KR102073737B1 - High-speed capacitance detection circuit - Google Patents

Abstract

The present invention relates to a high-speed capacitance detection circuit which adopts a DC-AC conversion method to increase the response rate. The detection circuit of the present invention comprises: a capacitance-voltage converter converting the capacitance of a sensor to a voltage; a frequency converter converting, into an AC signal of a predetermined frequency, a sensed signal converted into the voltage by the capacitance-voltage converter; an AC amplifier amplifying the frequency-converted sensed signal; a band-pass filter suppressing low-frequency noise at the output of the AC amplifier and passing only the sensed signal of the predetermined frequency; a synchronous detector detecting the signal having passed through the band-pass filter; a low-pass filter (LPF) suppressing high-frequency noise at the output of the synchronous detector; and a comparator detecting whether or not the capacitance is detected by comparing the output of the low-pass filter with a threshold voltage. According to an embodiment of the present invention, a signal in a high-frequency band and the noise in a low-frequency band are separated from each other with a large frequency difference so that the nose can be easily removed. The LPF can suppress carrier components so the cutoff frequency of the LPF can be shifted to a 10 to 100 times higher frequency than that of a conventional LPF. Therefore, the frequency response rate can be greatly increased.

Description

High-speed capacitance detection circuit

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a capacitive sensor circuit using charge and discharge of a capacitor, and more particularly, to a high speed capacitive detection circuit employing a DC-AC conversion method in order to increase the response speed.

In general, a proximity sensor is a sensor that can detect the approach of a detection object in a non-contact manner, a high frequency oscillation type using electromagnetic induction according to a sensing method, and an electrostatic capacitance that senses a change in capacitance between the detection object and the sensor. It is divided into capacitive type and magnetic type using magnet. The capacitive proximity sensor consists of a sensor and a detection circuit. The sensor has a sensor electrode and detects a change in capacitance between the sensor electrode and the ground when an object approaches the sensor.

In the conventional detection circuit for detecting a change in capacitance in the capacitive proximity sensor, as illustrated in FIGS. 1 to 3, the CV converter 11 converting the capacitance of the detection sensor 20 into a voltage. And peak detectors 12-1 and 12-2, DC amplifier 13, LPF 14, and comparator 15.

1 to 3, when the capacitance is not detected, there is almost no difference in voltage between Vref of V1 and Vsense of V2 as shown in FIG. 2A. As shown in b), the difference between V1 and V2 increases. This difference value is DC amplified and then the LPF 14 having a cutoff frequency fc of approximately 40 Hz, as shown in FIG. 3, with the amplified CV converted signal S1 and power noise N1 ranging from approximately 50 Hz to 120 Hz. Is detected as a signal containing

As described above, the conventional detecting circuit converts the change in the capacitance of the sensing sensor 20 into a voltage in the CV (capacitive-voltage) converting circuit and then outputs a determination signal from the DC amplifier 13 (DC Amp). ) Should be greatly amplified to the level of operation.

However, when a large DC amplification to obtain a desired output change in this way, several problems occur as follows. First, it is always easy to drift for large direct current amplification. Second, since a small signal is directly amplified by a large signal, it is susceptible to noise effects such as power supply from a high impedance terminal such as CV conversion, and in order to improve the low pass filter (LPF) having a low cutoff frequency (fc) in series. When inserted, there is a problem that the frequency response speed is slow.

KR 10-0653403 B1 KR 10-1723914 B1

The present invention has been proposed to solve the above problems, and an object of the present invention is to convert a voltage obtained from capacitance-to-voltage (CV) conversion into an AC signal of a predetermined frequency (the frequency-converted signal is thus In the embodiment of the present invention, a high-capacitance detection circuit is amplified to a level capable of operating a comparator by passing an amplification and a detector).

This embodiment discloses a high speed capacitance detection circuit employing a DC-AC conversion scheme in order to increase the response speed. The disclosed high speed capacitance detection circuit includes a capacitance-voltage converter for converting a capacitance of a sensor into a voltage, and a frequency converter for converting a sensing signal converted into voltage by the capacitance-voltage converter into an AC signal of a predetermined frequency. And an AC amplifier for amplifying the frequency-converted detection signal, a band pass filter for suppressing low frequency noise at the output of the AC amplifier and passing only a detection signal of a predetermined frequency, and detecting a signal passing through the band pass filter. A synchronous detector, a low pass filter for suppressing high frequency noise at the output of the synchronous detector, and a comparator for detecting whether the capacitance is sensed by comparing the output of the low pass filter with a threshold voltage.

The band pass filter has a frequency characteristic of removing power supply noise, and the frequency converter inserts a variable resistor VR1 for zero adjustment on the reference voltage Vref side.

When a high gain DC amplifier is configured from CV conversion to a comparator input as in the related art, parasitic oscillation occurs easily even if only small noise is fed back. However, an embodiment of the present invention may be composed of a carrier amplifier (AC amplifier) and a synchronous detector. Therefore, the amplification is separated into AC and DC respectively, so that parasitic oscillation does not occur, thus achieving a stable system.

In addition, in the embodiment of the present invention, since a DC signal is altered by amplifying a carrier signal of a predetermined frequency, a desired voltage can be obtained from a synchronous detector having an AC-converted carrier amplification and amplification function without amplification at once. The amplification degree of amplification can be lowered, and as a result, temperature drift can be suppressed small.

In addition, according to an embodiment of the present invention, the capacitance-voltage (CV) converted signal is converted into an AC signal of a specific frequency (3.9 kHz in the embodiment of the present invention), but the noise of the power supply is the reference voltage signal V1 and the sensing voltage. It is in phase with the signal Vsense, and is not detected as a signal change by the in-phase cancellation, nor is it frequency-converted. Therefore, the detection signal of the high frequency band and the noise of the low frequency band are separated to easily remove the noise, thereby greatly improving the frequency response speed.

1 is a block diagram showing a conventional capacitance detection circuit;
2 is a waveform diagram showing a change in voltage of a conventional capacitance detection circuit;
3 is a view showing the characteristics of the low pass filter (LPF) of the conventional capacitance detection circuit,
4 is a block diagram of a fast capacitance detecting circuit according to an embodiment of the present invention;
5 is a voltage change waveform diagram of a fast capacitance detection circuit according to an embodiment of the present invention;
6 is a diagram illustrating the spectral characteristics of the fast capacitance detecting circuit according to the embodiment of the present invention.

The technical problem achieved by the present invention and the practice of the present invention will be more clearly understood by preferred embodiments of the present invention described below. The following examples are merely illustrated to illustrate the present invention and are not intended to limit the scope of the present invention.

4 is a block diagram of a high speed capacitance detection circuit according to an embodiment of the present invention.

As shown in FIG. 4, the high-speed capacitive detection circuit 100 according to the embodiment of the present invention has a capacitance (C) -voltage (V) converter for converting the capacitance of the sensor 20 into a voltage. 110, first and second peak detectors 120-1 and 120-2, frequency converter 130, AC amplifier 140, bandpass filter 150, synchronous detector Zero V cramp det (160), low pass filter (LPF) 170, comparator (comp; 180).

Referring to FIG. 4, the capacitance-to-voltage (CV) converter 110 converts the capacitance C sensed by the sensor 20 into a voltage V, and includes a first peak detector 120-1; Peak det.) Outputs the reference voltage signal V1, and the second peak detector 120-2 (Pear det.) Outputs the sensing voltage signal V2.

The frequency converter 130 may include a first buffer 132-1 buffering and amplifying the reference voltage signal V1, a variable resistor VR1 for adjusting the output of the first buffer 132-1, and a first buffer. A first switch 134-1 for switching the output of the buffer 132-1 at a half cycle of 3.9 KHz, a second buffer 132-2 for buffering and amplifying the sensing voltage signal V2, and a second buffer ( The resistor R2 connected to the output terminal of the 132-2 and the second switch 134-2 for switching the output of the second buffer 132-2 in a half cycle of 3.9 KHz are composed of the DC voltage signals V1, V2) is converted into an AC voltage signal of 3.9 KHz (called 'carrier V3'). Here, the variable resistor VR1 is for adjusting the output voltage of the reference voltage signal V1 such that the input voltage V5 of the comparator 180 is 0V or more (specifically, 0.5V to 0.8V).

The AC amplifier 140 amplifies the carrier V3 converted into AC at a predetermined frequency, and the bandpass filter 150 suppresses power supply noise from the AC amplified carrier V3 to remove the carrier V4 from which the noise is removed. Output Typically, the band pass filter 150 is composed of a combination of HPF and LPF, the HPF side of the band pass filter 150 of the present embodiment passes a high frequency signal band of 3.9KHz and low frequency (approximately 50 ~ 120Hz) such as power supply noise ) Noise can be suppressed.

The synchronous detector 160 is a zero clamp type synchronous detector, and fixes the voltage on the Vsense side of the carrier V4 to zero (V), detects the carrier V4, directs it, and then performs a power failure through the LPF 170. The direct current component of the voltage signal whose capacitance is detected is extracted. In this way, the synchronous detector 160 detects exactly twice because it operates to clamp the Vsense side to the GND potential. In addition, since the LPF 170 of the present embodiment is for removing noise of a high frequency component rather than power noise, it is not necessary to set the cutoff frequency to 50 Hz or 60 Hz or less to remove power noise as in the prior art, thereby inevitably increasing the frequency response speed. Can be.

The comparator 180 compares the low pass input voltage V5 with a predetermined threshold voltage E to detect the presence of capacitance. In the present embodiment, since the Vsense side becomes zero (V) by the clamp, when the voltage obtained from the capacitance detection of the CV conversion is large, the crest of the carrier V4 to be input to the synchronous detector 160 becomes large, and thus the comparator 180 Input voltage V5 also increases. On the contrary, if there is no capacitive detection, since the wave height of the carrier V4 to be input to the synchronous detector 160 becomes small, the input voltage V5 of the comparator 180 also decreases. In the present embodiment, when there is no external capacitance sensing, the variable resistor VR1 is adjusted such that the input voltage V5 of the comparator 180 is about 0.8V to about 1.0V. In this embodiment, the threshold voltage (E) of the comparator 180 is set to 1.2V.

5 is a voltage change waveform diagram of a fast capacitance detection circuit according to an embodiment of the present invention.

Referring to FIG. 5, (a) is a waveform diagram of the reference voltage signal V1 and the sensing voltage signal V2 when the capacitance is not detected by the sensing sensor 20, where V1 is Vref and V2 is Vsense. It can be seen that the difference is small. (b) is a waveform diagram of the reference voltage signal V1 and the sensing voltage signal V2 when capacitance is detected in the sensing sensor 20, where V1 is Vref and V2 is Vsense, and the difference is large. have. (c) the frequency converter 130 alternately switches the reference voltage signal V1 and the detected voltage signal V2 to a clock of 3.9 kHz to band-pass filter the altered carrier V3 to remove noise. The waveform of the carrier V4 and the input voltage V5 of the comparator 180 low-filtered after synchronous detection are shown.

When the capacitance is not detected in FIG. 5, the variable resistor VR1 is adjusted such that the V sense voltage is slightly lower than the Vref voltage as shown in (a) so that the carrier V3 voltage is higher than the threshold voltage (E). When the voltage is lowered (V3 <E) and the capacitance is detected, as shown in (b), the V sense voltage drops, so that the difference with Vref is large and the carrier V3 waveform that is altered in the frequency converter 140 becomes the carrier V3 voltage. It becomes larger than this threshold voltage (E) (V3> E).

6 is a diagram illustrating the spectral characteristics of the fast capacitance detection circuit according to an embodiment of the present invention, wherein the upper side is the spectrum of the sensing voltage signal V2, the lower side is the spectrum of the carrier V4, and each spectrum is the horizontal axis. This frequency is represented and the vertical axis represents voltage.

Referring to FIG. 6, when an increase in capacitance is detected in C-V conversion, a difference between V1 and V2 is increased, and the voltage of the difference is converted into a carrier V3 by the frequency converter 130. Since the carrier V3 converted to alternating current has a small signal level, the carrier V3 amplifies the AC amplifier 140 to a level necessary for the synchronous detector 160. At this time, as shown in FIG. 6, the power source noise N1 and the desired carrier signal S1 have a separable frequency difference, and thus the power source noise N1 can be sufficiently suppressed by the inserted BPF 150. Only signal S1 can be extracted. That is, the reference voltage signal V1 and the sense voltage signal V2 are alternately output by the frequency conversion clock to become the carrier V3, but the noise N1 included in the carrier V3 is applied to the next stage BPF 150. ), Only the desired carrier V4 can be output.

On the other hand, in the embodiment of the present invention, the carrier is described as 3.9 kHz. However, if the frequency response speed is to be further increased, this frequency may be increased.

The circuit of the present invention can be applied to various fields such as a touch sensor, a fingerprint sensor, a keyboard (pad), a jog dial for a game machine, a remote controller, as well as an electrostatic proximity sensor, and is suitable for miniaturization, in particular, for ICization. .

Although the present invention has been described with reference to one embodiment shown in the drawings, it will be understood by those skilled in the art that various modifications and equivalent other embodiments are possible therefrom.

20: detection sensor 100: detection circuit
110: capacitive-voltage converter 120-1, 120-2: peak detector
130: frequency converter 132-1,132-1: buffer
134-1,134-2: switch 140: AC amplifier
150: bandpass filter 160: synchronous detector
170: low pass filter 180: comparator

Claims (3)

A capacitance-to-voltage converter for outputting a reference voltage signal V1 and a sensing voltage signal V2 obtained by converting the capacitance of the sensing sensor into voltage;
A frequency converter converting the sensing signal converted into voltage by the capacitance-voltage converter into an AC signal having a predetermined frequency;
An AC amplifier for amplifying the frequency converted sensed signal;
A band pass filter that suppresses low frequency noise at the output of the AC amplifier and passes only a detection signal having a predetermined frequency;
A synchronous detector for detecting a signal that has passed through the bandpass filter;
A low pass filter for suppressing high frequency noise at the output of the synchronous detector; And
Comparing the output of the low pass filter with a threshold voltage (Threshold Voltage) and detects whether the capacitance is detected,
The frequency converter
A first buffer for buffering and amplifying the reference voltage signal V1;
A variable resistor VR1 for zeroing the output of the first buffer;
A first switch for switching the output of the first buffer at a half period of a predetermined frequency;
A second buffer for buffering and amplifying the sensed voltage signal V2;
A resistor R2 connected to an output terminal of the second buffer;
And a second switch for switching the output of the second buffer at a half cycle of a predetermined frequency.
A high speed capacitance detection circuit, characterized by converting a detection signal into an AC signal of a predetermined frequency. The method of claim 1, wherein the band pass filter
A high-speed capacitance detection circuit having a frequency characteristic of removing power supply noise. delete

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