A chopper-stabilized amplifier including a parallel notch filter

Abstract

A chopper-stabilized amplifier including a parallel notch filter includes a first transconductance amplifier, a first chopper circuit whose output is coupled to the input of the first transconductance amplifier for chopping an input signal and applying the chopped input signal to the input of the first transconductance amplifier, and a second chopper circuit whose input is coupled to the output of the first transconductance amplifier for chopping an output signal generated by the first transconductance amplifier. A ping-pong notch filter is connected to the output of the second chopper circuit and performs an integration and transfer function on the chopped output signal generated by the second chopper circuit to filter a ripple voltage. The ping-pong notch filter includes first and second notch filters connected in parallel, each notch filter having an input coupled to the output of the second chopper circuit.

Description

Technical Field

[0001] The present invention relates to amplifier circuits and, more particularly, to chopper-stabilized amplifiers for reducing ripple noise. Background Art

[0002] Low offset voltage and low ripple noise are important for both instrumentation amplifiers and operational amplifiers. Inserting a chopper and increasing stage gain are common techniques that have been widely used to reduce amplifier offset voltage. However, the output ripple noise of traditional chopper-stabilized amplifiers is still high. Traditional chopper-stabilized amplifiers maintain broadband noise characteristics at the input stage, but transfer the input offset voltage to the chopping signal, resulting in a large ripple voltage at the amplifier output.

[0003] Figure 1 A prior art chopper stabilized amplifier 10 is shown with an offset and ripple correction loop 12. The ripple voltage component of the chopper stabilized amplifier output can be up to about 10 μV. However, it has the disadvantages of large offset capacitor area and slow transient response due to stability requirements because the feedback loop does not have a very fast transient response.

[0004] Figure 2 Another conventional chopper-stabilized amplifier 20 is shown having a single notch filter 22 for reducing ripple noise. The amplifier 20 includes a transconductance amplifier 24 and a first chopper circuit 26 located before the input of the transconductance amplifier 24 and a second chopper circuit 28 located after the output of the transconductance amplifier 24. Figure 3 Shows Figure 2 The waveform of the amplifier 20. Figure 2 and 3 As shown, the chopper stage and the notch filter stage are clocked at the same frequency and have a 90 degree phase shift. Figure 3 As shown, capacitors C6 and C7 sample the chopper stage output voltage for the positive chopping phase and the negative chopping phase of each chopping cycle, thereby sampling twice for each chopping cycle, and transferring the charge on capacitors C6 and C7 to capacitor C8 in each half notch phase. Due to process variations, the charging and discharging currents of amplifier GM1 are unbalanced. The rising and falling edges of the chopper stage output are asymmetric. Due to this asymmetry, a large ripple voltage is generated at capacitor C8 and thus at the output. For example, if a 10mV offset is inserted in the chopping phase, the ripple voltage component in the chopper-stabilized amplifier output is approximately 1.7mV, while the average output offset is less than 3μV. A single notch filter 22 can eliminate the offset, but it cannot eliminate the output ripple noise.

[0005] Therefore, it is desirable to implement a chopper-stabilized amplifier having low ripple noise and low offset voltage. Summary of the invention

[0006] This Summary provides a selection of concepts in a simplified form that are further described in the Detailed Description. This Summary is not intended to identify key features or essential features of the subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

[0007] According to one embodiment, the present invention provides a chopper-stabilized amplifier, including a first transconductance amplifier, a first and a second chopping circuit, and a ping-pong notch filter. The first chopping circuit is coupled to the input of the first transconductance amplifier for chopping an input signal and applying the chopped input signal to the input of the first transconductance amplifier. The second chopping circuit is coupled to the output of the first transconductance amplifier for chopping an output signal generated by the first transconductance amplifier. The ping-pong notch filter has first and second notch filters connected in parallel. Each of the first notch filter and the second notch filter has an input coupled to the output of the second chopping circuit. The ping-pong notch filter performs an integration and transfer function on the chopped output signal generated by the second chopping circuit to notch filter the ripple voltage.

[0008] In one embodiment, there is a 90° phase shift between the clocks of the first notch filter and the second notch filter.

[0009] In one embodiment, the chopper-stabilized amplifier further includes a second transconductance amplifier having an input coupled to the output of the first notch filter; a third transconductance amplifier having an input coupled to the output of the second notch filter; and a fourth transconductance amplifier having an input coupled to the output of the second transconductance amplifier, and then the combined output of the second and third transconductance amplifiers is provided to the input of the fourth transconductance amplifier.

[0010] In one embodiment, the chopper-stabilized amplifier further includes a fifth transconductance amplifier having an input receiving the input signal and having an output coupled to the combined output of the second and third transconductance amplifiers.

[0011] In another embodiment, the chopper-stabilized amplifier further includes a buffer coupled between the output of the third transconductance amplifier and the input of the first notch filter.

[0012] The present invention also provides a method for operating a chopper-stabilized amplifier. The method includes chopping an input signal and applying the chopped input signal to an input of a first transconductance amplifier, and chopping an output signal generated by the first transconductance amplifier at a chopping frequency to generate a chopped output signal. The chopped output signal generated by the second chopper circuit is subjected to an integration and transfer function by a ping-pong notch filter including first and second notch filters connected in parallel to notch filter the ripple voltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] So that the manner in which the features of the invention described above can be understood in detail, according to the specific description of the invention of the embodiments and the above invention content, some embodiments are shown in the accompanying drawings. It will be noted, however, that the drawings only show typical embodiments of the invention and are therefore not to be considered as limiting the scope, because the invention may have other equally effective embodiments. It should be understood that the drawings are only schematic and are not drawn to scale. The advantages of the claimed subject matter will be apparent to those skilled in the art upon reading this specification and the resulting drawings, in which reference numerals are used to represent elements such as:

[0014] Figure 1 shows a schematic diagram of a conventional chopper-stabilized amplifier with offset and ripple correction loops;

[0015] Figure 2 A schematic diagram of another conventional chopper-stabilized amplifier having a single notch filter for reducing ripple noise is shown;

[0016] Figure 3 Shows Figure 2 The circuit’s notch filter waveform diagram;

[0017] Figure 4 A schematic diagram of a chopper-stabilized amplifier according to an embodiment of the present invention is shown;

[0018] Figure 5 Shows Figure 4 An operating mode of a chopper-stabilized amplifier;

[0019] Figure 6 Shows Figure 4 Waveform diagrams of the circuit; and

[0020] Figure 7 A schematic diagram of a chopper-stabilized amplifier according to another embodiment of the present invention is shown. DETAILED DESCRIPTION

[0021] like Figure 1 As shown, the chopper-stabilized amplifier 400 includes a negative (-) input terminal and a positive (+) input terminal, through which an input signal Vin is applied to a first chopper circuit 401. The output terminal of the first chopper circuit 401 is connected to the input terminal of a transconductance amplifier 402. The first chopper circuit 401 chops the input signal Vin and applies the chopped input signal to the transconductance amplifier 402. The output of the transconductance amplifier 402 is connected to the input of a second chopper circuit 403. The second chopper circuit 403 chops the output signal generated by the transconductance amplifier 402. Chopper circuits are known in the art (see, for example, Figure 2, chopping circuits 26 and 28), so there is no need to further describe these elements here. The chopped output signal generated by the second chopping circuit 403 is provided to the first and second notch filters 404 and 405. The first and second notch filters 404 and 405 are connected in parallel.

[0022] Notch filters 404 and 405 receive respective clock signals and there is a 90° phase shift between these clock signals as will be discussed further below.

[0023] The first notch filter 404 includes a first switch 406A connected between a first node N1 and a second node N2, a second switch 406B connected between the second node N2 and a third node N3, a third switch 406C connected between a fourth node N4 and a fifth node N5, and a fourth switch 406D connected between the fifth node N5 and a sixth node N6. The first integral and transfer capacitor C1 is connected between the second node N2 and the fifth node N5. The first notch filter 404 also includes a fifth switch 407A connected between the first node N1 and a seventh node N7, a sixth switch 407B connected between the seventh node N7 and the third node N3, a seventh switch 407C connected between the fourth node N4 and the eighth node N8, and an eighth switch 407D connected between the eighth node N8 and the sixth node N6. The second integral and transfer capacitor C2 is connected between the seventh node N7 and the eighth node N8. The third capacitor C3 is connected between the third node N3 and the sixth node N6. The third capacitor C3 improves filtering and reduces ripple noise. The second transconductance amplifier 408 has inputs connected to the output of the first notch filter 404 at the third and sixth nodes N3 and N6 , and the third transconductance amplifier 409 has inputs connected to the output of the second transconductance amplifier 408 .

[0024] The second notch filter 405 is similar to the first notch filter 404. The second notch filter 405 includes a ninth switch 410A connected between a ninth node N9 and a tenth node N10, a tenth switch 410B connected between the tenth node N10 and the eleventh node N11, an eleventh switch 410C connected between the twelfth node N12 and the thirteenth node N13, and a twelfth switch 410D connected between the thirteenth node N13 and the fourteenth node N14. The fourth integration and transfer capacitor C4 is connected between the tenth node N10 and the thirteenth node N13. The second notch filter 405 also includes a thirteenth switch 411A connected between the ninth node N9 and the fifteenth node N15, a fourteenth switch 411B connected between the fifteenth node N15 and the eleventh node N11, a fifteenth switch 411C connected between the twelfth node N12 and the sixteenth node N16, and a sixteenth switch 411D connected between the sixteenth node N16 and the fourteenth node N14. The fifth integrating and transferring capacitor C5 is connected between the fifteenth node N15 and the sixteenth node N16. The sixth capacitor C6 is connected between the eleventh node N11 and the fourteenth node N14. The input of the fourth transconductance amplifier 412 is connected to the output of the second notch filter 405, more specifically, to the eleventh node N11 and the fourteenth node N14. The output of the fourth transconductance amplifier 412 is connected to the output of the second transconductance amplifier 408, and then the combined amplifier output is provided together to the input of the fourth transconductance amplifier 409. More specifically, the outputs (combined) of the second and fourth amplifiers 408 and 412 are provided to the negative (-) input terminal of the third amplifier 409, and the positive (+) input terminal of the third amplifier 409 is connected to the ground.

[0025] The first notch filter 404 is connected in parallel with the second notch filter 405 and is arranged as a ping-pong notch filter to allow the chopper-stabilized amplifier 400 to achieve low ripple, wherein the notch filters 404 and 405 suppress the ripple generated by the offset in the first transconductance amplifier 402. Figure 2 In the single notch filter arrangement shown, the output ripple is not effectively removed. However, the ping-pong arrangement of the present invention greatly reduces the output ripple.

[0026] The fifth feedforward transconductance amplifier 413 has a negative (−) input terminal and a positive (+) input terminal to receive an input signal (Vin). The output of the feedforward transconductance amplifier 413 is connected to the input of the third transconductance amplifier 409.

[0027] The transconductances of the first transconductance amplifier 402, the second transconductance amplifier 408, the third transconductance amplifier 409, the fourth transconductance amplifier 412 and the fifth transconductance amplifier 413 are GM1, GM2, GM3, 6M4 and GMFF respectively.

[0028] In one embodiment, the first node N1 of the first notch filter 404 is connected to one terminal of the seventh capacitor C7, and the third node N3 is connected to one terminal of the eighth capacitor C8. The other terminals of the seventh and eighth capacitors C7 and C8 are connected to the output of the third transconductance amplifier 409. The fourth node N4 is connected to one terminal of the fifth capacitor C9, and the node N6 is connected to one terminal of the tenth capacitor C10. The other terminals of the ninth and tenth capacitors C9 and C10 are grounded.

[0029] In one embodiment, the node N11 of the second notch filter 405 is connected to one terminal of the eleventh capacitor C11, and the other terminal of the eleventh capacitor C11 is connected to the output of the third transconductance amplifier 409. The fourteenth node N14 of the notch filter 405 is connected to one terminal of the twelfth capacitor C12, and the other terminal of the twelfth capacitor C12 is grounded.

[0030] In one embodiment, a thirteenth capacitor C13 is connected between the output of the third transconductance amplifier 409 and the output of the feed-forward (fifth) transconductance amplifier 413 .

[0031] Figure 5 Shows Figure 4 A timing diagram of an operation mode of the chopper-stabilized amplifier 400. A first clock signal (PHASE1), a second clock signal (PHASE2), a third clock signal (PHASE3), a fourth clock signal (PHASE4) and a fifth clock signal (PHASE5) are used to control the chopping signal and the notch filter switch, such as Figure 4 As shown. The first clock signal (PHASE1) controls the chopping signals of the chopping circuit 401 and the chopping circuit 403. The second clock signal (PHASE2) controls the switches 406A, 406C, 407B and 407D. The third clock signal (PHASE3) controls the switches 406B, 406D, 407A and 407C. PHASE4 controls the switches 410B, 410D, 411A and 411C. Phase 5 controls the switches 410A, 410C, 411B and 411D. In Figure 5 In the embodiment of the present invention, the second clock signal (PHASE2) and the third clock signal (PHASE3) are notch filtered clock signals that are out of phase with each other and are operated at half the first clock signal (PHASE1) of the chopping signal frequency with a delay of one quarter of the chopping signal period. The quarter cycle delay allows a 90 degree delay, which means that the chopping signal and the notch filter clock signal are orthogonal to each other, which reduces high-order harmonics and is useful for reducing flicker noise. Each of the first and second notch filters 404 and 405 includes two parallel signal paths, and the switching operation frequency of each signal path is half the chopping signal frequency.

[0032] Switches 410A, 410B, 410C and 410D are controlled by PHASE4, and switches 411A, 411B, 411C and 411D are controlled by PHASE5. PHASE4 and PHASE5 are out of phase with each other. PHASE2 and PHASE3 are out of phase with each other. The chopping output signal generated by the second chopping circuit 403 is sampled by notch filters 404 and 405 in the positive chopping phase and the negative chopping phase, respectively. For example, when the first clock signal (PHASE1) is at a high level, the voltage of the chopping output signal is sampled by C1 and c2 at the midpoint of the high level of the first clock signal (PHASE1), and the charges on C1 and c2 are transferred to C3. When the first clock signal (PHASE1) is at a low level, the voltage of the chopping output signal is sampled by C4 and c5 at the midpoint of the low level state of the first clock signal (PHASE1), and the charges on C4 and C5 are transferred to c6. The voltages on C3 and C6 are provided to the second and fourth transconductance amplifiers 408 and 412, respectively, and the outputs of the second and fourth transconductance amplifiers 408 and 412 are combined. Because the positive and negative phases are sampled by two notch filters 404 and 405, respectively, the asymmetry of GM1 is suppressed, thus achieving very small ripples.

[0033] Figure 6 Shows Figure 4 Output waveforms of the first and second notch filters 404 and 405 of the circuit 400. In one embodiment, all positive outputs NOTCH2_OUTP of the notch filter 404 are located at the falling edge of the positive notch input signal NOTCH_INP generated by the second chopping circuit 403. All negative outputs NOTCH2_OUTN of the first notch filter 404 are located at the rising edge of the negative notch input signal NOTCH_INN generated by the second chopping circuit 403. Similarly, the positive output NOTCH1_OUTP of the second notch filter 405 is located at the rising edge of the positive notch input signal NOTCH_INP generated by the second chopping circuit 403. The negative output NOTCH1_OUTN of the second notch filter 405 is located at the falling edge of the negative notch input signal NOTCH_INN generated by the second chopping circuit 403. In one embodiment, if a 10mV offset is inserted in the chopping stage (mainly in the first transconductance amplifier), the output of the ping-pong notch filter of the present invention can be realized as a ripple voltage of about 70μV, while the conventional single notch filter ( Figure 2 ) has a ripple voltage of about 1.77 mV. Therefore, the ripple noise of the ping-pong notch filter in the present invention can be effectively reduced.

[0034] Figure 7 is a schematic diagram of a chopper-stabilized amplifier 700 according to another embodiment of the present invention. Figure 7 and Figure 4The difference between the embodiments is that the chopper-stabilized amplifier 700 includes a buffer 714 coupled between the input of the first notch filter 404 and the output of the third transconductance amplifier 409. The buffer 714 is provided to further reduce the ripple voltage. For example, if the inserted offset is 10 mV, the chopper-stabilized amplifier 700 can reduce the ripple voltage from about 70 μV to 15 μV.

[0035] The present invention also provides a method of operating a chopper-stabilized amplifier. The method includes the steps of chopping an input signal and applying the chopped input signal to an input of a first transconductance amplifier, and chopping an output signal generated by the first transconductance amplifier at a chopping frequency to generate a chopped output signal. Then, first and second notch filters connected in parallel are used to perform integration and transfer functions on the chopped output signal generated by the second chopper circuit, thereby reducing the ripple voltage.

[0036] Therefore, the present invention provides a chopper-stabilized amplifier and a method of operating the chopper-stabilized amplifier using a ping-pong notch filter to reduce the offset voltage and ripple noise of the chopper-stabilized amplifier.

[0037] "A" and "this" and similar terms describing the subject matter herein (especially in the claims) are considered to cover both the singular and the plural, unless otherwise indicated or clearly contradicted by the context. The numerical ranges recorded here are merely a convenient way to represent discrete values ​​falling within the range, and unless otherwise indicated, each discrete value is included in the specification as if it were recorded separately. In addition, the above description is only an embodiment and is not intended to be limiting, and the scope of protection is defined by the claims. Any and all embodiments provided herein, or the use of illustrative terms (such as "for example"), are only for the purpose of better illustrating the subject matter, and unless otherwise claimed, this is not a limitation on the scope of the subject matter. The use of the term "based on" and other similar phrases recorded in the claims and the specification indicates conditions for producing a result, and is not intended to exclude any other conditions for producing the result. Any term in the specification is not considered to indicate an element that is not necessarily claimed as an implementation claimed by the present invention.

[0038] Preferred embodiments are described herein. Of course, variations of those preferred embodiments are apparent to those skilled in the art based on what is described herein. Those skilled in the art appropriately use these variations, and the inventors intend to implement the claimed claims in different ways specifically described herein. Therefore, the claimed subject matter includes all modifications of the subject matter described in the claims and their equivalents as permitted by applicable law. In addition, unless otherwise specified or explicitly contradicted herein, combinations of the elements described above in all possible variations are encompassed herein.

Claims

1. A chopper-stabilized amplifier including a parallel notch filter, comprising: a first transconductance amplifier; a first chopping circuit coupled to the input of the first transconductance amplifier for chopping an input signal and applying the chopped input signal to the input of the first transconductance amplifier; a second chopping circuit coupled to the output of the first transconductance amplifier for chopping an output signal generated by the first transconductance amplifier; and a ping-pong notch filter including a first notch filter and a second notch filter connected in parallel, wherein each of the first notch filter and the second notch filter has an input coupled to the output of the second chopping circuit, and wherein the ping-pong notch filter performs an integration and transfer function on the chopped output signal generated by the second chopping circuit to filter a ripple voltage in the chopped output signal, the ping-pong notch filter suppressing a ripple generated due to an offset in the first transconductance amplifier.

2. The chopper-stabilized amplifier of claim 1, wherein there is a 90 degree phase shift between the clocks of the first notch filter and the second notch filter.

3. The chopper-stabilized amplifier of claim 1 , wherein the first notch filter has a first input and a second input, and a first output and a second output, and wherein the first notch filter comprises: A first path includes a first switch coupled between the first input terminal and the first node, a second switch coupled between the first node and the first output terminal, a third switch coupled between the first input terminal and the second node, and a fourth switch coupled between the second node and the second output terminal; a second path including a fifth switch connected between the first input terminal and the third node, a sixth switch connected between the third node and the first output terminal, a seventh switch coupled between the second input terminal and the fourth node, and an eighth switch coupled between the fourth node and the second output terminal; and A first capacitor is coupled between the first and second nodes and a second capacitor is coupled between the third and fourth nodes.

4. The chopper-stabilized amplifier of claim 3 , wherein the chopper output signal is controlled by a first clock signal, the first, third, sixth and eighth switches are controlled by a second clock signal, and the second, fourth, fifth and seventh switches are controlled by a third clock signal, wherein the frequencies of the second and third clock signals are half the frequency of the first clock signal.

5. The chopper-stabilized amplifier of claim 4, wherein the second clock signal and the third clock signal are out of phase with each other.

6. The chopper-stabilized amplifier of claim 3, wherein the second notch filter has third and fourth input terminals and third and fourth output terminals, and wherein the second notch filter comprises: a third path including a ninth switch coupled between the third input terminal and the fifth node, a tenth switch coupled between the fifth node and the third output terminal, an eleventh switch coupled between the fourth input terminal and the sixth node, and a twelfth switch coupled between the sixth node and the fourth output terminal; a fourth path, comprising a thirteenth switch coupled between the third input terminal and the seventh node, a fourteenth switch coupled between the seventh node and the third output terminal, a fifteenth switch coupled between the fourth input terminal and the eighth node, and a sixteenth switch coupled between the eighth node and the fourth output terminal; as well as A third integrating and transferring capacitor is coupled between the fifth and sixth nodes, and a fourth integrating and transferring capacitor is coupled between the seventh and eighth nodes.

7. The chopper-stabilized amplifier of claim 6, wherein the tenth, twelfth, thirteenth and fifteenth switches are controlled by a fourth clock signal, and the ninth, eleventh, fourteenth and sixteenth switches are controlled by a fifth clock signal.

8. The chopper-stabilized amplifier of claim 7, wherein the fourth clock signal and the fifth clock signal have the same frequency.

9. The chopper-stabilized amplifier of claim 7, wherein the frequencies of the fourth and fifth clock signals are half the frequency of the first clock signal.

10. The chopper-stabilized amplifier of claim 1 , further comprising: a second transconductance amplifier having an input coupled to the output of the first notch filter; a third transconductance amplifier having an input coupled to the output of the second notch filter; and A fourth transconductance amplifier has an input coupled to the output of the second transconductance amplifier and the output of the third transconductance amplifier.

11. The chopper-stabilized amplifier of claim 10, further comprising a fifth transconductance amplifier having an input receiving the input signal and having an output coupled to the outputs of the second and third transconductance amplifiers.

12. The chopper-stabilized amplifier of claim 10, further comprising a buffer coupled between an input of the first notch filter and an output of the fourth transconductance amplifier.

13. A method of operating a chopper-stabilized amplifier, the method comprising: chopping an input signal and applying the chopped input signal to an input of a first transconductance amplifier; chopping the output signal generated by the first transconductance amplifier at a chopping frequency to generate a chopped output signal; and performing an integration and transfer function on a chopped output signal generated by the second chopping circuit using a ping-pong notch filter to filter a ripple voltage in the chopped output signal; The ping-pong notch filter includes first and second notch filters connected in parallel, and the ping-pong notch filter suppresses ripples generated by an offset in the first transconductance amplifier.

14. The method of operating a chopper-stabilized amplifier of claim 13, wherein there is a 90 degree phase shift between the clocks to the first and second notch filters.

15. The method of operating a chopper-stabilized amplifier of claim 13, wherein a clock frequency of the first and second notch filters is half a frequency of the chopped output signal.

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