JPH0511926A - Noise eliminating circuit - Google Patents

Abstract

PURPOSE:To provide the noise eliminating circuit at low cost without damaging responsiveness. CONSTITUTION:The above circuit is composed of an S/H circuit 1 to fetch an input signal ei synchronously with a pulse T1, S/H circuit 2 to fetch the input signal ei synchronously with a pulse T2 inverting the phase of the pulse T1, subtration circuit 3 to calculate the change rate of the input signal ei according to difference between the output values of the S/H circuits 1 and 2, comparator 4 to compare the change rate of the input signal ei with a fixed value eR, AND gate 5 not to pass the pulse T1 when the input signal ei is remarkably changed, and S/H circuit 6 to fetch the output of the S/H circuit 1 only when the AND gate 5 outputs the pulse T1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a noise removing circuit for removing impulse noise contained in an analog signal, and more specifically to a circuit for removing noise superimposed on an output signal of a sensor.

[0002]

2. Description of the Related Art When a sensor circuit that outputs an analog amount is affected by power line impulse noise or the like, impulsive noise is superimposed on the signal output from the circuit. When it is desired to remove the impulsive noise from the output signal, a low pass filter has been exclusively used in the past. FIG. 5 is an example of the simplest low-pass filter, and shows a circuit including a resistor 20 and a capacitor 21. The analog input signal e _i on which the impulsive noise is superimposed is applied to the resistor 20.
Since the capacitor 21 and the capacitor 21 form an integration circuit, the capacitor 21 performs charging or discharging operation according to the level of the analog input signal e _i . Since this operation requires a certain time, as a result, the impulsive noise superimposed on the analog signal is reduced.

[0003]

However, in order to effectively reduce the impulsive noise, the time constant of the circuit must be increased, which causes a problem that the response of the circuit is deteriorated. Although the simple low-pass filter shown in FIG. 5 has been described here, the same is true of the fact that even if the circuit configuration is more complicated, the response of the circuit deteriorates if noise is effectively reduced.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a low-cost noise elimination circuit which does not impair the response.

[0005]

To achieve the above object, the present invention provides a change rate detecting means for detecting a time change rate of an analog input signal, and an output of the change rate detecting means as a judgment reference value. Controlled by comparing means for comparison and the output of this comparing means, if the temporal change rate of the analog input signal is smaller than the judgment reference value, the analog input signal is taken in, and if the temporal change rate is larger than the judgment reference value. Is composed of a sample and hold circuit that holds the previous value.

[0006]

The rate-of-change detecting means detects the rate of temporal change of the analog input signal, so that, for example, when impulse noise is superimposed on the analog input signal, its output becomes large. The output of the change rate detecting means is applied to the comparing means and compared in magnitude with the judgment reference value. Then, the determination reference value is set to the maximum value of the rate of change of the analog input signal in the normal state. Therefore, the comparison means determines whether or not impulse noise or the like is superimposed on the analog input signal.

The sample-and-hold circuit is controlled by the output of the comparison means, and takes in the analog input signal when the rate of change of the analog input signal is below the judgment reference value, but when the change rate exceeds the judgment reference value, the previous value is obtained. The input signal remains held. Therefore, the portion where the analog input signal changes drastically is not output from the sample hold circuit, which means that the impulse noise is removed from the analog input signal.

[0008]

FIG. 1 is a circuit block diagram showing an embodiment of the present invention. This circuit includes a sample hold circuit (hereinafter referred to as an S / H circuit) 1 that captures an analog input signal e _i in synchronization with a _first pulse T ₁ and the analog input signal e in synchronization with a _second pulse T _{2. The} S / H circuit 2 that takes in _i , the subtraction circuit 3 that obtains the difference between the output values of the S / H circuits 1 and 2 described above, and the output value of the subtraction circuit 3 that is the judgment reference value + e _R ,
-E _R , the comparator 4, which compares the output of the comparator 4 and the first pulse T _1, and the output of the S / H circuit 1 in synchronization with the output signal of the AND gate 5. The S / H circuit 6 is incorporated. The S / H circuit 1, the S / H circuit 2 and the subtraction circuit 3 constitute a change rate detecting means.

FIG. 2 is a waveform diagram showing operation waveforms of main parts of the circuit of FIG. That is, (a) to (h) of FIG.
Are (a) first pulse T ₁ and (b) first pulse T ₁
A second pulse T ₂ whose phase is inverted, (c) an analog input signal e _i on which impulse noise is superimposed, (d) S /
Output signal e ₁ and S / H circuit 2 the output signal e ₂ of H circuit 1, (e) the output signal of the subtracting circuit 3, (f) the output signal of the comparator 4, (g) the output signal of the AND gate 5, And (h) shows the output signal e _O of the S / H circuit 6.

The operation of the circuit of FIG. 1 will be described below with reference to the waveform diagram of FIG. The analog input signal e _i has two S /
Although it is added to the H circuits 1 and 2, the S / H circuits 1 and 2
Is a circuit which takes in the analog input signal e _i when the input pulses T ₁ and T ₂ (hereinafter, collectively referred to as T) are at the H level, and holds the input value at that time when the input pulse T changes to the L level. .. Then, two types of phase-inverted pulses T ₁ and T ₂ are applied to each S / H circuit. Therefore,
When one of the S / H circuits takes in the analog input signal, the other S / H circuit holds the input signal value one cycle before the pulse T.
The output signals of the H circuit 1 and the S / H circuit 2 are e in FIG.
It changes like ₁ and e ₂ .

Since the two signals e ₁ and e ₂ in which the timing holding the signal is shifted back and forth by one cycle of the pulse T are subtracted by the subtraction circuit 3, the output of the subtraction circuit 3 is shown in FIG. It becomes the waveform of the solid line. As described above, the outputs e ₁ and e ₂ of the S / H circuits 1 and ₂ are obtained by inputting the input signal e _i at a timing that moves back and forth by one cycle of the pulse T. Therefore, the outputs e ₁ -e ₂ of the subtraction circuit 3 are
The value is almost proportional to the rate of change of the analog input signal e _i . The outputs e ₁ -e ₂ of the subtraction circuit 3 are applied to the comparator 4 and compared with the judgment reference voltages + e _R , -e _R. Here, in the comparator 4, the input voltage e ₁ -e ₂ is + e _R ~
This is a so-called window comparator that determines whether or not the value is within the range of −e _R. Therefore, the comparator 4 has the subtraction circuit 3
The output value is set to the L level only when the output e ₁ -e _{2 of} is out of the range of + e _R to -e _R (see e and f in FIG. 2).

By the way, the fact that the outputs e ₁ -e ₂ of the subtraction circuits are out of the range of the judgment reference voltage e _R means that the rate of change of the input signal e _i is large, that is, the input signal e _i.
It means that impulsive noise is superimposed on. In such a case, the output of the comparator 4 (L
Depending on the level, the AND gate 5 is turned off and the pulse T ₁ is not output from the AND gate. Therefore, at the timing when the analog input signal e _i changes drastically (for example, when impulsive noise is superimposed), S
The / H circuit 6 retains the previous value without taking in the input signal e ₁ , and consequently the output signal e _O of the S / H circuit 6 does not include the impulse noise. ..

FIG. 3 is a circuit block diagram showing another embodiment of the present invention. This circuit uses the analog input signal e _i
Receiving a low-pass filter 7, a subtracting circuit 8 for obtaining a difference between the output value e ₃ of the low-pass filter 7 and the analog input signal e _i , and an output of the subtracting circuit 8 for judging reference voltage + e _R ,
It is composed of a comparator 9 for comparing with −e _R and a hold circuit 10 for holding the output of the low-pass filter 7 in response to a change in the output of the comparator 9. Here, the low-pass filter 7 is a filter having the lowest responsiveness allowed in the circuit of FIG. In other words, it is a filter that does not impair the responsiveness required for the circuit, although its noise removal effect is not sufficient. It should be noted that in the above configuration, the low-pal filter 7 and the subtraction circuit 8 correspond to the change rate determination means.

FIG. 4 shows the waveform of the main part of the circuit of FIG. That is, (a) to (d) of FIG.
(A) Analog input signal e _i (solid line) and output signal e ₃ of low-pass filter 7 (broken line), (b) output signal of subtraction circuit 8, (c) output signal of comparator 9, and (d) hold circuit The output signal e _{0 of} 10 is shown. The operation of the circuit of FIG. 3 will be described below with reference to the waveform of FIG. The analog input signal e _i is delayed in phase and smoothed by being applied to the low-pass filter 7, and e _{3 in} FIG.
It becomes a waveform like. Output e of this low pass filter 7
_{Since 3} is added to the subtraction circuit 8 and subtracted together with the input signal e _i , the output of the subtraction circuit 8 has a waveform proportional to e _i −e ₃ (see b in FIG. 4). The output e ₃ of the low-pass filter 7 has a phase slightly delayed with respect to the input signal e _i . Therefore, the output of the subtraction circuit 8 has a value corresponding to the rate of change of the input signal e _i . That is, when the input signal e _i changes extremely as a result of superposition of impulsive noise and the like, the output of the subtraction circuit 8 is large, and conversely, the input signal e _i is large.
_{When i} changes gradually, the output of the subtraction circuit 8 is small. The output of the subtraction circuit 8 is applied to the comparator 9. The comparator 9 is a so-called window comparator, and the output value of the subtraction circuit 8 and the judgment reference voltages + e _R , -e _R.
Make a comparison. Then, the comparator 9 outputs the L level voltage when the output of the subtraction circuit is large, that is, when the change of the input signal e _i is large. In addition, the hold circuit 10
This circuit receives the output e ₃ of the low-pass filter 7 and holds the input signal e ₃ when the output of the comparator 9 is at L level. Therefore, when the change of the input signal e _i is sharp, the hold circuit 10 holds the previous input value e ₃ , and eventually, the impulsive noise is removed from the output e _{0 of} the circuit. (See a and d in FIG. 4).

[0015]

As described above, in the noise removing circuit according to the present invention, the period in which impulse noise is superimposed is detected and the input signal in that period is removed.
The noise removal rate is good, but the responsiveness of the circuit is not impaired. Further, noise is removed without using a D / A converter, an A / D converter, a CPU, etc. Therefore, a simple and low-cost circuit is realized.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is an operation waveform diagram of a main part of the circuit of FIG.

FIG. 3 is a block diagram showing another embodiment of the present invention.

4 is an operation waveform diagram of a main part of the circuit of FIG.

FIG. 5 is a circuit diagram of a typical low pass filter.

[Explanation of symbols]

1, 2, 6 S / H circuit 3 Subtraction circuit 4 Comparator 5 AND gate e _i Analog input signal T ₁ , T ₂ Sample and hold pulse e _R Judgment reference voltage

Claims (1)

Claim: What is claimed is: 1. A change rate detecting means for detecting a time change rate of an analog input signal, a comparing means for comparing an output of the change rate detecting means with a judgment reference value, and a comparing means of the comparing means. A sample hold that is controlled by the output and takes in the analog input signal when the temporal change rate of the analog input signal is smaller than the judgment reference value and holds the previous value when the temporal change rate is larger than the judgment reference value. And a circuit for eliminating noise.