CN112751542B - Second-order multifunctional switch capacitor filter - Google Patents
Second-order multifunctional switch capacitor filter Download PDFInfo
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- CN112751542B CN112751542B CN201911041243.9A CN201911041243A CN112751542B CN 112751542 B CN112751542 B CN 112751542B CN 201911041243 A CN201911041243 A CN 201911041243A CN 112751542 B CN112751542 B CN 112751542B
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
- H03H7/00—Multiple-port networks comprising only passive electrical elements as network components
- H03H7/01—Frequency selective two-port networks
Abstract
The invention provides a second-order multifunctional switch capacitor filter which is characterized by comprising four resistor-capacitor switching circuits with the same circuit structure, an open-circuit capacitor switching circuit, a logic control circuit, an operational amplifier amplifying circuit and a low-pass filter circuit. The filter has the remarkable advantages that the switch between the resistor and the capacitor is realized through the switch capacitor structure, the switch among the low pass, the high pass and the band pass can be realized, and the adjustment of the filter characteristic frequency can be realized through changing the clock frequency.
Description
Technical Field
The invention belongs to the technical field of signal processing, and relates to a switched capacitor filter, in particular to a switched capacitor filter with adjustable characteristic frequency, which can realize low-pass, high-pass and band-pass conversion.
Background
In signal processing operations, filters are a widely used analog circuit block. In practical applications, different kinds of filters often need to be designed due to different requirements of circuits, so it is critical to find a circuit structure capable of implementing multiple filtering modes.
Filters can be classified into various types according to principle or topology, and continuous time filters and switched capacitor filters are widely used in integrated circuits. Compared to continuous time filters, switched capacitor filters have the advantage that their transfer function depends on the ratio between the capacitors, with a higher accuracy. And the equivalent resistance of the switch capacitor module can be changed by changing the clock frequency of two non-overlapping phases, so as to adjust the transfer function of the filter. More importantly, the capacitor is used for replacing the resistor, so that the area can be effectively saved, the power consumption is reduced, and the integration level is improved.
Currently, in the switch capacitor filter chip on the market, the TLC04 of texas instruments is a fourth-order butterworth filter based on switch capacitors, and can realize low-pass filtering with cut-off frequency from 0.1Hz to 30 kHz. More preferably, MF10-N is a switched capacitor multifunctional filter that can perform high pass, low pass, band pass, all pass and notch functions, similar to the structure of LTC1060 in Adenode semiconductor. However, these two types of chips still require external circuitry to implement the various second order filtering, and are not truly integrated into one circuit. In addition, the accuracy and parasitic capacitance of the external circuit can adversely affect the switched capacitor filter.
The research directions of the prior invention patents are mostly focused on the aspects of high order and high precision and are oriented to special application filters, so that how to realize the filter with multiple functions, such as low-pass, high-pass and band-pass integrated design, is a problem to be solved at present.
Disclosure of Invention
In order to solve the problems that the existing filter cannot be switched between low pass, high pass and band pass and has single function and ensure a larger working range, the invention provides a second-order multifunctional switched capacitor filter which can freely switch filter types and has adjustable characteristic frequency.
The technical scheme adopted by the invention is as follows:
a second-order multifunctional switch capacitor filter, as shown IN figure 1, comprises LOW, HIGH, BAND, CLK, IN five input pins and OUT one output pin, wherein LOW, HIGH, BAND is used for controlling the state of the filter, CLK is a clock signal, IN is an input signal, OUT is an output signal, and the filter is characterized by comprising four resistance-capacitance switching circuits with the same circuit structure, an open-circuit-capacitance switching circuit, a logic control circuit, an operational amplifier amplifying circuit and a low-pass filter circuit;
the resistance-capacitance switching circuit has four input ports CLK1, CLK2, IN, RC and an output port OUT, wherein CLK1, CLK2 are clock signals with opposite phases, IN is an input signal, OUT is an output signal, RC is a control signal, the circuit is represented as a capacitor at a high level, and the circuit is represented as a resistor at a low level. The input terminals CLK1 and CLK2 of the four resistance-capacitance switching circuits are connected to each other, and externally input clocks are connected in parallel. The input end IN of the first resistance-capacitance switching circuit is connected with an external input signal, and the output end OUT is connected with the input ends IN of the second resistance-capacitance switching circuit, the third resistance-capacitance switching circuit and the open-circuit-capacitance switching circuit. The output end OUT of the second resistance-capacitance switching circuit is connected with the output end of the operational amplifier amplifying circuit. The output end OUT of the third resistance-capacitance switching circuit is connected with the input end of the operational amplifier amplifying circuit. The input end IN of the fourth resistance-capacitance switching circuit is connected with the output end OUT of the third resistance-capacitance switching circuit, and the output end OUT is grounded.
The open-circuit-capacitor switching circuit is provided with two input ports OC, IN and an output port OUT, wherein IN is an input signal, OUT is an output signal, OC is a control signal, the circuit is represented as a capacitor at a high level, and the circuit is represented as an open circuit at a low level. The input end OC is connected with the bandpass port of the logic control circuit, the input end IN is connected with the output end OUT of the first resistance-capacitance switching circuit, and the output end OUT is grounded.
The logic control circuit is provided with three external control ports LOW, HIGH, BAND and a clock signal input port CLK, and when the port LOW is connected with a HIGH level and the port HIGH and the port BAND are connected with a LOW level, the filter circuit is in a LOW-pass state; when the port HIGH is connected with a HIGH level, the port LOW and the port BAND are connected with a LOW level, and the filter circuit is in a HIGH-pass state; when port BAND is HIGH and port LOW and port HIGH are LOW, the filter circuit exhibits a bandpass state. In the control circuit, the port LOW is connected to the port RC of the second and fourth resistance-capacitance switching circuits and to the port RC of the third resistance-capacitance switching circuit after passing through one inverter, the port HIGH is connected to the port RC of the first resistance-capacitance switching circuit, the port BAND is connected to the port OC of the open-circuit-capacitance switching circuit, the port CLK is connected to the port CLK1 of the four resistance-capacitance switching circuits and to the port CLK2 of the four resistance-capacitance switching circuits after passing through one inverter.
And the input end of the operational amplifier amplifying circuit is connected with the output end OUT of the third resistance-capacitance switching circuit, and the output end of the operational amplifier amplifying circuit is connected with the input end of the low-pass filter circuit.
The input end of the low-pass filter circuit is connected with the output end of the operational amplifier amplifying circuit, and the output end is used as final output.
Further, the resistance-capacitance switching circuit is composed of nine switches, two capacitors and an inverter, and the schematic diagram is shown in fig. 2. The inverter is used for inverting the input RC signal to control the opening and closing of the switch. The four switches S1, S2, S3 and S4 are controlled by clock signals CLK1 and CLK2, and the capacitor forms a switched capacitor structure to replace a resistor, the other four switches S5, S6, S7 and S8 are controlled by RC signals, the S5 and S6 are respectively connected with the S1 and S2 in parallel, and the S7 and S8 are respectively connected with the S3 and S4 in series, so that the switching between the switched capacitor structure and the common capacitor structure is realized. The last switch S9 and capacitor controlled by RC signal is used to switch the switch capacitor structure and the common capacitor structure.
Further, the open-capacitor switching circuit is composed of a switch, an inverter and a capacitor, and the schematic diagram is shown in fig. 3. The inverter forms two-phase non-overlapping clocks which are respectively connected with the positive port and the negative port of the switch to control the on-off of the switch, and the on-off of the switch controls whether the whole circuit is in an open circuit or a capacitor.
Further, the operational amplifier amplifying circuit is composed of an operational amplifier and two resistors. The positive input end of the operational amplifier is connected with the input end of the operational amplifier amplifying circuit, and the output end of the operational amplifier amplifying circuit. One end of the first resistor is connected with the reverse input end of the operational amplifier, and the other end of the first resistor is grounded. One end of the second resistor is connected with the inverting input end of the operational amplifier, and the other end is connected with the output end of the operational amplifier.
Further, the low-pass filter circuit is composed of an operational amplifier, a resistor and a capacitor. One end of the resistor is connected with the input end of the low-pass filter circuit, and the other end of the resistor is connected with the capacitor and the positive input end of the operational amplifier. One end of the capacitor is connected with the resistor and the positive input end of the operational amplifier, and the other end of the capacitor is grounded. The positive input end of the operational amplifier is connected with the resistor and the capacitor, and the negative input end is connected with the output end.
Further, the switch is composed of a PMOS tube and an NMOS tube, the schematic diagram is shown in fig. 4, the gate ends of the two tubes are respectively connected with opposite clock signals, the drain ends are connected with the input end, the source ends are connected with the output end.
When the low-pass input terminal is connected with high level and the high-pass and band-pass terminals are connected with low level, the RC terminals of the first resistance-capacitance switching circuit and the third resistance-capacitance switching circuit are low level, so that the switches S5, S6 and S9 are opened, the switches S7 and S8 are closed, the circuit is in a switched capacitor structure, and the switches S1, S2, S3 and S4 are controlled by CLK signals to display resistance characteristics which are R1 and R2 respectively. The RC terminals of the second and fourth resistance-capacitance switching circuits are high, so that the switches S5, S6, S9 are closed, the switches S7, S8 are opened, the switches S1, S2, S3, S4 are shielded, and the circuits exhibit capacitance characteristics, C1, C2 respectively. The RC terminal of the open-capacitor switching circuit is low, and thus the switch is opened, exhibiting an open characteristic. Thus, the overall circuit structure is shown in FIG. 5 as a second order low pass filter with a characteristic frequency of
When the high-pass input terminal is connected with a high level and the low-pass and band-pass terminals are connected with a low level, RC terminals of the first resistor-capacitor switching circuit and the third resistor-capacitor switching circuit are high levels, so that the switches S5, S6 and S9 are closed, the switches S7 and S8 are opened, the switches S1, S2, S3 and S4 are shielded, and the circuits show capacitance characteristics which are C1 and C2 respectively. The RC terminals of the second and fourth resistance-capacitance switching circuits are low, so that the switches S5, S6, S9 are opened, the switches S7, S8 are closed, the circuit is in a switched capacitor structure, and the switches S1, S2, S3, S4 are controlled by the CLK signal to display resistance characteristics, R1, R2 respectively. The RC terminal of the open-capacitor switching circuit is low, and thus the switch is opened, exhibiting an open characteristic. Thus, the overall circuit structure is shown in FIG. 6, and is characterized by a second-order high-pass filter with a characteristic frequency of
When the band-pass input terminal is connected with high level and the low-pass terminal is connected with high-pass terminal and low-level, the RC terminals of the first resistance-capacitance switching circuits, the second resistance-capacitance switching circuits and the fourth resistance-capacitance switching circuits are low level, so that the switches S5, S6 and S9 are opened, the switches S7 and S8 are closed, the circuits are in a switched capacitor structure, and the switches S1, S2, S3 and S4 are controlled by CLK signals to display resistance characteristics which are R1, R2 and R3 respectively. The RC terminal of the third RC switching circuit is at a high level, so that the switches S5, S6, S9 are closed, the switches S7, S8 are open, the switches S1, S2, S3, S4 are shielded, and the circuit exhibits a capacitive characteristic, i.e. a capacitance C1. The RC terminal of the open-capacitance switching circuit is at a high level, and thus the switch is closed, exhibiting a capacitive characteristic, i.e., a capacitance C2. Thus, the overall circuit structure is shown in FIG. 7 as a second order bandpass filter with a characteristic frequency of
In the switched capacitor configuration, the switch is controlled by two non-overlapping clock signals, which are repeated between open and closed. Assuming that the voltage across the resistor does not change much in one period and is approximately constant, the magnitude of the resistance of the simulated resistor is deduced to be
Therefore, the clock frequency directly controls the size of the analog resistor of the switched capacitor structure, and further can control the transfer function of the constructed second-order filter circuit.
Compared with the prior art, the invention has the following advantages:
1. the invention realizes the filtering by using the switched capacitor structure, and can effectively change the equivalent resistance of the switched capacitor module by adjusting the frequency of the two-phase non-overlapping clock so as to adjust the transfer function of the filter.
2. The invention adopts the switch capacitor trans-impedance equivalent circuit, so that the parasitic effect of the capacitor can not influence the circuit.
3. The invention adopts a low-pass filter circuit to improve the noise brought by the switch.
4. The invention uses the follower structure at the output end, ensures the isolation between the follower structure and the load, and greatly reduces the output impedance.
5. The invention can effectively save area by using the capacitor to replace the resistor, reduce power consumption and be beneficial to improving the integration level.
6. The invention can realize multiple filtering functions of low pass, high pass and band pass, and can be switched freely among the three.
Drawings
FIG. 1 is a schematic diagram of the present invention.
Fig. 2 is a schematic diagram of a resistor-capacitor switching circuit.
Fig. 3 is a schematic diagram of an open-capacitor switching circuit.
Fig. 4 is a schematic diagram of the switch.
Fig. 5 is a schematic diagram of the present invention operating in a low pass mode.
Fig. 6 is a schematic diagram of the present invention operating in a high pass mode.
Fig. 7 is a schematic diagram of the present invention operating in a bandpass mode.
Fig. 8 is a waveform diagram of input and output at 20kHz in the example.
Fig. 9 is a frequency response of the filter at 20kHz for the clock signal in the example.
Fig. 10 is a waveform diagram of input and output when the clock signal is 10kHz in the example.
Fig. 11 is a frequency response of the filter at 10kHz for the clock signal in the example.
Detailed Description
The following further illustrates the technical solution of the present invention with reference to the accompanying drawings, but not limited thereto, and all modifications and equivalents of the technical solution of the present invention are included in the scope of protection of the present invention without departing from the spirit and scope of the technical solution of the present invention.
In this embodiment, the specific design parameter is that the capacitance Ca is 0.5pF and the capacitance Cb is 9.5pF.
In this embodiment, taking the HIGH-pass state as an example, the port HIGH is set to the HIGH level, and the ports LOW and BAND are set to the LOW level. The input signal is 100mV of peak-to-peak value, the frequencySinusoidal signal with 100Hz, clock signal is square wave signal with 20kHz frequency, and power supply voltage V dd Is 2V.
At this time, the schematic diagram of the circuit is shown in fig. 6, the first and third resistance-capacitance switching circuits are shown as capacitors with a size of 10pF, the second and fourth resistance-capacitance switching circuits are shown as resistors, and the first and third resistance-capacitance switching circuits are shown as resistors according to formula 7 at the clock frequency
Calculate its cut-off frequency as per equation 4
In this example, the waveforms of the input end and the output end are as shown in fig. 8, and the waveform of the output signal is normal, and no cut-off or distortion occurs. Further, the frequency response of this example is shown in fig. 9, where the cut-off frequency of the high pass filter is about 150Hz, which is substantially consistent with the calculation result.
Next, the frequency of the clock signal is changed from 20kHz to 10kHz, and the waveforms of the input terminal and the output terminal are shown in fig. 10.
After the frequency change, the waveform of the output signal remains normal, and no cut-off or distortion occurs. Further, the results obtained are shown in FIG. 11. The cut-off frequency of the high-pass filter is 75Hz at this time, which maintains consistency with the calculation result, and this result also demonstrates the feasibility of achieving characteristic frequency adjustability by varying the input clock signal.
Claims (6)
1. The second-order multifunctional switch capacitor filter comprises LOW, HIGH, BAND, CLK, IN five input pins and an OUT (output) pin, wherein LOW, HIGH, BAND is used for controlling the state of the filter, CLK is a clock signal, IN is an input signal, OUT is an output signal, and the filter is characterized by comprising four resistance-capacitance switching circuits with the same circuit structure, an open-circuit-capacitance switching circuit, a logic control circuit, an operational amplifier amplifying circuit and a low-pass filter circuit;
the resistance-capacitance switching circuit is provided with four input ports CLK1, CLK2, IN, RC and an output port OUT, wherein the CLK1 and CLK2 are clock signals with opposite phases, the IN is an input signal, the OUT is an output signal, the RC is a control signal, the circuit is represented as a capacitor at a high level, the circuit is represented as a resistor at a low level, the input ends CLK1 and CLK2 of the four resistance-capacitance switching circuits are respectively connected and connected with an external input clock, the input end IN of the first resistance-capacitance switching circuit is connected with an external input signal, the output end OUT is connected with the input ends IN of a second resistance-capacitance switching circuit, a third resistance-capacitance switching circuit and an open-circuit-capacitance switching circuit, the output end OUT of the second resistance-capacitance switching circuit is connected with the output end OUT of the operational amplifier amplifying circuit, the input end IN of the fourth resistance-capacitance switching circuit is connected with the output end OUT of the third resistance-capacitance switching circuit, and the output end OUT of the output end OUT is grounded;
the open-circuit-capacitor switching circuit is provided with two input ports OC, IN and an output port OUT, wherein IN is an input signal, OUT is an output signal, OC is a control signal, the circuit is represented as a capacitor at a high level, the circuit is represented as an open circuit at a low level, the input end OC is connected with a bandpass port of the logic control circuit, the input end IN is connected with the output end OUT of the first resistance-capacitance switching circuit, and the output end OUT is grounded;
the logic control circuit has three external control ports of LOW pass (LOW), HIGH pass (HIGH), BAND pass (BAND) and clock signal input ports CLK, when the LOW pass end is connected to HIGH level, the HIGH pass and BAND pass are connected to LOW level, the filter circuit is in LOW pass state, when the HIGH pass end is connected to HIGH level, the LOW pass and BAND pass are connected to LOW level, the filter circuit is in HIGH pass state, when the BAND pass is connected to HIGH level, the LOW pass and HIGH pass ends are connected to LOW level, the filter circuit is in BAND pass state, in the control circuit, the LOW pass end is connected to the RC ends of the second and fourth resistance-capacitance switching circuits, and after passing through an inverter, the RC end of the third resistance-capacitance switching circuit is connected to the HIGH pass end, the BAND pass end is connected to the RC end of the first resistance-capacitance switching circuit, the BAND pass end is connected to the OC end of the open-capacitance switching circuit, the end is connected to the CLK1 end of the four resistance-capacitance switching circuits, and after passing through an inverter, the control circuit is connected to the 2 ends of the four resistance-capacitance switching circuits;
the input end of the operational amplifier amplifying circuit is connected with the output end OUT of the third resistance-capacitance switching circuit, and the output end of the operational amplifier amplifying circuit is connected with the input end of the low-pass filter circuit;
the input end of the low-pass filter circuit is connected with the output end of the operational amplifier amplifying circuit, and the output end is used as final output.
2. The filter according to claim 1, wherein the resistor-capacitor switching circuit is composed of nine switches, two capacitors and an inverter, wherein the inverter is used for inverting an input RC signal to control the switching of the switches, four switches S1, S2, S3, S4 are controlled by clock signals CLK1, CLK2 and the capacitors form a switched capacitor structure to replace resistors, the other four switches S5, S6, S7, S8 are controlled by RC signals, S5, S6 are respectively connected in parallel with S1, S2, and S7, S8 are respectively connected in series with S3, S4 to realize the switching between the switched capacitor structure and a common capacitor structure, and the last switch S9 and capacitor controlled by the RC signals are used for realizing the switching of the switched capacitor structure and the common capacitor structure.
3. The filter of claim 1, wherein the open-capacitor switching circuit is composed of a switch, an inverter and a capacitor, the inverter forms two non-overlapping clocks and is respectively connected with positive and negative ports of the switch to control the on-off of the switch, and the on-off of the switch is used for controlling whether the whole circuit is in an open circuit or a capacitor.
4. The second-order multifunctional switch capacitor filter according to claim 1, wherein the operational amplifier amplifying circuit is composed of an operational amplifier and two resistors, the forward input end of the operational amplifier is connected with the input end of the operational amplifier amplifying circuit, the output end of the operational amplifier amplifying circuit is connected with the output end of the operational amplifier amplifying circuit, one end of the first resistor is connected with the reverse input end of the operational amplifier, the other end of the first resistor is connected with the reverse input end of the operational amplifier, and the other end of the second resistor is connected with the output end of the operational amplifier.
5. The second-order multifunctional switch capacitor filter according to claim 1, wherein the low-pass filter circuit is composed of an operational amplifier, a resistor and a capacitor, one end of the resistor is connected with the input end of the low-pass filter circuit, the other end of the resistor is connected with the capacitor and the positive input end of the operational amplifier, one end of the capacitor is connected with the resistor and the positive input end of the operational amplifier, the other end of the capacitor is grounded, the positive input end of the operational amplifier is connected with the resistor and the capacitor, and the negative input end of the operational amplifier is connected with the output end.
6. The second-order multifunctional switched capacitor filter of claim 2 wherein said switch is comprised of a PMOS and an NMOS tube having gate terminals connected to opposite clock signals, drain terminals connected to the input terminal, and source terminals connected to the output terminal.
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| CN201911041243.9A CN112751542B (en) | 2019-10-29 | 2019-10-29 | Second-order multifunctional switch capacitor filter |
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| GB8511218D0 (en) * | 1984-05-04 | 1985-06-12 | Secr Defence | Switched capacitor filter circuits |
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