CN107104656A - Filter time constant changes circuit and D/A change-over circuits - Google Patents

Abstract

The present invention discloses a filter time constant change circuit and a D/A conversion circuit, in which the time constant of the filter is changed according to the change of the input signal, and the ripple of the output of the filter is reduced. The filter time constant change circuit (4) includes: a variation calculation unit (40, 41) which calculates the value f(t) of the input signal from the current time point and the value of the input signal T time before from the current time point The amount of change h(t, T)=f(t)-f(t-T) of each specified time T obtained by f(t-T); , T) and a predetermined threshold to output a control signal for controlling the time constant of the filter (2).

Description

Filter time constant change circuit and D/A conversion circuit

technical field

The present invention relates to a filter time constant changing circuit which changes the time constant of a filter according to a change of an input signal, and a D/A conversion circuit using the filter time constant changing circuit.

Background technique

In the output of the D/A conversion circuit, generally, the pulse waveform is smoothed by a low-pass filter and output. Depending on the size of the time constant of the filter at this time, the response speed and the ripple cannot be balanced at the same time. In fact, when determining the time constant of the filter, for example, the response speed is determined as fast as possible within the allowable ripple value. On the other hand, when a large change in the output of the D/A conversion circuit is performed step by step, there are cases where a follow-up delay due to the limitation of the response speed becomes a problem.

As a solution to such a problem, a technique has been proposed in which, when there is a large and sharp change in the output of the D/A conversion circuit, by reducing the time constant of the filter, the limitation of the ripple is realized and the response is quickened (see patent Literature 1). FIG. 14 is a block diagram showing the configuration of a D/A conversion circuit disclosed in Patent Document 1. As shown in FIG. The D/A conversion circuit of FIG. 14 uses an ASIC (Application Specific Integrated Circuit, application specific integrated circuit) 300 to improve responsiveness. When the digital data for generating the PWM signal changes, it is switched to an analog filter with a small time constant formed by the resistor 302 and the capacitor 303 to improve responsiveness. When there is no change in the digital data, it is switched to an analog filter with a relatively large time constant composed of the resistor 301 and the capacitor 303 to suppress the output ripple.

When changing the time constant of the filter according to the magnitude of the change in the input signal from the D/A conversion circuit to the filter, for example, a configuration such as that shown in FIG. 15 is conceivable as common sense. In the example of FIG. 15 , the change amount calculation unit 100 calculates the change amount Δf(t)/Δt of the input signal f(t), and the time constant change unit 101 changes the time constant of the filter 102 based on the change amount Δf(t)/Δt. time constant. In the example of FIG. 16, when the input signal f(t) changes, the time constant of the filter 102 is changed. On the other hand, in the example of FIG. 17 , when the amount of change Δf(t)/Δt exceeds the threshold value TH, the time constant of the filter 102 is changed.

Generally, the processing performed in the change amount calculation unit 100 is equivalent to differentiation, and the generation of a control signal for changing the time constant of the filter 102 (filter time constant change flags in FIGS. 16 and 17 ) is limited to the input signal f( t) has a period of inclination. Therefore, when the time width in which the input signal f(t) has an inclination is very short, the time width in which the control signal for changing the time constant is generated also becomes short. The appropriate time width for changing the time constant of the filter 102 depends on the time constant of the filter 102 and the purpose of the filter 102. Therefore, if the time width for generating a control signal for changing the time constant is shortened, there may be The function for changing the time constant of the filter 102 may not be fully exhibited.

For example, consider a case where the time constant of the filter 102 is temporarily reduced in response to a change in the input signal f(t) in order to respond to a sudden change in the input signal f(t) greater than a certain level. In this case, the time width of the changed time constant needs to be such that the time constant of the filter 102 after the change can sufficiently follow the input signal f(t). However, even if the time constant of the filter 102 is reduced only during the change period of the input signal f(t) (only during the period when the differential value exceeds the threshold value TH), the output of the filter 102 becomes as shown in FIG. The function of device 102. In the example of Fig. 18, g(t) represents the actual output of the filter 102, and g'(t) represents the ideal output. It can be seen from FIG. 18 that the output g(t) of the filter 102 cannot follow the change of the input signal f(t).

With regard to the problem of such a configuration in FIG. 15 , it is easy to think that using a timer and using the output of the variation calculation unit 100 (the differential value of the input signal f(t)) as a trigger, when the output of the variation calculation unit 100 exceeds the threshold value In this case, the time constant of the filter 102 is changed and maintained for a certain period of time, so that the function of the filter 102 can be fully exerted. In the example of FIG. 19 , by providing a timer 103 between the variation calculating unit 100 and the time constant changing unit 101, the timer 103 creates a signal with a time width necessary to follow the input signal f(t), and As shown in FIG. 20, this provides a time width for changing the time constant necessary for tracking the input signal f(t).

However, in the configuration shown in FIG. 19 , when the output of the change amount calculation unit 100 (the differential value of the input signal f(t)) is used as the trigger for changing the time constant of the filter 102, there is an unintentional increase. The risk of misoperation in the scene. For example, as shown in FIG. 21, the time constant of the filter 102 is changed for a certain period of time corresponding to the change of the instantaneous input signal f(t), and there is an output g(t) of the filter 102 in an unintended area. The ripple problem.

prior art literature

patent documents

Patent Document 1: Japanese Patent Laid-Open No. 2003-101413

Contents of the invention

The problem to be solved by the invention

As described above, in the configuration of changing the time constant of the filter according to the change of the input signal, there is a problem that if the output of the filter is made to sufficiently follow the change of the input signal, a Larger ripple.

Note that the above problems are not limited to filters that receive the output of the D/A conversion circuit, and similarly occur in filters that filter an analog signal and input it to the A/D conversion circuit, for example.

The present invention was made to solve the above-mentioned problems, and an object of the present invention is to provide a filter time constant change that can reduce ripple in the output of the filter in a configuration that changes the time constant of the filter according to changes in the input signal. circuit.

technical means to solve problems

The present invention is a filter time constant change circuit for determining a time constant of a filter for filtering an input signal. The filter time constant change circuit is characterized in that a change amount calculation unit calculates the time constant obtained from the current time point. The value f(t) of the input signal and the value f(t-T) of the input signal before T time from the current time point, the change amount h(t, T) for each specified time T = f(t)- f(t-T); and a comparison unit that outputs a control signal for controlling a time constant of the filter based on a comparison result between the change amount h(t, T) and a predetermined threshold.

In addition, in a configuration example of the filter time constant changing circuit of the present invention, when the change amount h(t, T) is equal to or less than the threshold value, the comparison unit outputs the The time constant of the filter is a control signal of the first filter time constant τ1; when the amount of change h(t, T) exceeds the threshold, the comparison unit outputs a signal to change the time constant of the filter is a control signal of a second filter time constant τ2 smaller than the first filter time constant τ1.

In addition, in an example configuration of the filter time constant changing circuit of the present invention, the predetermined time T is from the time when the change of the input signal starts until the second filter time constant In τ2, the time until the value of the output signal of the filter reaches a predetermined ratio with respect to the final value of the change.

In addition, the D/A conversion circuit of the present invention is characterized by including: a D/A conversion unit that converts a digital input signal into an analog signal; a filter that smoothes an output signal of the D/A conversion unit; And the filter time constant changing circuit according to any one of claims 1 to 3, which receives the digital input signal as an input and outputs a control signal for controlling the time constant of the filter.

The effect of the invention

According to the present invention, by providing the variation calculating unit and the comparing unit, the output of the filter can be made to follow the variation of the input signal, and the ripple of the output of the filter can be reduced. In addition, in the present invention, since the ripple of the output of the filter can be reduced without using a conventional timer, circuit mounting can be easily performed.

Description of drawings

FIG. 1 is a waveform diagram illustrating the principle of variation calculation according to the present invention.

FIG. 2 is a waveform diagram illustrating the principle of variation calculation according to the present invention.

Fig. 3 is a waveform diagram illustrating the operation of changing the time constant of the filter in the present invention.

FIG. 4 is a waveform diagram showing another example of variation calculation according to the present invention.

FIG. 5 is a waveform diagram showing another example of variation calculation according to the present invention.

Fig. 6 is a waveform diagram illustrating the operation of changing the time constant of the filter in the present invention.

FIG. 7 is a waveform diagram showing another example of variation calculation according to the present invention.

FIG. 8 is a waveform diagram showing another example of variation calculation according to the present invention.

Fig. 9 is a waveform diagram illustrating the operation of changing the time constant of the filter in the present invention.

FIG. 10 is a block diagram showing the configuration of a D/A conversion circuit according to the embodiment of the present invention.

FIG. 11 is a diagram illustrating a method of setting a predetermined time in the embodiment of the present invention.

FIG. 12 is a waveform diagram showing an example of the response of the D/A conversion circuit according to the embodiment of the present invention.

13 is a waveform diagram showing actual measurement results of the response of the D/A conversion circuit according to the embodiment of the present invention.

FIG. 14 is a block diagram showing the configuration of a conventional D/A conversion circuit.

FIG. 15 is a block diagram showing a configuration for changing the time constant of a filter.

FIG. 16 is a waveform diagram illustrating the operation of changing the time constant of the filter in the configuration of FIG. 15 .

Fig. 17 is a waveform diagram illustrating another operation of changing the time constant of the filter in the configuration of Fig. 15 .

FIG. 18 is a waveform diagram illustrating a problem with the configuration of FIG. 15 .

FIG. 19 is a block diagram showing another configuration for changing the time constant of the filter.

FIG. 20 is a waveform diagram illustrating the operation of changing the time constant of the filter in the configuration of FIG. 19 .

FIG. 21 is a waveform diagram illustrating a problem with the configuration of FIG. 19 .

detailed description

[Principle of invention]

1(A) to 1(C) and FIG. 2(A) to 2(C) are waveform diagrams illustrating the principle of variation calculation according to the present invention. In the present invention, the conventional configuration for calculating the amount of change (differential value) of the input signal is replaced with a configuration for calculating the amount of change for each time width T corresponding to the change of the time constant that is the purpose of the filter, thereby , so that the effect of changing the time constant of the filter is properly reflected. Specifically, calculate the value f(t) of the input signal at the current time point at which the amount of change is desired to be calculated and the value f(t-T) of the input signal before T time from the current time point for each specified time The variation h(t, T)=f(t)-f(t-T), and the time constant of the filter is determined according to the calculation result. In the examples of Fig. 1 (A) to Fig. 1 (C) and Fig. 2 (A) to Fig. 2 (C), it is shown that the change amount h(t , T) situation.

If the time constant of the filter is reduced (the filter time constant change flag is set to "1") when the amount of change h(t, T) calculated as above exceeds the predetermined threshold value TH, the filter The output g(t) of is as shown in FIG. 3 . g'(t) in Fig. 3 represents the ideal output.

Comparing with the situation in Fig. 20, it can be seen that the output g(t) of the filter of the present invention follows the input signal f(t) earlier.

In addition, in the present invention, since the amount of change h(t, T) is in a format that includes the total amount of change in the input signal f(t) as information, ripples as described in FIG. 21 do not occur. question. 4(A) to 4(D) and FIG. 5 show the results of calculation of the amount of change of the present invention for the instantaneous input signal f(t) change as shown in FIG. 21 .

If the time constant of the filter is reduced when the calculation result in Fig. 4(A) to Fig. 4(D) and Fig. 5 exceeds the threshold value TH, the output g(t) of the filter becomes as shown. In the present invention, the threshold TH is defined based on the maximum value of the difference (f(t)-f(t-T) between two points of the input signal, so that malfunctions caused by noise below the threshold TH do not occur. .

In addition, in the present invention, even when very sharp and large-value noise is mixed into the input signal f(t), since the time width of the peak of the noise is within ×2, the filter Since the operation of changing the time constant of , it can be expected that the influence on the filter output will be smaller than the case of using the timer in FIG. 19 . 7(A) to 7(C), FIG. 8(A), and FIG. 8(B) show the results of calculation of the amount of change in the present invention for such a noise input.

If the calculation results of (A) to (C) in Fig. 7, (A) in Fig. 8, and (B) in Fig. 8 exceed the threshold value TH, the time constant of the filter is reduced, The output g(t) of the filter becomes as shown in FIG. 9 .

As described above, in the present invention, it is possible to solve the conventional problem that ripples are generated in the output g(t) of the filter in an unintended region.

[implementation mode]

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 10 is a block diagram showing the configuration of a D/A conversion circuit according to the embodiment of the present invention. The D/A conversion circuit of the present embodiment is made up of following parts: D/A conversion part 1, it converts the digital input signal f(t) of 16bit into analog signal; The output signal of 1 is smoothed; the buffer circuit 3 is connected to the output terminal of the low-pass filter 2; and the filter time constant change circuit 4 is used to determine the low-pass filter based on the change amount of the input signal f(t). The time constant of device 2.

Various configurations such as a ΔΣ modulator, a PWM (Pulse Width Modulation, pulse width modulation) modulator, and a T-shaped resistance type D/A converter can be applied as the D/A converter 1, and the present invention is not limited thereto. The way of D/A conversion.

The low-pass filter 2 is composed of the following parts: resistors R1, R2, which are arranged in series between the output terminal of the D/A conversion part 1 and the input terminal of the buffer circuit 3; The control signal of the circuit 4 is used to short-circuit the resistor R1; and the capacitor C1 is arranged between the input terminal of the buffer circuit 3 and the ground.

The filter time constant changing circuit 4 is composed of a delay unit 40 , a subtraction unit 41 , and a comparison unit 42 (comparator). The delay unit 40 and the subtraction unit 41 constitute a variation calculation unit.

The delay unit 40 delays the digital input signal f(t) for a predetermined time T. As the delay unit 40 , a multi-stage cascaded flip-flop can be used.

The subtraction unit 41 subtracts the digital output signal of the delay unit 40 from the digital input signal f(t). In this way, the change amount of the input signal per time T can be calculated, that is, h(t, T)=f(t)-f(t-T) can be calculated.

The comparison unit 42 compares the digital output signal of the subtraction unit 41 with a predetermined threshold value TH. When the value of the digital output signal of the subtraction part 41 exceeds the threshold value TH, the comparison part 42 sets the control signal (filter time constant change flag) for controlling the switch SW1 of the low-pass filter 2 to "1" (High, High), and when the value of the digital output signal of the subtraction unit 41 is equal to or less than the threshold TH, the comparison unit 42 sets the control signal to “0” (Low). The threshold TH may be set in advance in consideration of the maximum possible value of the difference (f(t)-f(t-T) between two points of the input signal and the magnitude of the noise to be removed.

The low-pass filter 2 is an active filter whose time constant can be changed. When the control signal output from the comparator 42 is "1", the switch SW1 is turned on. Accordingly, since the resistor R1 is short-circuited, the time constant of the low-pass filter 2 becomes small. When the control signal output from the comparator 42 is "0", the switch SW1 is turned off. Accordingly, since the resistors R1 and R2 are connected in series, the time constant of the low-pass filter 2 becomes large.

As shown in FIG. 11 , the predetermined time T serving as the delay time of the delay unit 40 may be set in advance as T such that the time from the change start time of the input signal f(t) to the time when the D/A The time until the value of the output signal g(t) of the conversion circuit reaches a predetermined ratio A (for example, 63.2% or 90%) to the final value (100%) of the change. The ratio A may be set according to the desired performance of the D/A conversion circuit.

In this embodiment, the speed at which the output signal g(t) of the D/A conversion circuit follows the input signal f(t) and the ripple level of the output signal g(t) are determined by the time constant of the low-pass filter 2, The speed and the level of ripple cannot be taken into account at the same time. In the present embodiment, the low-pass filter 2 is based on, for example, a time constant of τ1 = 10 msec, but on the other hand, a high-speed response of 1 msec may be required. However, if the time constant of the low-pass filter 2 is τ2 = 1 msec, the ripple of the output signal g(t) becomes large. Therefore, T=1 msec, A=63.2%, and a high-speed response mode capable of following a sudden change of the input signal f(t) are prepared.

According to the present embodiment, as shown in FIG. 12 , when the input signal f(t) changes, the operation is as follows: at time t1 when the change amount h(t, T) exceeds the threshold value TH, the low-pass filter The time constant of 2 is changed from, for example, τ1=10msec to τ2=1msec (switch SW1 is turned on); at the time point t2 when the output signal g(t) reaches the ratio A=63.2% of the input signal f(t), low-pass filtering The time constant of the device 2 is changed from τ2 = 1 msec to τ1 = 10 msec (the switch SW1 is turned off).

FIG. 13 is a waveform diagram showing actual measurement results of the response of the D/A conversion circuit of the present embodiment. 130 in FIG. 13 represents the output signal g(t) of the D/A conversion circuit when the time constant of the low-pass filter 2 is fixed at 1 msec, and 131 represents the output signal g(t) when the time constant of the low-pass filter 2 is fixed at 10 msec. The output signal g(t) of the D/A conversion circuit, 132 represents the output signal g(t) of the D/A conversion circuit of this embodiment.

It can be seen from FIG. 13 that if the time constant of the low-pass filter 2 is reduced, large ripples appear in the output signal g(t). Therefore, it is intended to reduce the time constant of the low-pass filter 2 only during following the change of the input signal f(t), but it is not enough to reduce the time constant only at the moment of the change of the input signal f(t). Therefore, in this embodiment, by calculating the variation h(t, T) of the input signal f(t), the time of the low-pass filter 2 is reduced only when the variation h(t, T) exceeds the threshold value TH Constant, realizes the time of sufficient time constant change.

In the present embodiment, since output ripple can be reduced without using the timer shown in FIG. 19 , mounting to a D/A conversion circuit of an integrated circuit can be easily performed.

In addition, in this embodiment, as an example of the low-pass filter 2, a first-order filter is used as an example for description, but it is not limited thereto. The present invention can also be applied to other filters such as filters.

In addition, in this embodiment, an example of application to a D/A conversion circuit was described, but the filter time constant changing circuit 4 of the present invention may be applied to an input unit of an A/D conversion circuit. In this case, it is sufficient to input the analog input signal f(t) to the low-pass filter 2 and input the output signal of the low-pass filter 2 to the A/D conversion circuit. The filter time constant changing circuit 4 is composed of an analog circuit. An example of the delay unit 40 is, for example, a CCD (Charge-Coupled Device, charge-coupled device).

【Industrial Utilization Possibility】

The present invention can be applied to a technique of changing the time constant of a filter based on an input signal.

Symbol Description

1...D/A conversion section, 2...Low-pass filter, 3...Buffer circuit, 4...Filter time constant change circuit, 40...Delay section, 41...Subtraction section, 42...Comparison section, R1, R2...Resistor, C1...capacitor, SW1...switch.

Claims (4)

1. a kind of filter time constant changes circuit, the time that its determination is used for the wave filter being filtered to input signal is normal Number, the filter time constant changes circuit and is characterised by, including：

Variable quantity calculating part, its calculate by current point in time the input signal value f (t) and from current time light T when Between before input signal value f (t-T) and obtain each stipulated time T variable quantity h (t, T)=f (t)-f (t-T)；And

Comparing section, its wave filter according to the comparative result of the variable quantity h (t, T) and defined threshold value carrys out output control The control signal of time constant.

2. filter time constant as claimed in claim 1 changes circuit, it is characterised in that

In the case where the variable quantity h (t, T) is below the threshold value, the comparing section output makes the time of the wave filter Constant is the 1st filter time constant τ 1 control signal；

In the case where the variable quantity h (t, T) exceedes the threshold value, the comparing section output makes the time of the wave filter normal Number is changed to the control signal of the 2nd filter time constant τ 2 less than the 1st filter time constant τ 1.

3. filter time constant as claimed in claim 2 changes circuit, it is characterised in that

The stipulated time T is lights from the change time started of the input signal, until normal in the 2nd filter temporal In number τ 2, the value of the output signal of the wave filter relative to the end value of the change reach as defined in time untill ratio.

4. a kind of D/A change-over circuits, it is characterised in that including：

D/A converter sections, digital input signals are converted to analog signal by it；

Wave filter, its output signal to the D/A converter sections is smoothed；And

Filter time constant change circuit any one of claims 1 to 3, it is using the digital input signals to be defeated Enter, and the control signal of filter time constant described in output control.