WO2015066879A1

WO2015066879A1 - Sigma-delta modulator

Info

Publication number: WO2015066879A1
Application number: PCT/CN2013/086718
Authority: WO
Inventors: 谢宁; 张宇; 王晖; 林晓辉
Original assignee: 深圳大学; 谢宁; 张宇; 王晖; 林晓辉
Priority date: 2013-11-07
Filing date: 2013-11-07
Publication date: 2015-05-14

Abstract

The present invention is suitable for use in the technical field of signal processing and provides a sigma-delta modulator, comprising a primary modulation unit, used for converting an inputted analog signal to a primary digital signal; the primary digital signal comprising quantization noise and multi-bit DAC nonlinear noise; a secondary modulation unit, cascaded with the primary modulation unit and used for converting the quantization noise in the primary digital signal and the multi-bit DAC nonlinear noise to a secondary signal, and feeding said secondary digital signal back to said primary modulation unit; a noise shaping unit, connected to the primary modulation unit and the secondary modulation unit and used for cancellation of the quantization noise in the primary digital signal and the multi-bit DAC nonlinear noise, to obtain a noise-cancelled digital signal and output same. The present invention uses a combined configuration of inter-stage feedback and dual quantization to offset the multi-bit DAC nonlinear error of the last stage and the quantization noise of the previous stage.

Description

Sigma-delta modulator

Technical Field

The invention belongs to the technical field of signal processing, and in particular relates to a sigma-de lta modulator.

Background Art

Analog-to-digital converters (ADCs) play a very important role in signal processing. A large number of data converters are required in the fields of digital audio, digital television, image coding, and frequency synthesis. As the size and bias of VLSIs continue to decrease, the accuracy and dynamic range of analog devices continue to decrease, posing a challenge for high resolution ADCs. High-order multi-sigma-delta Since the ADC does not require a sample-and-hold circuit, the circuit scale is small, and a high resolution can be realized, so that it is widely used in practice.

Sigma-delta Since the birth of the modulation technology in the 1960s, after several years of development, it has become one of the mainstream technologies for implementing high-performance analog-to-digital conversion interface circuits in very large-scale integrated circuit systems. The sigma-delta data converter based on sigma-delta modulation technology, combined with the application of oversampling technology and noise shaping technology, can push the quantization noise to the high frequency end, thereby significantly improving the signal to noise ratio of the data converter. In short, the Sigma-delta modulator is used to convert a continuous-time, continuous-amplitude input signal into a discrete-time, discrete-amplitude output sequence.

As mentioned above, Sigma-delta The ADC uses oversampling technology combined with noise shaping technology to double suppress quantization noise to achieve high-precision analog-to-digital conversion. Oversampling techniques and noise shaping techniques are introduced as follows:

Oversampling---sigma-delta The ADC samples the input signal with a clock much higher than the Nyquist frequency, so that the power of the quantization noise is distributed over a wider frequency band, thus reducing noise in the signal band.

Noise shaping - noise shaping can further improve the signal to noise ratio of the converter. By using the characteristics of the high-pass filter, the quantization noise of the low-frequency portion is shifted to the high-frequency portion, and noise in the signal bandwidth is reduced. The higher the order and sampling frequency of the high pass filter, the less noise there is in the signal bandwidth. But the higher the order, the more unstable the system becomes. A common way to achieve noise shaping is to use sigma-delta Modulator.

In the actual design, compromises need to be made based on design specifications, stability, and dynamic range. For single-loop high-order (third-order or higher) sigma-delta For ADC, the biggest problem is stability. In order to maintain high-order SDM (sigma-delta Modulator), we can use multi-bit quantizer, but this will increase the design difficulty of the subsequent internal DAC, if it is not handled properly, it will generate a lot of harmonic components, but make sigma-delta The performance of the ADC is degraded.

The Sigma-delta modulator is mainly composed of an A/D converter and a D/A The converter consists of a series of integrators. The number of integrators determines the order of the sigma-delta modulator. For example, if there are three integrators connected in series in a single loop modulator, then this single loop The sigma-delta modulator is called a single-loop third-order sigma-delta modulator.

The main performance indicators of the Sigma-delta modulator are: dynamic range (DR), signal-to-noise ratio (SNR), and signal-to-noise ratio ( SNDR ), effective number of bits ( ENOB ), and overload ( OL ).

In traditional multi-bit sigma-delta ADC architectures, the feedback loop requires the use of a multi-bit DAC due to The mismatch of internal components (such as capacitors) of the multi-bit DAC results in a decrease in linearity, and the DAC produces nonlinear noise, which degrades the performance of the entire sigma-delta modulator system. In traditional multi-bit In the sigma-delta ADC structure, the multi-bit DAC nonlinear noise is not processed, which affects the modulator SNR and further improvement of SNDR.

In order to make a single ring sigma-delta The modulator can be stable. The maximum passband gain of the empirically quantized noise transfer function must be less than 1.5 in a one-bit quantizer design, less than 2.5 in a two-bit quantizer design, and must be less than a three-bit quantizer design. 3.5, and must be less than 5 in a four-bit quantizer design.

Figure 1 is a three-stage multi-bit sigma-delta modulator of the conventional embodiment. First input analog signal X Feeding to an input port of the first adder; the output port of the first adder is coupled to the input port of the first integrator. An output port of the first integrator is respectively connected to the second adder and an input port of the first analog to digital converter; the second adder The output port of 2 is connected to the input port of the third adder 3; the output port of the second integrator 2 is connected to one input port of the fourth adder 4; the output port of the fourth adder 4 is connected to the third integrator 3 The input port is connected; the third integrator 3 The output port is connected to the input port of the second analog-to-digital converter; one output of the first analog-to-digital converter and the second analog-to-digital converter is output as a digital signal, and the other output is respectively sent to the first digital-to-analog conversion And the second digital-to-analog converter; the two digital-to-analog converters divide the converted analog signal into three paths: the first analog signal is fed back to the other input port of the first adder at the input end; the second analog The signal is fed back to the other input port of the third adder at the input; the third analog signal passes the gain coefficient Gain1 feeds back to the other input port of the fourth adder. The digital signals output by the first analog-to-digital converter and the second analog-to-digital converter respectively obtain the final digital signal through the first digital circuit 1 and the second digital circuit. And output.

Traditional sigma-delta modulators have the following disadvantages:

1. Multi-bit digital-to-analog converter (DAC) is used in each stage of the feedback loop. For process reasons, multi-bit DAC Non-linear noise is generated, which can seriously affect the performance of the entire sigma-Delta modulator when the nonlinear noise is large.

2, no relevant measures are used to deal with multi-bit DAC nonlinear noise, making multi-bit DAC Direct output of nonlinear noise.

Technical Problem

The technical problem to be solved by the present invention is to provide a sigma-delta modulator It is designed to perform noise shaping on the nonlinear noise of a multi-bit DAC, thereby suppressing it so that the noise floor is as low as possible.

Technical Solution

The present invention is implemented in such a manner that a sigma-delta modulator includes:

a first-order modulation unit for converting an input analog signal into a first-order digital signal; the first-order digital signal includes quantization noise and multi-bit DAC nonlinear noise;

a secondary modulation unit cascading with the primary modulation unit for converting quantization noise and multi-bit DAC nonlinear noise in the primary digital signal into a secondary digital signal, and the secondary digital The signal is fed back to the first-level modulation unit, and the first-order modulation unit modulates and converts the input analog signal according to the feedback second-level digital signal;

a noise shaping unit coupled to the primary modulation unit and the secondary modulation unit for canceling the quantization noise in the secondary digital signal and the primary digital signal from a multi-bit DAC nonlinear noise, A digital signal after noise cancellation is obtained and output.

further, The primary modulation unit includes: a first adder, a second adder, a third adder, a first integrator, a first analog to digital converter, and a second digital to analog converter; the first adder is in phase The input end is connected to an external analog signal, the output end of the first adder is connected to the input end of the first integrator, and the output end of the first integrator is connected to the non-inverting input end of the second adder, An output of the second adder is connected to an input end of the first analog to digital converter, and an output end of the first analog to digital converter is simultaneously connected to the input of the noise shaping unit and the first digital to analog converter The output end of the first digital-to-analog converter is connected to the inverting input end of the first adder, and the non-inverting input end of the third adder is connected to the output end of the first digital-to-analog converter. The inverting input terminal of the third adder is connected to the output end of the second adder;

The second modulation unit includes: a fourth adder, a fifth adder, a sixth adder, a second integrator, a third integrator, a second analog to digital converter, a third analog to digital converter, and a second number And a non-inverting input of the fourth adder is connected to an output end of the third adder, an output end of the fourth adder is connected to an input end of the second integrator, the second integral The output of the fifth integrator is connected to the input of the third integrator, the output of the fifth integrator is connected to the input of the third integrator, and the output of the third integrator is connected to the third An input end of the analog to digital converter, an output of the third analog to digital converter is coupled to a first non-inverting input of the sixth adder, and an input of the second analog to digital converter is coupled to the second An output of the integrator, an output of the second analog-to-digital converter being coupled to a second non-inverting input of the sixth adder, an output of the sixth adder simultaneously connecting the noise shaping unit and the An input of a second digital to analog converter, the second digital to analog converter The output terminal is simultaneously connected to the inverting input terminal of the fourth adder, the inverting input terminal of the fifth adder, and the inverting input terminal of the second adder.

further, The second modulation unit further includes a first gain module, a second gain module, and a third gain module; an output end of the fifth integrator is connected to an input end of the third integrator through the first gain module; An input end of the second analog-to-digital converter is connected to an output end of the second integrator through the second gain module; an output end of the sixth adder is connected to an input end of the third gain module, where The output of the third gain module is simultaneously connected to the input of the noise shaping unit and the second digital to analog converter.

further, The first gain module and the second gain module are both 2 times gain, and the third gain module is 1/2 times gain.

Further, the noise shaping unit includes: a first matching module, a second matching module, and a seventh adder;

The first matching module and the second matching module are respectively connected to the output ends of the primary modulation unit and the second modulation unit, and the first matching module and the second matching module are used to cooperate with each other. And modulating the secondary digital signal and the quantization noise in the primary digital signal with the multi-bit DAC nonlinear noise into a size-matched noise; the outputs of the first matching module and the second matching module are respectively connected to the a first non-inverting input and a second non-inverting input of the seventh adder.

Advantageous Effects

this The invention uses interstage feedback combined with a double quantization structure to cancel out the last-order multi-bit DAC nonlinear error and the previous-level quantization noise in the cascade structure, and the final-stage quantization noise and the previous-stage multi-bit DAC nonlinearity. The error is also shaped by noise, which greatly improves the The sigma-delta modulator outputs the signal-to-noise ratio of the digital signal. The experimental results also show that when the internal capacitance of the multi-bit DAC does not match and the nonlinear noise is generated, the performance of the improved structure is significantly better than the conventional structure.

Description of Drawings

1 is a logical structural diagram of a three-stage multi-bit sigma-delta modulator provided by the prior art;

2 is a schematic structural diagram of a sigma-delta modulator provided by the present invention;

Figure 3 is a diagram showing an example of a specific logic of the sigma-delta modulator shown in Figure 2;

Figure 4A is a sigma-delta modulator and a conventional sigma-delta provided by the present invention. The signal-to-noise-distortion ratio of the output digital signal of the modulator with a fixed input signal amplitude;

Figure 4B is a sigma-delta modulator and a conventional sigma-delta modulator provided by the present invention. The signal-to-noise distortion ratio of the output digital signal with different input signal amplitudes.

Best Mode

The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It is understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

In the present invention, the input signal is added to each integrator output and input to the quantizer. After quantization, the output signal is converted by the DAC and subtracted from the input signal. Since the actual circuit, these operations are completed in one cycle. Therefore, the subtracted signal is equivalent to the quantization error of the current period input signal. The integrator simply integrates the quantization error (ie, the integrator only processes the quantization noise), thus improving the modulator's structure to suppress multi-bit DAC nonlinear noise, thereby increasing the SNR and SNDR of the entire sigma-delta modulator.

Fig. 2 shows the structural principle of the sigma-delta modulator provided by the present invention, and for the convenience of description, only the parts related to the present invention are made. Referring to FIG. 2, the sigma-delta modulator provided by the present invention comprises a primary modulation unit 1, a secondary modulation unit 2, and a noise shaping unit 3, wherein the primary modulation unit 1 and the secondary modulation unit 2 are cascaded and form a closed loop structure. The noise shaping unit 3 is in turn connected to the primary modulation unit 1 and the secondary modulation unit 2.

The primary modulation unit 1 is used to convert the input analog signal into a primary digital signal, as described in the background section above (ie, due to a mismatch of internal components (eg, capacitance) of the multi-bit DAC, its linearity is reduced, The DAC produces nonlinear noise, which degrades the performance of the entire delta-sigma modulator system.) Therefore, the first-order digital signal contains multi-bit DAC nonlinear noise, and the first-order digital signal also includes quantization noise.

The second modulation unit 2 converts the quantization noise in the primary digital signal and the multi-bit DAC nonlinear noise into a secondary digital signal, and feeds the secondary digital signal back to the primary modulation unit, The primary modulation unit modulates and converts the input analog signal according to the feedback secondary digital signal. Finally, the noise shaping unit 3 cancels the quantization noise in the second-level digital signal and the first-order digital signal and the multi-bit DAC nonlinear noise to obtain a digital signal after noise cancellation and outputs the digital signal.

Through the combination of the above-mentioned interstage feedback and the double quantization structure, the multi-bit DAC nonlinear error and the upper-level quantization noise of the last stage in the cascade structure are cancelled, and the final-stage quantization noise and the previous-stage multi-bit DAC nonlinearity are eliminated. The error is also shaped by noise, which greatly improves the signal-to-noise ratio of the digital signal output by the sigma-delta modulator.

FIG. 3 is a specific structural example of the modulator shown in FIG. 2. It should be understood that the specific implementation is not limited to the structure described in FIG. 3, as long as the functions of the various units in FIG. 2 can be implemented.

Referring to FIG. 3, the primary modulation unit 1 includes a first adder 11, a second adder 13, a third adder 16, a first integrator 12, a first analog to digital converter 14, and a second digital to analog converter 15. . The non-inverting input of the first adder 11 is input to the external analog signal X, the output end is connected to the input end of the first integrator 12, and the output end of the first integrator 12 is connected to the non-inverting input end of the second adder 13, the second addition The output end of the first analog-to-digital converter 14 is connected to the input end of the first analog-to-digital converter 14, and the first digital-to-analog conversion is connected to the input end of the first analog-to-digital converter 15 The output of the first adder 11 is connected to the inverting input of the first adder 11, the non-inverting input of the third adder 16 is connected to the output of the first digital-to-analog converter 15, and the inverting input is connected to the second adder 13. Output.

The second modulation unit 2 includes: a fourth adder 21, a fifth adder 23, a sixth adder 27, a second integrator 22, a third integrator 24, a second analog to digital converter 25, and a third analog to digital conversion The second digital to analog converter 28. The non-inverting input of the fourth adder 21 is connected to the output of the third adder 16, the output of the fourth adder 21 is connected to the input of the second integrator 22, and the output of the second integrator 22 is connected to the fifth adder. At the non-inverting input of 23, the output of the fifth integrator 23 is connected to the input of the third integrator 24, and the output of the third integrator 24 is connected to the input of the third analog-to-digital converter 26, the third analog-to-digital converter The output of 26 is connected to the first non-inverting input of the sixth adder 27, the input of the second analog-to-digital converter 25 is connected to the output of the second integrator 22, and the output of the second analog-to-digital converter 25 is connected. The second non-inverting input of the sixth adder 27, the output of the sixth adder 27 is simultaneously connected to the input ends of the noise shaping unit 3 and the second digital-to-analog converter 28, and the output of the second digital-to-analog converter 28 is simultaneously connected. The inverting input terminal of the quad-adder 21, the inverting input terminal of the fifth adder 23, and the inverting input terminal of the second adder 13.

Further, in order to improve the overall signal-to-noise ratio, a first gain module 29, a second gain module 20, and a third gain module 211 are further disposed. The other connection relationship is that the output end of the fifth integrator 23 is connected through the first gain module 29. The input end of the third integrator 24; the input end of the second analog-to-digital converter 25 is connected to the output end of the second integrator 22 through the second gain module 20; the output end of the sixth adder 27 is connected to the third gain module At the input of 211, the output of the third gain module 211 is simultaneously connected to the input of the noise shaping unit 3 and the second digital to analog converter 28.

The present invention recommends that both the first gain module 29 and the second gain module 20 are preferably 2x gain, and the third gain module 211 preferably has 1/2 gain.

The noise shaping unit 3 includes a first matching module 31, a second matching module 32, and a seventh adder 33. The first matching module 31 and the second matching module 32 are respectively connected to the output terminals 2 of the first-level modulation unit 1 and the second-level modulation unit. Specifically, the first matching module 31 is connected to the output end of the first analog-to-digital converter, and the second matching is performed. Module 32 is coupled to the output of the third gain module. The first matching module 31 and the second matching module 32 are configured to modulate the quantization noise in the second-level digital signal and the first-level digital signal with the multi-bit DAC nonlinear noise into a matching noise by interworking; the first matching module 31 and The output ends of the second matching module 32 are respectively connected to the first non-inverting input and the second non-inverting input of the seventh adder 33.

Take the first-level and second-level digital outputs as an example:

The digital output of the first stage and the second stage are respectively input to the digital circuit 1, the digital circuit 2, and the final digital output is obtained by the adder 7:

Where Y ₁ (z), Y ₂ (z) are the first stage, the second stage digital output, E ₁ (z), E ₂ (z), E ₃ (z) are quantizer 1, quantizer 2 The quantization noise of the quantizer 3, E _d1 (z), E _d2 (z) are the first-order DAC1 and the second-stage DAC2 nonlinear noise, respectively.

We can see from the expression, the second multi-bit non-linear noise is DAC2 second order noise shaping and quantization noise _{_{E 1 (z), E 2}} (z) is also effectively suppressed (i.e., multiplied by a gain 1 / 2).

Figure 4A compares the signal-to-noise distortion ratio of the sigma-delta modulator output digital signal with an input signal amplitude of 6 dB, where the dashed line represents the signal-to-noise distortion ratio curve of the present invention, and the solid line represents the conventional signal-to-noise distortion ratio curve. . It can be seen from the comparison of FIG. 4A that the conventional embodiment causes system distortion due to multi-bit DAC nonlinear noise, thereby degrading the performance of the modulator system; however, in the embodiment of the present invention, the nonlinear noise of the multi-bit DAC is effectively suppressed. Thereby improving the modulator signal-to-noise ratio.

Figure 4B compares the signal-to-noise distortion ratio of the sigma-delta modulator output digital signal with different input signal amplitudes. It can be seen from the comparison of FIG. 4B that the input dynamic range of the embodiment of the present invention is not reduced under the same simulation conditions, wherein the above one curve (Improved) is the signal-to-noise distortion of the output digital signal of the sigma-delta modulator of the present invention. The following curve (Conventional) is the signal-to-noise ratio of the digital signal output by the conventional sigma-delta modulator.

In summary, the present invention feeds back the second stage analog input signal back to the first stage on the basis of the conventional Sigma-delta modulator structure, and adds a multi-bit quantizer to the second stage, and the second stage two The digital outputs of the multi-bit quantizers are summed to obtain the final digital output of the second stage. This feedback double quantization structure helps to reduce the overall quantization noise value, thereby improving the system SNR, and performing second-order noise shaping on the nonlinear noise of the last-stage multi-bit DAC, thereby effectively suppressing the nonlinear noise of the multi-bit DAC.

The invention relates to the field of analog signal to digital signal with high precision and high signal to noise ratio, which mainly improves the stability of the converter and reduces the power consumption of the converter, thereby improving the performance of the converter. Suitable for some portable products such as mobile phones, PDAs.

The above is only the preferred embodiment of the present invention, and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection of the present invention. Within the scope.

Claims

A sigma-delta modulator characterized by comprising:

a first-order modulation unit for converting an input analog signal into a first-order digital signal; the first-order digital signal includes quantization noise and multi-bit DAC nonlinear noise;

a secondary modulation unit cascading with the primary modulation unit for converting quantization noise and multi-bit DAC nonlinear noise in the primary digital signal into a secondary digital signal, and the secondary digital The signal is fed back to the first-level modulation unit, and the first-order modulation unit modulates and converts the input analog signal according to the feedback second-level digital signal;

a noise shaping unit coupled to the primary modulation unit and the secondary modulation unit for canceling the quantization noise in the secondary digital signal and the primary digital signal from a multi-bit DAC nonlinear noise, A digital signal after noise cancellation is obtained and output.
The sigma-delta modulator of claim 1 wherein:

The primary modulation unit includes: a first adder, a second adder, a third adder, a first integrator, a first analog to digital converter, and a second digital to analog converter; the first adder is in phase The input end is connected to an external analog signal, the output end of the first adder is connected to the input end of the first integrator, and the output end of the first integrator is connected to the non-inverting input end of the second adder, An output of the second adder is connected to an input end of the first analog to digital converter, and an output end of the first analog to digital converter is simultaneously connected to the input of the noise shaping unit and the first digital to analog converter The output end of the first digital-to-analog converter is connected to the inverting input end of the first adder, and the non-inverting input end of the third adder is connected to the output end of the first digital-to-analog converter. The inverting input terminal of the third adder is connected to the output end of the second adder;

The second modulation unit includes: a fourth adder, a fifth adder, a sixth adder, a second integrator, a third integrator, a second analog to digital converter, a third analog to digital converter, and a second number And a non-inverting input of the fourth adder is connected to an output end of the third adder, an output end of the fourth adder is connected to an input end of the second integrator, the second integral The output of the fifth integrator is connected to the input of the third integrator, the output of the fifth integrator is connected to the input of the third integrator, and the output of the third integrator is connected to the third An input end of the analog to digital converter, an output of the third analog to digital converter is coupled to a first non-inverting input of the sixth adder, and an input of the second analog to digital converter is coupled to the second An output of the integrator, an output of the second analog-to-digital converter being coupled to a second non-inverting input of the sixth adder, an output of the sixth adder simultaneously connecting the noise shaping unit and the An input of the second digital to analog converter, the second digital to analog converter The end of the inverting input terminal while connecting the fourth adder, the inverting input of a fifth adder, the inverting input of the second adder.
The sigma-delta of claim 2 a modulator, wherein the second modulation unit further includes a first gain module, a second gain module, and a third gain module; and an output end of the fifth integrator is connected to the first through the first gain module An input end of the third integrator; an input end of the second analog to digital converter is connected to an output end of the second integrator through the second gain module; an output end of the sixth adder is connected to the third end An input end of the gain module, an output end of the third gain module is simultaneously connected to the input ends of the noise shaping unit and the second digital to analog converter.
The sigma-delta of claim 3 The modulator is characterized in that the first gain module and the second gain module are both 2 times gain, and the third gain module is 1/2 times gain.
The sigma-delta of claim 1 a modulator, wherein the noise shaping unit comprises: a first matching module, a second matching module, and a seventh adder;

The first matching module and the second matching module are respectively connected to the output ends of the primary modulation unit and the second modulation unit, and the first matching module and the second matching module are used to cooperate with each other. And modulating the secondary digital signal and the quantization noise in the primary digital signal with the multi-bit DAC nonlinear noise into a size-matched noise; the outputs of the first matching module and the second matching module are respectively connected to the a first non-inverting input and a second non-inverting input of the seventh adder.