The present disclosure relates to a continuous-time system delta-sigma (ΔΣ) modulator and a signal processing system which are applied to audio equipment and medical instruments in addition to receivers in wireless communication.
Referring to FIG. 1A and FIG. 1B, there are shown conceptual diagrams illustrating general continuous-time system quadrature lowpass type ΔΣ modulators. FIG. 1A shows a feedback-type ΔΣ modulator and FIG. 1B shows a feedforward-type ΔΣ modulator.
A ΔΣ modulator 1A shown in FIG. 1A is made up of integrators INT1, INT2, INT3, and INT4, a quantizer Quan, adders ADD1, ADD2, ADD3, and ADD4, and digital-to-analog (DA) converts DAC1, DAC2, DAC3, and DAC4.
A ΔΣ modulator 1B shown in FIG. 1B is made up of integrators INT1, INT2, INT3, and INT4, a quantizer Quan, adders ADD1 and ADD2, and a DA converter DAC1.
In FIGS. 1A and 1B, u is representative of an analog input signal and v is representative of a digital output signal. a1, a2 and a3 are each representative of a feedback gain or a gain in feedforward path. Q is representative of an effective gain of the quantizer. The gain of the DA converts DAC4 and DAC1 that give feedback to the integrator INT4 is 1.
A noise transmission function (NTF) to digital output signal v of quantization noise generated in the quantizer Quan is expressed by equation (1) below in both FIG. 1A and FIG. 1B, indicating a frequency characteristic of highpass type.
                              N          ⁢                                          ⁢          T          ⁢                                          ⁢          F                =                              s            4                                              s              4                        +                                          a                1                            ⁢                              Qs                3                                      +                                          a                2                            ⁢                              Qs                2                                      +                                          a                3                            ⁢              Qs                        +            Q                                              (        1        )            
To be more specific, a quantization noise generated in the quantizer by a feedback effect in the ΔΣ modulator is noise-shaped to be shifted to high-frequency area, thereby providing a high S/N ratio within a signal band.
Referring to FIG. 2, there is shown a circuit diagram illustrating an exemplary configuration of an integrator providing an element block of the ΔΣ modulator.
Referring to FIG. 3, there is shown a circuit diagram illustrating an exemplary configuration of an adder providing an element block of the ΔΣ modulator.
The integrator INT shown in FIG. 2 has an OTA (Operational Transconductance Amplifier) 1, a resistor element R, and a capacitor C.
The adder ADD shown in FIG. 3 has an OTA2, and a resistor elements R1 and R2.
A transfer characteristic between input and output of the integrator INT is expressed as equation (2) below by use of a resistor (R) and a capacitor (C).
                              T          ⁡                      (            s            )                          =                                            v              out                                      v              in                                =                      1            sCR                                              (        2        )            
Likewise, a transfer characteristic between input and output of the adder ADD is expressed as equation (3) below by use of two resistors R1 and R2.
                              T          ⁡                      (            s            )                          =                                            v              out                                      v              in                                =                                    R              2                                      R              1                                                          (        3        )            
As shown in FIG. 2 and FIG. 3, the integrator INT and the adder ADD can each be configured by use of one amplifier OTA.
Only if the adder ADD is arranged immediately before the integrator INT, one amplifier can realize both an integrator and an adder.
Referring to FIG. 4, there is shown a circuit diagram illustrating a circuit realized by an adder and an integrator having two inputs and one output.
Output voltage vout of this circuit can be expressed as equation (4) below, providing an operation realized by an adder and an integrator combined.
                                              ⁢                              V            out                    =                                                    1                                  sCR                  1                                            ⁢                              V                                  in                  ⁢                                                                          ⁢                  1                                                      +                                          1                                  sCR                  2                                            ⁢                              V                                  in                  ⁢                                                                          ⁢                  2                                                                                        (        4        )            
Now, paying attention to the ΔΣ modulator shown in FIG. 1, it is seen that the modulator can be configured by four amplifiers because all adders are arranged immediately before the integrator in the case of feedback type shown in FIG. 1A.
On the other hand, in the case of the feedforward type shown in FIG. 1B, the adder arranged immediately before the quantizer Quan cannot form a circuit with the integrator, so that five amplifiers are required, thereby leading to increased power and increased circuit scale.
In order to avoid a situation where one more amplified is required, a ΔΣ modulator of feedforward type that does not use the adder arranged immediately before the quantizer may be considered.
Referring to FIG. 5, there is shown a configuration of a ΔΣ modulator that does not use any adder immediately before a quantizer as a variation to the configuration shown in FIG. 1B.
In a ΔΣ modulator 1C shown in FIG. 5, every add operation is executed in front of an integrator INT1 of the last stage, so that every feedback signal passes two integrators.
Passing two integrators raises the degree by two degrees, so that the feedforward path including a1 is set to sa1 rather than a constant, thereby realizing first-degree coefficient by providing a differential characteristic.
Referring to FIG. 6, there is shown a circuit for realizing a portion enclosed by dotted lines shown in FIG. 5.
A feedforward path from the output of the integrator INT4 to the input of the integrator INT1 can be realized by coupling, by use of a capacitor, the input and the output of an amplifier OTA configuring a corresponding integrator.