JPH01155709A - Switched capacitor filter circuit - Google Patents

Abstract

PURPOSE:To eliminate the need for the analog pre-filter and postfilter having been required for a conventional circuit by providing the roll of the pre-filter and post-filter of a consecutive time system filter on a block being a part in the switched capacitor filter. CONSTITUTION:At least a primary or secondary block of a first stage and a last stage in the switched capacitor filter(SCF) 14 having a main characteristic is constituted by an analog filter 12 using a MOS transistor as a resistor. The analog filter 12 uses the MOSFET as a resistor and it is set to a required resistance by a resistance calibration circuit 13 as a bias voltage calibration circuit being the gate-source circuit. Thus, the pre-filter and the post-filter having a cut-off frequency of fs/2 are not required in the filter circuit of this invention.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a filter circuit that can be implemented as a monolithic IC, and particularly relates to a low-pass filter.

(Prior art) Conventional filter circuits that can be implemented as monolithic ICs include an RC active filter that uses a resistor and a capacitor, and a switched capacitor integrator that replaces the resistance of the RC integrator circuit in the RC active filter with a switch and a capacitor. There is a switched capacitor filter (hereinafter referred to as "5CFJ"), the former is a continuous time filter, and the latter is a discrete time filter (sample value system).
It's a filter.

FIG. 9 shows the configuration of the RC active low-pass filter. In the figure, 1 is a signal input terminal, 2 is a signal output terminal, 3 is a resistor, 4 is a capacitor, and 5 is an operational amplifier. The frequency characteristics of this filter are determined by the absolute values of the resistor 3 and capacitor 4. When implemented as an IC, it is difficult to manufacture highly accurate resistors and capacitors because the element sensitivity is large, and therefore there are drawbacks such as large errors from design values and large temperature deviations of the element. Furthermore, the resistance and capacitance values are extremely large for signals in the audio frequency band, and when placed on an IC chip, the area will be extremely large.

The SCF equivalently realizes resistance by periodically opening/closing switches and capacitors, and the transfer characteristics of the SCF are determined only by the capacitance ratio between the sampling capacitor and the integrating capacitor and the switching frequency, so current MOS process technology By using this, the required characteristics can be achieved with high precision.

FIG. 10 shows the configuration of an example of a switched capacitor integrator constituting the SCF. Analog switch (AS) 8a
, 8b are input with complementary clock signals whose logic "1" phases do not overlap, and by alternately turning on the analog switches 8a and gb connected to the sampling capacitor 6, charge is accumulated in the integrating capacitor 7. Ru.

FIG. 11 is a block diagram showing the SCF circuit. In the figure, 9 is a brifilter, 10 is an SCF block, and 11 is a post filter. Since the SCF is a sample value filter, an aliasing phenomenon (aliasing) occurs in which signal components appear at integral multiples of the clock frequency f, so it is called a continuous time filter that band-limits the input signal to f3/2 in advance. An analog filter is required.

Further, a continuous-time post filter is required for the output, which extracts only the desired signal and attenuates the aliased signal.

Normally, the pre-filter and post-filter are composed of RC active filters, and since the accuracy of the filter characteristics is determined by the absolute values of the resistors and capacitors, it is not very accurate when integrated into an IC. In principle, the clock frequency fs of the SCF should be at least twice the highest signal frequency. In order to reduce the power consumption of the circuit, it is advantageous to keep the clock frequency as low as possible under these conditions.

[Problem that the invention seeks to solve]

However, when the clock frequency is lowered, the cut-off characteristics required of the pre-filter and post-filter become steeper, and the circuit becomes significantly more complex.

Therefore, the characteristics of the analog brifilter and post are SC
The lower limit of the clock frequency used by F was determined.

Furthermore, since the filter characteristics of SCF are determined by the capacitance ratio, it is possible to realize extremely high-precision filters, but on the other hand, as mentioned above, signal aliasing occurs, which is typical of sample value filters, and high-frequency noise falls into the fundamental band. There was a drawback.

The present invention has been made in view of these points, and its purpose is to provide a high-precision filter that can be easily fabricated into a monolithic IC.

The object of the present invention is to obtain a switched capacitor filter circuit that does not require a post filter.

[Means for solving problems]

In order to achieve such an object, the present invention provides a switched capacitor of second order or higher order, which is composed of a switched capacitor filter block and an analog filter of first order or higher order, which is used in the first stage, the final stage, or both the first stage and the final stage. In the filter circuit, the analog filter includes a capacitor that determines the capacitance value, a MOS transistor as a resistor, a capacitor that maintains the bias voltage between the gate and source of the MOS transistor, and a switch circuit that switches the MOS transistor. It is constructed from a bias voltage setting circuit that applies a required bias between the gate and source of the MOS transistor.

[Effect]

In the switched capacitor filter circuit according to the present invention, a pre-filter and a post-filter are not necessary, and the circuit scale is reduced.

〔Example〕

First, the features of the present invention will be described. The main feature of the present invention is that in a switched capacitor filter which is a sample value filter, a part of the SCF has functions of a bristle filter for preventing overlap and a post filter for removing aliased signals. . That is, S.C.
The first stage of F.

R where the last stage uses a MOS transistor as a resistor
This takes advantage of the fact that no aliasing signal occurs due to the configuration of C active filters.

Next, the differences from the conventional technology will be described. In the conventional technology, the bris filter and post filter are composed of RC active filters, so in addition to the SCF, a bris filter and post filter with a blocking wave number of f,/2 are required, but the filter according to the present invention The circuit differs from the prior art in that it does not require a pre-filter or post-filter with a cut-off wave number of f,/2.

Figure 4 shows the amplitude frequency characteristics of the conventional SCF circuit.
The vertical axis shows the amplitude, and the horizontal axis shows the frequency and clock frequency.Since the brifilter and post filter are composed of RC active filters, the blocking wave number is f,/2 to avoid complicating the SCF circuit. . In FIG. 4, Sl shows the SCF characteristic, S2 shows the characteristics of the brifilter and post-filter, and S3 shows the image.

FIG. 5 shows the amplitude frequency characteristics of the SCF circuit according to the present invention. The vertical axis shows amplitude, and the horizontal axis shows frequency and clock frequency. The SCF circuit according to the present invention has the main characteristic S
At least the first and last primary or secondary blocks of the CF are constituted by analog filters using MOS transistors as resistors. All filters can be implemented with the same order as required for the main SCF.

An SCF circuit (switched capacitor filter circuit) according to the present invention is shown in FIG. In FIG. 1, l is a signal input terminal, 2 is a signal output terminal, and 12 is an analog filter using a MOSFET as a resistor, and the required resistance value is determined by a resistance value calibration circuit 13 as a bias voltage calibration circuit between the gate and source. is set to 14 is an SCF block (switched capacitor filter block), the above M
An analog filter that uses the unsaturated region between the drain and source of an OSFET as a resistor takes advantage of the fact that the resistance between the drain and source is determined with considerable accuracy by the bias voltage between the MOSFET's gate and source. is a voltage-controlled resistor (Voltage-C
It operates as a controlled Re5istor). There are two methods for setting the bias voltage between the gate and source: an indirect method and a direct method. In the indirect method, a reference filter or oscillator is created on an IC chip along with the required main filter using an RC having the same ratio as the Re product of the main filter. Then, by inputting a reference clock signal from the outside and comparing the phase of the reference clock with the output signal of the reference filter or the oscillation frequency of the oscillator, the RC values of the reference filter and oscillator as well as the RC value of the main filter are corrected. This is the way to do it. Methods of directly setting the bias voltage include a method of interrupting the operation of the main filter and a method of preparing a plurality of MOSFETs and using them alternately.

Analog filter 12 using a MOSFET shown in FIG. 1 as a resistor. As an example of the configuration of the resistance value calibration circuit 13, "IEE Newsletter on Circuits and Systems,
CAS-Volume 29, No. 5, May 1982 (18E1!
TRANSACTIONS ON CIRCUITS
ND SYSTEMS, vol, CAS-29, No5
．． MaV 1982) J. Below, an example of this configuration will be described as a switched resistor filter (S RF
). FIG. 2 shows a resistor that constitutes this switched resistor filter. In the figure, 15 is an analog switch (AS) for switching FE718.19 as a resistor, 16.17 is a resistance terminal, 20.
21 is a bias voltage holding capacitor, 22 is a drain connection terminal, 23 is a gate connection terminal, 24 is a source connection terminal, 25 is a gate-source voltage setting circuit (pre-chain circuit) as a bias voltage setting circuit; The circuit 25 corresponds to the resistance value calibration circuit 13 in FIG. The SRF is obtained by replacing the resistor 3 shown in FIG. 9 with the circuit shown in FIG. 2, and serves as an analog filter 12 constituting the first and last stages of the switched capacitor filter circuit according to the present invention. Also, the capacity 4 in Figure 9
is a capacitor that determines the capacitance value.

This switched resistor filter applies a bias voltage between the gate and source of FE718.19, and the resistance region between the drain and source determined by the bias voltage is R.
This is used as a resistor for the C active filter. When viewed from the resistor terminals 16 and 17, a resistor that is always biased with the same voltage can be seen even if the FET is replaced, and the switched resistor filter can be treated as a continuous time filter.

The switched resistor (S R) circuit in Figure 2 is a FET
It consists of a pre-chain circuit that sets the bias voltage of , and a switching circuit that alternately switches the FET into a pre-tune circuit and a filter circuit.

In the pretune circuit, if the equivalent resistance of SRF is R,
The gate-source voltage V so that the resistance value of the FET becomes R. stipulates.

FIG. 3 shows a case where the pretune circuit is constructed from a switched capacitor (SC) integrator. In the same figure, 25
is an FET, 26 is a bias voltage holding capacitor, 27 is a sampling capacitor, 28 is an integrating capacitor, 29 is an analog switch, and Vst is a reference DC voltage. In the figure, the SC integrator consists of an operational amplifier 5, capacitors 27 and 28, and an analog switch (AS) 29. In Figure 3, S
When the output voltage v0 of the C integrator becomes constant, that is, F
When the gate-source voltage VCS of ET1B and 19 becomes constant, the equivalent resistance Rvt+ of FET1B and 19 and the resistance Rrt of the SC integrator. become equal. At this time, Rvtt= Rvto= I / Co ° f
-, and the equivalent resistance value can be determined by the sampling capacitance Cu and the clock frequency f.

Next, a circuit design example will be explained. The SCF transfer function is represented by two functions. On the other hand, the transfer function of an analog filter is represented by an S function, and in order to construct an SCF from an analog filter, the S function is converted into two functions by LD conversion or bilinear conversion.

In this way, the transfer function expressed by two functions can be expressed by a cascade or parallel connection of first-order or second-order blocks.

When a filter is divided into quadratic blocks, the value of Q representing the steepness of the filter within each block varies, and Q
There are filters with high Q and filters with low Q, and the desired filter characteristics are achieved as a whole.

As an example, the transfer function F (s) of a ninth-order low-pass filter (LPF) is shown in the following equation.

s" + (2923,80)"s" + (1555,72)" St+ (1154,70)"s" 10 (1015,43) "The entire filter is composed of secondary or primary blocks. , among these, the primary section with low Q is placed at the front stage, the section with Q = 0.57 is placed at the last stage, a switched resistor filter configuration is created in which the resistance region of the FET is used as a resistor, and the remaining SCF Examples of the configuration of this ninth-order switched capacitor low-pass filter (SCLPF) are shown in Figures 6 and 7. Figure 6 shows the configuration of a conventional SCF circuit, and Figure 7 shows the configuration of a conventional SCF circuit. In FIGS. 6 and 7 showing the configuration of the SCF circuit according to the present invention, 30 is a brifilter, 31 is a ninth-order SCF block, 32 is a post filter, 33 is a first-order section,
34 is a section with Q=0.79, 35 is a section with Q=1.35
section, 36 is the section of Q=4.15, 37
is the section with Q=0.57, and 34-36 are SC
Configure F block. In this way, the entire filter can be configured with the main ninth-order characteristics. Since the overall characteristics of the filter are determined by the characteristics of the block with a large Q, that is, the SCF block, a highly accurate filter can be realized as in the conventional case. It is well known that the frequency characteristics of the SCF can be varied by changing the clock frequency, but a switched resistor filter can also be realized by changing the clock frequency of the switched capacitor integrator of the bias circuit. Therefore, not only a filter with fixed frequency characteristics but also a filter with variable frequency characteristics can be realized.

FIG. 8 shows the amplitude frequency characteristics of the filter of formula (1).

In the same figure, the vertical axis is the attenuation amount, the horizontal axis is the frequency, the solid line curve 310 shows the calculated value of the transfer function of equation (1), and the dotted line curve S20 is a partial section of the first stage in FIG. shows the value when the Q=0.57 section at the last stage is replaced with SRF. As shown in FIG. 8, replacing the low-Q section with a MOS FET filter has little effect on the in-band characteristics.

〔Effect of the invention〕

As explained above, the present invention allows some blocks in the SCF to play the roles of the continuous-time filter and the post-filter, which were conventionally required. Eliminates the need for a pre-filter and post-filter, thus reducing the need for SCF monolithic ICs.
The conventional RC active filter, which occupied a large amount of chip area, is no longer necessary, and a switched capacitor filter circuit can be realized with the same order as the conventional switched capacitor filter block, and the resistor of the active filter can also be configured with a MOSFET. The circuit scale can be significantly reduced. This effect is particularly noticeable when constructing a filter for the audio frequency band.

Another advantage is that the clock frequency of the switched capacitor filter, which conventionally determined the characteristics of the pre-filter and post-filter, can be set to an arbitrary value that is twice or more the highest signal frequency.

[Brief explanation of the drawing]

FIG. 1 is a system diagram showing a switched capacitor filter circuit according to the present invention, FIG. 2 is a circuit diagram showing a resistor constituting the switched resistor filter, and FIG. 3 is a circuit diagram showing a gate-source voltage setting circuit. FIG. 4 is a graph showing amplitude versus frequency characteristics of a conventional switched capacitor filter circuit, FIG. 5 is a graph showing amplitude versus frequency characteristics of a switched capacitor filter circuit according to the present invention, and FIG. 6 is a conventional configuration of a ninth-order filter. Fig. 7 is a block diagram showing an example of the structure of a ninth-order filter according to the present invention;
The figure is a graph comparing the frequency characteristics and design characteristics according to the configuration example shown in Fig. 7, Fig. 9 is a circuit diagram showing an RC active low-pass filter that constitutes a conventional switched capacitor filter circuit, and Fig. 10 is a graph showing a switched capacitor filter circuit. A circuit diagram showing an integrator, and FIG. 11 is a system diagram showing a conventional switched capacitor filter circuit. 1...Signal input terminal, 2...Signal output terminal, 12.
...Analog filter, 13...Resistance value calibration circuit, 1
4...SCF block, 141-144...SCF
section.

Claims (1)

[Claims] In a switched capacitor filter circuit of second order or higher order, which is composed of a switched capacitor filter block and an analog filter of first order or higher order, which is used in the first stage, the last stage, or both the first stage and the last stage, the analog filter determines the capacitance value. Capacitor and M as a resistor
An OS transistor, a capacitor that holds a bias voltage between the gate and source of the MOS transistor, a switch circuit that switches the MOS transistor, and a bias voltage setting circuit that applies a required bias between the gate and source of the MOS transistor. A switched capacitor filter circuit comprising: