delta modulator; the input end of the second summator is respectively coupled with the negation signal of the output signal and the input signal, and the output end of the second summator is coupled to the input end of the first-stage integrator so as to form a first feedback path; and the input end of the third adder is respectively coupled with the negation signal of the output signal, the negation signal of the input signal and the output end of the first-stage integrator so as to form a second feedback path. Through the double-feedback structure, the signal transfer function of the modulator has a roll-off characteristic out of band, so that out-of-band overshoot is effectively suppressed.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of integrated circuits, and in particular to a cascaded continuous-time Sigma-Delta modulator with anti-out-of-band overshoot. BACKGROUND

[0002] This section is intended to provide background or context to the embodiments of the application and is not to be considered as admission that this section is prior art to the application.

[0003] A Sigma-Delta modulator is a key structure for implementing high-resolution analog-to-digital conversion using oversampling and noise shaping techniques, and is widely used in fields such as audio signal processing, high-precision data acquisition, communication systems, and radio frequency receivers. According to the different integrator structures, Sigma-Delta modulators can be divided into two categories: discrete-time type and continuous-time type. Among them, the continuous-time Sigma-Delta modulator has the advantages of low power consumption, strong anti-clock jitter capability, and high bandwidth utilization, and has become an important implementation form of high-performance analog-to-digital converters (ADCs).

[0004] In the design of high-order continuous-time Sigma-Delta modulators, in order to achieve higher noise shaping order while maintaining system stability, a multi-stage cascaded structure is usually used. This structure connects multiple low-order modulation loops in sequence to obtain equivalent high-order noise shaping performance. Traditional cascaded Sigma-Delta modulators need to use filters in the digital domain to cancel the quantization noise generated by the previous modulator, thereby achieving high-order noise shaping effect. However, in actual circuits, due to the limited gain of analog integrators, device mismatch, clock jitter and other non-ideal factors, the analog domain noise transfer function and the digital filter transfer function cannot be completely matched, resulting in quantization noise leakage, reducing the signal-to-noise ratio and linearity of the modulator.

[0005] In addition, for continuous-time Sigma-Delta modulators, the signal transfer function (STF) usually has an overshoot phenomenon at out-of-band frequencies. When the input signal frequency is high, the out-of-band overshoot will cause the modulator output to saturate, and in severe cases, it can even lead to loop instability, thereby limiting the maximum swing of the input signal and the system dynamic range. Existing improvement methods mainly focus on optimizing the noise cancellation path or adjusting the feedback topology, and most schemes can reduce quantization noise leakage, but cannot effectively suppress the out-of-band overshoot problem of continuous-time modulators.

[0006] Therefore, how to reduce the out-of-band overshoot effect of the continuous-time Sigma-Delta modulator while maintaining high precision and high stability of the cascade structure becomes a problem to be solved by those skilled in the art. SUMMARY

[0007] The purpose of the present application is to provide an anti-out-of-band overshoot cascade continuous-time Sigma-Delta modulator, aiming at solving the problems of existing continuous-time cascade Sigma-Delta modulators, such as out-of-band overshoot of signal transfer function (STF) under high-frequency input condition, and easy saturation of output. Although the traditional cascade structure can realize high-order noise shaping, it depends on digital filter to offset the quantization noise of the previous stage, and has defects such as complex structure, difficult transfer function matching and noise leakage. The present application proposes a cascade continuous-time Sigma-Delta modulator with a double feedback path structure, which realizes out-of-band overshoot suppression and quantization noise offset through analog domain structure improvement, thereby improving the linearity, stability and adaptability of the modulator.

[0008] The first aspect of the present application discloses an anti-out-of-band overshoot cascade continuous-time Sigma-Delta modulator, comprising: a first modulator, the first modulator comprising a first integrator and a first quantizer coupled in sequence; a second modulator, the second modulator comprising a second integrator and a second quantizer coupled in sequence; a first adder, the input end of the first adder being coupled to the output end of the first quantizer and the output end of the second quantizer respectively, for adding the output signals of the first modulator and the second modulator in the digital domain and outputting the output signal of the cascade continuous-time Sigma-Delta modulator; a second adder, the input end of the second adder being coupled to the negated signal of the output signal of the cascade continuous-time Sigma-Delta modulator for forming a first feedback path, the second adder adding the negated signal of the output signal of the cascade continuous-time Sigma-Delta modulator and the input signal of the cascade continuous-time Sigma-Delta modulator and then outputting to the input end of the first integrator; and a third adder, the input end of the third adder being coupled to the negated signal of the output signal of the cascade continuous-time Sigma-Delta modulator for forming a second feedback path, the third adder adding the negated signal of the output signal of the cascade continuous-time Sigma-Delta modulator, the negated signal of the input signal of the cascade continuous-time Sigma-Delta modulator, and the output signal of the first integrator and then outputting to the input end of the second integrator.

[0009] In one preferred embodiment, the signal transfer function of the cascade continuous-time Sigma-Delta modulator is H1 H2=1 / (s 2 τ1τ2), wherein H1 is the transfer function of the first integrator, τ1 is the time constant of the first integrator, H2 is the transfer function of the second integrator, τ2 is the time constant of the second integrator, and s is the Laplace operator.

[0010] In one preferred embodiment, the first integrator comprises a first operational amplifier, a first resistor and a first capacitor; wherein the positive input terminal of the first operational amplifier is grounded, the first capacitor is coupled between the negative input terminal and the output terminal of the first operational amplifier, the first resistor is connected in series to the negative input terminal of the first operational amplifier, and the time constant τ1 of the first integrator is equal to 1 / R1C1, wherein R1 is the resistance value of the first resistor and C1 is the capacitance value of the first capacitor.

[0011] In one preferred embodiment, the second integrator comprises a second operational amplifier, a second resistor and a second capacitor; wherein the positive input terminal of the second operational amplifier is grounded, the second capacitor is coupled between the negative input terminal and the output terminal of the second operational amplifier, the second resistor is connected in series to the negative input terminal of the second operational amplifier, and the time constant τ2 of the second integrator is equal to 1 / R2C2, wherein R2 is the resistance value of the second resistor and C2 is the capacitance value of the second capacitor.

[0012] In one preferred embodiment, the signal transfer function of the cascade continuous-time Sigma-Delta modulator has a roll-off characteristic at high frequency band to suppress the out-of-band overshoot of the input signal at high frequency band.

[0013] In one preferred embodiment, the transfer function of the first integrator is equal to the transfer function of the second integrator.

[0014] In one preferred embodiment, the first integrator and the second integrator are continuous-time integrators.

[0015] In one preferred embodiment, the input signal is a differential signal, and the negated signal of the input signal is the phase-inverted differential signal.

[0016] Compared with the prior art, the application has at least the following beneficial effects: In the embodiments of the application, the double feedback path structure makes the STF naturally roll off out of band by introducing two feedback path channels in the continuous-time cascade Sigma-Delta modulator, effectively prevents output saturation caused by high-frequency input, suppresses the STF out-of-band overshoot, and improves the linearity and dynamic range of the modulator.

[0017] Further, the quantization noise cancellation is realized by the analog domain feedback path structure, without the need for a digital filter to realize the quantization noise cancellation, avoiding noise leakage caused by digital filter matching errors.

[0018] Further, it is not dependent on the integrator transfer function matching, and is effective for any noise transfer function and any order Sigma-Delta modulator, and thus can be adapted to different process and structure implementations, has stronger system robustness, and improves system stability and design adaptability.

[0019] Further, the structure of the application is simple, does not require additional digital filter modules, is convenient for system integration and application expansion, and can be directly applied to high-precision analog-to-digital conversion, audio signal acquisition and radio frequency receiving systems.

[0020] In summary, the application effectively suppresses the out-of-band overshoot while realizing high-order noise shaping of the continuous-time Sigma-Delta modulator, improves the signal precision and system stability, and has significant technical innovation and application value.

[0021] Each of the technical features disclosed in the above summary, each of the technical features disclosed in the various embodiments and examples below, and each of the technical features disclosed in the drawings, can be freely combined with each other to form various new technical solutions (which should all be considered to have been described in this specification), unless such a combination of technical features is technically infeasible. For example, features A+B+C are disclosed in one example, features A+B+D+E are disclosed in another example, features C and D are equivalent technical means that play the same role, and can only be used in one way in technology, and feature E can be combined with feature C in technology. Therefore, the scheme of A+B+C+D should not be considered to have been described because it is technically infeasible, and the scheme of A+B+C+E should be considered to have been described. BRIEF DESCRIPTION OF DRAWINGS

[0022] In order to more clearly illustrate the technical solutions of the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed in the embodiments or prior art description. It should be understood that the drawings in the following description are only some embodiments of the present application, and those skilled in the art can also obtain other embodiments from these drawings without creative labor.

[0023] Figure 1 is a structure diagram of a cascade continuous-time Sigma-Delta modulator according to an embodiment of the present application.

[0024] Figure 2 is a structure diagram of a first integrator according to an embodiment of the present application.

[0025] Figure 3 is a structure diagram of a second integrator according to an embodiment of the present application. DETAILED DESCRIPTION

[0026] In the following description, many technical details are presented in order to make the reader better understand the present application. However, those skilled in the art can understand that the technical solutions claimed by the present application can be realized even without these technical details and based on various changes and modifications of the following embodiments.

[0027] In order to make the purposes, technical solutions and advantages of the present application more clear, the following will further describe the embodiments of the present application in combination with the drawings.

[0028] The first embodiment of the present application relates to a cascade continuous-time Sigma-Delta modulator against out-of-band overshoot, the structure of which is shown in Figure 1 The Sigma-Delta modulator includes a first modulator and a second modulator. The first modulator includes a first integrator 101 and a first quantizer 102 coupled in sequence. The second modulator includes a second integrator 103 and a second quantizer 104 coupled in sequence. The transfer function of the first integrator 101 in the first modulator is H1, the output signal of the first quantizer (Q1) 102 in the first modulator is V1, and the quantization noise is E1. The transfer function of the second integrator 103 in the second modulator is H2, the output signal of the second quantizer (Q2) 104 in the second modulator is V2, and the quantization noise is E2. V in is an input signal of the Sigma-Delta modulator, V out is an output signal of the Sigma-Delta modulator. The input signal V in and the output signal V out are differential signals, i.e. the input signal Vin comprising V inn and V inp , the output signal V out comprising V outn and V outp , only single-ended is shown in the figures for simplicity of illustration.

[0029] The Sigma-Delta modulator further comprises a first adder 105, a second adder 106, and a third adder 107. The second adder 106 outputs V x1 , which is the input signal of the first integrator 101101 in the first modulator loop, and the third adder 107 outputs V x2 , which is the input signal of the second integrator 103103 in the second modulator loop.

[0030] The input terminals of the first adder 105 are coupled to the output terminals of the first quantizer 102 and the second quantizer 104, respectively. The first adder 105 receives the output signal V1 of the first modulator and the output signal V2 of the second modulator, respectively, and adds them in the digital domain to obtain and output the output signal V out of the cascaded continuous-time Sigma-Delta modulator.

[0031] The input terminals of the second adder 106 are coupled to the negated output signal V out of the cascaded continuous-time Sigma-Delta modulator and the input signal V in of the cascaded continuous-time Sigma-Delta modulator, respectively. The output terminal of the second adder 106 is coupled to the input terminal of the first integrator 101. The second adder 106 adds the negated output signal V out and the input signal V in and outputs V x1 to the input terminal of the first integrator 101.

[0032] The input terminals of the third adder 107 are coupled to the negated output signal V out of the cascaded continuous-time Sigma-Delta modulator, the negated input signal V in of the cascaded continuous-time Sigma-Delta modulator, and the output signal V x1 of the first integrator 101, respectively. The third adder 107 adds the negated output signal V out , the negated input signal V in , and the signal V x1 and outputs Vx2 To the input terminal of the second-stage integrator 103.

[0033] It is understandable that the input signal V in It is a differential signal, and the input signal V in The inverted signal is the phase of the reversed differential signal; for example, a pair of differential signals V can be used to reverse the phase of the differential signal. inn and V inp The correspondence is reversed and then connected to the input of the third adder 107. Output signal V out It is a differential signal, and the output signal V out The inverted signal is the phase of the reversed differential signal; for example, a pair of differential signals V can be used to reverse the phase of the differential signal. outn and V outp After the correspondence is reversed, it is connected to the input of the second adder 106 and the input of the third adder 107.

[0034] The first-stage integrator 101 is a continuous-time integrator. (The structural reference for the first-stage integrator 101 is...) Figure 2 As shown. The first-stage integrator 101 includes a first operational amplifier 201, a first resistor 202, and a first capacitor 203. The positive input terminal of the first operational amplifier 201 is grounded. The first capacitor 203 is coupled between the negative input terminal and the output terminal of the first operational amplifier 201. The first resistor 202 is connected in series with the negative input terminal of the first operational amplifier 201. The time constant τ1 of the first-stage integrator 101 is 1 / R1C1, where R1 is the resistance value of the first resistor 202, and C1 is the capacitance value of the first capacitor 203. The negative input terminal of the first operational amplifier 201 is coupled to the output signal V of the third adder 107. x2 The integral result Vout1 is obtained and output to the first-stage quantizer 102.

[0035] The second-stage integrator 103 is a continuous-time integrator. (The structural reference for the second-stage integrator 103 is...) Figure 3 As shown. The second-stage integrator 103 includes a second operational amplifier 201', a second resistor 202', and a second capacitor 203'. The positive input terminal of the second operational amplifier 201' is grounded. The second capacitor 203' is coupled between the negative input terminal and the output terminal of the second operational amplifier 201'. The second resistor 202' is connected in series with the negative input terminal of the second operational amplifier 201'. The time constant τ2 of the second-stage integrator 103 is 1 / R2C2, where R2 is the resistance value of the second resistor 202' and C2 is the capacitance value of the second capacitor 203'. The negative input terminal of the second operational amplifier 201' is coupled to the output signal V of the second adder 106. x2 The integral result Vout2 is obtained and output to the second-stage quantizer 104.

[0036] The signal transfer function of the cascaded continuous-time Sigma-Delta modulator is H1 H2=1 / (s 2 τ1τ2), wherein H1 is the transfer function of the first integrator 101, τ1 is the time constant of the first integrator 101, H2 is the transfer function of the second integrator 103, τ2 is the time constant of the second integrator 103, and s is the Laplace operator. The transfer function H1 of the first integrator 101 and the transfer function H2 of the second integrator 103 can be equal. The signal transfer function H1 H2 of the cascaded continuous-time Sigma-Delta modulator has a roll-off characteristic in the high-frequency band to suppress the out-of-band overshoot of the input signal in the high-frequency band.

[0037] In order to better understand the technical solutions of the present application, a specific example will be described below, and the details listed in the example are mainly for the purpose of understanding and do not limit the protection scope of the present application.

[0038] Compared with the single-loop high-order Sigma-Delta modulator, the cascaded Sigma-Delta modulator uses multiple low-order noise shaping loops instead of one high-order noise shaping loop, which improves the loop stability while achieving the same noise shaping effect. Taking a two-stage cascade as an example, a common cascaded Sigma-Delta modulator needs a digital filter to offset the analog noise transfer function of the first loop of the modulator, so that the noise transfer function of the modulator only contains high-order noise shaping effect.

[0039] However, the analog noise transfer function depends on the finite operational amplifier gain of the integrator, the matching of circuit passive devices, etc., which changes the ideal noise transfer function. Therefore, it cannot be completely offset by the transfer function of the digital filter, resulting in noise leakage and reducing the performance of the modulator.

[0040] In addition, for the traditional feedforward continuous-time Sigma-Delta modulator, the out-of-band signal transfer function (STF) has an overshoot phenomenon, which causes the modulator output to saturate when the input is high frequency.

[0041] The present application provides a dual feedback path cascaded continuous-time Sigma-Delta modulator, which adds two feedback paths in the cascaded Sigma-Delta modulator loop, so that the input transfer function of the modulator has a certain roll-off when the input signal is high frequency, thereby resisting the out-of-band overshoot effect. Moreover, high-order noise shaping effect can be achieved without digital filter, and two-stage filter transfer function matching is not required, which has better adaptability, and the order of the two-stage modulator can be arbitrarily selected.

[0042] With two-stage cascade Sigma-Delta modulator as an example, the core idea of the present application is to introduce a feedback path outputting to the first-stage integrator in the first-stage modulator; to introduce a feedback path outputting to the second-stage integrator in the second-stage modulator, which is different from the traditional cascade structure; and to introduce the output of the first-stage integrator of the cascade modulator to the input of the second-stage integrator, and to introduce the input signal V in to the input of the second-stage integrator.

[0043] Reference Figure 1 The above technical problem is solved by the following features: 1. The output signal V out is fed back to the first-stage integrator 101 as the first feedback path.

[0044] 2. A feedback path outputting to the second-stage integrator 103 is introduced in the second-stage modulator.

[0045] 3. The output of the first-stage integrator 101 of the cascade modulator is introduced to the input of the second-stage integrator 103.

[0046] 4. The input signal V in is connected to the input of the second-stage integrator 103 after being inverted.

[0047] 5. The output V1 of the first-stage quantizer 102 and the output V2 of the second-stage quantizer 104 are added in the digital domain to obtain the final output V out of the cascade Sigma-Delta.

[0048] The final output V out of the system is derived as shown in the following formulas (1) and (2): (1) (2) Substitute formula (2) into formula (1) to obtain the following formula (3): (3) Simplify formula (3) to obtain the following formula (4): (4) If the transfer functions of the two-stage integrators are equal, i.e. the transfer function H1 of the first-stage integrator is equal to the transfer function H2 of the second-stage integrator, formula (5) can be obtained: (5) For the Sigma-Delta modulator, the noise transfer function NTF is as shown in the following formula (6): (6) where H is the transfer function of the integrator.

[0049] For equation (5), it can be rewritten as equation (7) as follows: (7) From equation (7), it can be seen that the output V out of the Sigma-Delta modulator is divided into two parts, one part is the input signal V in x STF, and the other part is the quantization noise E1 of the first quantizer 102 and the quantization noise E2 of the second quantizer 104 added together after two-stage noise shaping (NTF1 x NTF2). That is, equation (8) is obtained as follows: (8) Generally, the transfer function of the continuous-time integrator is , , τ1, τ1 are the time constants of the two-stage integrator.

[0050] From the above calculation process, it can be found that there is a filter function of the signal transfer function STF being H1H2=1 / (s 2 τ1τ2), so as to reduce the out-of-band overshoot of the input signal at high frequencies.

[0051] It should be noted that the relational terms such as first and second and the like are used only to differentiate one entity or action from another, and do not necessarily require or imply that these entities or actions are in any way mutually exclusive, directly or indirectly, unless the context clearly suggests otherwise. In addition, the terms "comprises", "comprising", or any other variations thereof are intended to cover a non-exclusive inclusion, so that a process, method, article, or apparatus that includes a list of elements does not include only those elements, but can include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without more limitations, an element defined by the statement "comprising a" does not exclude the existence of additional identical elements in the process, method, article, or apparatus that includes the element. In this application, if it is mentioned that a certain behavior is performed according to a certain element, it means that the behavior is performed at least according to the element, which includes two cases: the behavior is performed only according to the element, and the behavior is performed according to the element and other elements. The expressions of multiple, multiple times, multiple, etc. include 2, 2 times, 2 and more than 2, more than 2 times, more than 2.

[0052] The term "coupled to" and its derivatives can be used herein. "Coupled" can mean two or more elements are in direct physical or electrical contact. However, "coupled" can also mean that two or more elements indirectly contact each other, but yet are still in cooperation or interaction with each other, and can mean that one or more other elements are coupled or connected between the elements that are said to be coupled to each other.

[0053] All documents mentioned in this specification are herein incorporated by reference in their entirety to disclose and describe the methods and / or materials in connection with which the documents are cited. The citation of any document is not to be construed as an admission that it is prior art with respect to the present description. In addition, it should be understood that the descriptions just given are illustrative of the general nature of the description described herein and are in no way to be taken as limiting the scope of the description. Any modification, equivalent replacement or improvement made during the spirit and principles of one or more embodiments described in the specification shall be included in the scope of protection of one or more embodiments described in the specification.

Claims

1. A cascaded continuous-time Sigma-Delta modulator with anti-out-of-band overshoot, characterized in that, include: The first-stage modulator includes a first-stage integrator and a first-stage quantizer coupled in sequence. The second-stage modulator includes a second-stage integrator and a second-stage quantizer coupled in sequence. The first adder, whose input is coupled to the output of the first-stage quantizer and the output of the second-stage quantizer, is used to add the output signals of the first-stage modulator and the second-stage modulator in the digital domain and output the output signal of the cascaded continuous-time Sigma-Delta modulator. The second adder has its input terminals coupled to the inverted signals of the output signals of the cascaded continuous-time Sigma-Delta modulator to form a first feedback path. The second adder adds the inverted signals of the output signals of the cascaded continuous-time Sigma-Delta modulator and the input signals of the cascaded continuous-time Sigma-Delta modulator and outputs the result to the input terminal of the first-stage integrator. as well as The third adder has its input terminals coupled to the inverted output signals of the cascaded continuous-time Sigma-Delta modulator to form a second feedback path. The third adder adds the inverted output signals of the cascaded continuous-time Sigma-Delta modulator, the inverted input signals of the cascaded continuous-time Sigma-Delta modulator, and the output signal of the first-stage integrator, and outputs the sum to the input terminal of the second-stage integrator.

2. The cascaded continuous-time Sigma-Delta modulator with anti-out-of-band overshoot as described in claim 1, characterized in that, The signal transfer function of the cascaded continuous-time Sigma-Delta modulator is H1. H2=1 / (s 2 τ1τ2), where H1 is the transfer function of the first-stage integrator, τ1 is the time constant of the first-stage integrator, H2 is the transfer function of the second-stage integrator, τ2 is the time constant of the second-stage integrator, and s is the Laplace operator.

3. The cascaded continuous-time Sigma-Delta modulator with anti-out-of-band overshoot as described in claim 2, characterized in that, The first-stage integrator includes a first operational amplifier, a first resistor, and a first capacitor; The positive input terminal of the first operational amplifier is grounded, the first capacitor is coupled between the negative input terminal and the output terminal of the first operational amplifier, the first resistor is connected in series with the negative input terminal of the first operational amplifier, and the time constant τ1 of the first stage integrator is 1 / R1C1, where R1 is the resistance value of the first resistor and C1 is the capacitance value of the first capacitor.

4. The cascaded continuous-time Sigma-Delta modulator with anti-out-of-band overshoot as described in claim 2, characterized in that, The second-stage integrator includes a second operational amplifier, a second resistor, and a second capacitor; The positive input terminal of the second operational amplifier is grounded, the second capacitor is coupled between the negative input terminal and the output terminal of the second operational amplifier, the second resistor is connected in series with the negative input terminal of the second operational amplifier, and the time constant of the second integrator is τ2=1 / R2C2, where R2 is the resistance value of the second resistor and C2 is the capacitance value of the second capacitor.

5. The cascaded continuous-time Sigma-Delta modulator with anti-out-of-band overshoot as described in claim 1, characterized in that, The signal transfer function of the cascaded continuous-time Sigma-Delta modulator has a roll-off characteristic in the high-frequency band to suppress out-of-band overshoot of the input signal in the high-frequency band.

6. The cascaded continuous-time Sigma-Delta modulator with anti-out-of-band overshoot as described in claim 1, characterized in that, The transfer function of the first-stage integrator is equal to the transfer function of the second-stage integrator.

7. The cascaded continuous-time Sigma-Delta modulator with anti-out-of-band overshoot as described in claim 1, characterized in that, The first-stage integrator and the second-stage integrator are continuous-time integrators.

8. The cascaded continuous-time Sigma-Delta modulator with anti-out-of-band overshoot as described in claim 1, wherein the input signal is a differential signal, and the inverted signal of the input signal is the phase inverted signal of the differential signal.

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