CN103308773A - Mutual capacitance variation measuring circuit with high precision and low power consumption - Google Patents

Mutual capacitance variation measuring circuit with high precision and low power consumption Download PDF

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Publication number
CN103308773A
CN103308773A CN2012100599131A CN201210059913A CN103308773A CN 103308773 A CN103308773 A CN 103308773A CN 2012100599131 A CN2012100599131 A CN 2012100599131A CN 201210059913 A CN201210059913 A CN 201210059913A CN 103308773 A CN103308773 A CN 103308773A
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CN
China
Prior art keywords
voltage
mutual capacitance
circuit
discharge
detected
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CN2012100599131A
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Chinese (zh)
Inventor
刘华
贾朝辉
卢昌鹏
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上海海栎创微电子有限公司
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Priority to CN2012100599131A priority Critical patent/CN103308773A/en
Publication of CN103308773A publication Critical patent/CN103308773A/en

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Abstract

The invention relates to a mutual capacitance variation measuring method with a high precision and a high stability. The system is composed of a switch discharge current source, a voltage follower and an impulse generator. The impulse generator generates an impulse waveform with a fixed length of time so that a detected mutual capacitance on a high clock edge is charged. If a voltage of a detected end is more than a reference voltage, the switch discharge current source discharges so as to reduce the voltage of the detected end until the voltage is lower than the reference voltage.<{EN4}>As for different discharge time periods of a switch current source caused by different detected mutual capacitances, the variation of the detected capacitances can be detected through measuring the variation of the time periods or the ratio variation of the time periods.<{EN5}>An error accumulation and mutual elimination method is applied by the circuit so that effects of an impact of a voltage fluctuation, an error of a measuring clock and an error of a detecting circuit itself on a measuring result are minimized to a lowest point. Therefore, the measuring method is great in measuring precision and stability. Because of the error accumulation and mutual elimination, the detecting circuit does not need a circuit with the high precision, and thus a simple circuit with the low power consumption can be adopted to realize. As for different discharge time periods of a switch current source caused by different detected mutual capacitances, the variation of the detected capacitances can be detected through measuring the variation of the time periods or the ratio variation of the time periods. An error accumulation and mutual elimination method is applied by the circuit so that effects of an impact of a voltage fluctuation, an error of a measuring clock and an error of a detecting circuit itself on a measuring result are minimized to a lowest point. Therefore, the measuring method is great in measuring precision and stability. Because of the error accumulation and mutual elimination, the detecting circuit does not need a circuit with the high precision, and thus a simple circuit with the low power consumption can be adopted to realize.

Description

A kind of mutual capacitance measure of the change circuit with high-precision low-power consumption

Technical field

The present invention relates to a kind of mutual capacitance metering circuit and its implementation with high-precision low-power consumption.The method can be used for touch control to be used, touch key-press for example, capacitive touch screen, mouse touch pad etc.Be particularly useful for referring to touch detect more.

Background technology

Traditional mutual capacitance touches the ultimate principle that detects and utilizes exactly pulse producer that tested mutual capacitance is is periodically discharged and recharged, and use current converter circuit that this charging and discharging currents is copied on the inner filter capacitor, and judge the variation of tested mutual capacitance by the magnitude of voltage that detects this filter capacitor.If when not touching, its voltage is V1.If with tested mutual capacitance of contact such as finger or felt pens, because many magnetic lines of force will rest on the finger, therefore tested mutual capacitance will reduce, its pulse producer charging and discharging electric current will correspondingly reduce, and the voltage on the final filter capacitor also can correspondingly reduce.By measuring the variation of its voltage, just can detect touch action.

The shortcoming of traditional detection method is: 1) need larger chip internal filter capacitor to guarantee its stability, and chip realizes that these filter capacitors need to take larger area so that final chip area becomes large, cost up; 2) detection of the interior voltage of chip must be used the ADC of a high-speed, high precision, and this can increase again area and the power consumption of chip.3) since in the practical application parameter consistency of tested mutual capacitance and PCB be difficult to guarantee, this causes the tested voltage can larger variation range even exceed the constraint of supply voltage so that measures unsuccessfully, so the control of volume production adjustment is difficult.

Summary of the invention

The object of the present invention is to provide a kind of mutual capacitance change detecting method of high production reliability of high-precision low-power consumption, can be widely used in during touch control etc. uses.For reaching above purpose, solution of the present invention is:

Tested mutual capacitance is around changing near the reference voltage Vref, by pulse producer tested mutual capacitance is is periodically discharged and recharged, utilize a discharge current source and pair of switches in the high cycle of clock tested mutual capacitance to be discharged, utilize a voltage follower and other pair of switches to make it avoid the impact of pulse producer negative edge discharge at the low tested mutual capacitance voltage of cycle stability of clock, by a comparer controlled discharge current source discharge process, the method for utilizing different tested mutual capacitance will change discharge time is come the variation of Detection capacitance.

Suppose that the voltage of tested mutual capacitance (1) near reference level Vref, periodically discharges and recharges tested mutual capacitance by pulse producer, can produce a charging current in the test side of measured capacitance (1), its size is:

Isc=Fsc*Cc*Vtx (formula 1)

Wherein, Fsc is that pulse producer A (4) is applied to the switching frequency in the tested mutual capacitance (1), and Cc is the mutual capacitance value of tested mutual capacitance (1), and Vtx is the output level of pulse producer A (4).

Per cycle of this electric current can be injected certain electric charge to tested mutual capacitance, and its size is:

Q1=Isc/Fsc=Vtx*Cc (formula 2)

By a discharge current source (7), the discharge current Icc to tested mutual capacitance is discharged by suitable setting, and guarantees Icc>Isc simultaneously, and this also is an important prerequisite of this measuring method.

This per cycle of discharge current source can bleed off certain electric charge over the ground to tested mutual capacitance, and its size is:

Q2=Icc*T (formula 3)

Wherein T is the time span that open in the discharge current source.

At the beginning detection-phase, the quick pre-charging electricity is carried out in tested mutual capacitance (1), the initial voltage of test side is based upon on the reference voltage Vref.Then in high cycle of clock of pulse producer, actuating switch (3), cut-off switch (2), at this moment the rising edge of pulse producer will charge to tested mutual capacitance, so that the voltage of test side surpasses Vref, and so that voltage comparator (6) output level change, producing an enable signal startup discharge current source discharges over the ground to tested mutual capacitance (1), through after the certain time interval T, the voltage drop of tested mutual capacitance (1) test port is to Vlow and below Vlow<Vref, and so that voltage comparator (6) output level changes, close the discharge current source.In the low cycle of clock of pulse signal generator, make switch (2) closure, switch (3) disconnects, at this moment the negative edge of pulse producer will discharge to tested mutual capacitance, so that the voltage of test side is far below Vref, but voltage follower will be so that test side voltage be got back to the voltage Vlow before the clock negative edge.And so forth until detect and to stop.

From the above description we can find out all the time can be so that pulse producer (4) reaches balance to the charging current total charging charge that produces and total discharge charge that the electric current that discharges by the discharge current source produces of tested mutual capacitance (1), that is: through N all after date voltage comparators (6)

N*Q1=Q2*Tn (formula 4)

By formula 1, formula 2, formula 3, formula 4 can obtain

Cc=Icc*Tn/ (N*Vtx) (formula 5)

Icc in the formula 5, Vtx, N are predefined system constants, and we can see that the detected complete linearity of mutual capacitance depends on Tn this of variable discharge time, detects this time variable by usage counter and just can realize accurately measuring mutual capacitance.Can simply adjust Icc in the specific implementation, Vtx, these system's constants that preset of N just can carry out precision to measuring system, the adjustment of speed or power consumption.

Adopted such scheme, the present invention has following characteristics: 1) measuring accuracy is high, and the size by increasing N is improving measurement accuracy very easily.2) circuit is simple and low in energy consumption, easily realizes at integrated circuit.Do not need complicated mimic channel and a large amount of interior filter capacitors of chip, only need basic Digital Logic, current source, voltage follower and a comparer just can be realized.3) volume production control is very easy to, even we can find out very large deviation is arranged Cc it also can work from formula 5, is not subjected to the constraint of supply voltage, so it is very easy to realize in volume production.

Description of drawings

The system chart of Fig. 1 testing circuit

Fig. 2 testing circuit is at the equivalent circuit diagram in high cycle of pulse producer clock

Fig. 3 testing circuit is at the pulse producer clock equivalent circuit diagram in low cycle

The voltage waveform of the tested mutual capacitance of Fig. 4 testing circuit test side

The testing circuit wave form varies that Fig. 5 mutual capacitance causes when reducing

Embodiment

The present invention is further illustrated below in conjunction with the accompanying drawing illustrated embodiment.

Fig. 1 is the ultimate system circuit diagram of metering circuit.Can be divided into three steps a measuring period of this metering circuit, is respectively:

1) the high phase of the cycles of pulse producer clock

In Fig. 1, cut-off switch (2), Closing Switch (3), its equivalent electrical circuit is as shown in Figure 2.The rising edge clock of this one-phase pulse producer can charge to the test side of tested mutual capacitance, after its voltage surpasses Vref comparer can start to discharge to it in the discharge current source until its voltage less than Vref.The voltage waveform of test side changes t1 as shown in Figure 4, t2 stage.

2) the low phase of the cycles of pulse producer clock

In Fig. 1, cut-off switch (3), Closing Switch (2), its equivalent electrical circuit is as shown in Figure 3.The clock negative edge of this one-phase pulse producer can discharge to the test side of tested mutual capacitance, and voltage follower can finally be stabilized in test side voltage negative edge voltage before.

3) repetitive process

Be to improve measuring accuracy, can repeating step 1 and step 2 many times.Utilize counter (9) that the time of the each discharge process in the repetitive process is added up, obtain time T n=T1+T2+.....The Time Calculation of integrated circuit all is to calculate with clock periodicity, so more if measure the total periodicity that continues, the quantization error of its clock is just less, and the precision of its measurement is also just higher.

Claims (8)

1. the detection method that changes of a mutual capacitance, its method of work is characterised in that: utilize one group of on-off circuit, current source and tested mutual capacitance produce a discharge circuit, to detected mutual capacitance in the high semiperiod intermittent discharge of clock.Utilize other one group of on-off circuit and a voltage follower to consist of a loop of voltage regulation and keep the magnitude of voltage that is examined mutual capacitance in the low semiperiod of clock, utilize simultaneously other one group of on-off circuit and detected mutual capacitance to consist of a pulse charge-discharge circuit, detected mutual capacitance is fixed the cycle discharges and recharges.When the high semiperiod of clock, the voltage on being detected electric capacity was less than reference voltage, on-off circuit will stop the discharge current source to detecting the discharge of mutual capacitance.Before the rising edge of clock arrives, on-off circuit will couple together the test side and disconnect being connected of mutual capacitance and voltage follower with detected mutual capacitance, so the rising edge of clock will charge to the test side of this mutual capacitance, its test side voltage will rise at once, and discharge switch control will be discharged to detected mutual capacitance when its voltage is higher than reference voltage.After this because the continuous discharge of the discharge switching circuit of tested mutual capacitance, voltage will descend gradually until be lower than reference voltage and close discharge path, before arriving in the low edge of clock, on-off circuit can disconnection detection being connected and voltage follower linked to each other with detected mutual capacitance of end and detected mutual capacitance, so because the effect of voltage follower repeats process before so that the negative edge of clock can not affect the magnitude of voltage of detected mutual capacitance.By measuring length discharge time in the detected mutual capacitance in the certain hour length, the variation by analyzing this time length or the variation of their ratio can measure the variation of tested mutual capacitance.
2. method according to claim 1, its measuring process is characterised in that:
1) precharge is carried out in detected mutual capacitance, made it initial voltage and be based upon on the reference voltage (Vref).
2) at rising edge clock the periodicity charging is carried out in tested mutual capacitance by pulse producer.
3) when measured capacitance is higher than reference voltage, the starting switch discharge circuit discharges to measured capacitance.And so that the charging current that discharge current produced greater than the pulse producer monocycle, like this, the voltage on the measured capacitance reduces gradually, after voltage is lower than reference voltage, stops discharge process.
4) pulse producer will be to tested mutual capacitance discharge on the low edge of clock, and on-off circuit will use voltage follower to stablize the voltage of measured point this moment, after this, can repeating step 2), 3) and step 4) repeatedly.
5) one or more discharges and recharges the time span that open in the discharge current source in the cycle to utilize counter measures.By observing the variation of discharge time, can learn the variation of tested mutual capacitance.
3. method according to claim 1, circuit of the present invention comprise tested mutual capacitance and corresponding on-off circuit, current source, pulse producer and corresponding voltage follower, comparer and timer.
4. circuit according to claim 3, its circuit comprises a tested mutual capacitance (1), it is characterized in that:
1) voltage of this mutual capacitance (1) is to change round near the reference voltage level
2) this mutual capacitance (1) is to discharge by discharge current source (7) and corresponding on-off circuit
3) this mutual capacitance (1) is to discharge and recharge by pulse producer A (4) and corresponding on-off circuit
4) this mutual capacitance (1) can be stablized its voltage by voltage follower (5) and corresponding on-off circuit
5) voltage of this mutual capacitance (1) is input to comparer (6), compares with reference voltage (Vref), and output is used for the discharge process of controlled discharge current source.
5. circuit according to claim 3, its circuit comprises a pulse producer A (7), it is characterized in that:
1) its pulsed frequency can be adjusted, to control the charging current of tested mutual capacitance (1).
6. circuit according to claim 3, its circuit comprises a voltage comparator (6), it is characterized in that:
1) it inputs the test lead voltage that an end is tested mutual capacitance, and other one terminates on the reference voltage Vref; These voltage inputs can be directly accesses or pass through other indirect cut-in methods such as resistance, voltage buffer
2) work of its output controlled discharge current source (7) enables, and when filter capacitor voltage was lower than Vref, discharge current source (7) quit work
3) counting that its output simultaneously also can control counter (9) is with the discharge time of record to tested mutual capacitance (1).
7. circuit according to claim 3, its circuit comprises a counter (9), it is characterized in that:
1) counter (9) is used for the discharge time of record tested mutual capacitance (1) in measuring period
2) work of counter (9) enables the output control by comparer (6).
8. circuit according to claim 3 comprises a voltage follower in the circuit, it is characterized in that:
1) stablizes the voltage of tested mutual capacitance test lead at the clock negative edge.
CN2012100599131A 2012-03-07 2012-03-07 Mutual capacitance variation measuring circuit with high precision and low power consumption CN103308773A (en)

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Application Number Priority Date Filing Date Title
CN2012100599131A CN103308773A (en) 2012-03-07 2012-03-07 Mutual capacitance variation measuring circuit with high precision and low power consumption

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN108169658A (en) * 2017-12-14 2018-06-15 上海华虹宏力半导体制造有限公司 The output current test circuit and test method of device blocks
CN109709613A (en) * 2018-12-24 2019-05-03 福建联迪商用设备有限公司 A kind of method, terminal and circuit whether detection wearable device is dressed
CN110418970A (en) * 2017-03-08 2019-11-05 赛普拉斯半导体公司 Ratio formula mutual capacitance-code converter

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH04138381A (en) * 1990-09-28 1992-05-12 Smc Corp Capacitance measuring circuit
US20080068029A1 (en) * 2006-09-15 2008-03-20 Agilent Technologies, Inc. Capacitance measuring apparatus and capacitance measuring method
CN101958090A (en) * 2009-07-21 2011-01-26 上海天马微电子有限公司 Touch detection circuit and method
CN102193033A (en) * 2010-03-08 2011-09-21 上海海栎创微电子有限公司 Self-capacitance change measuring circuit with quick response

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH04138381A (en) * 1990-09-28 1992-05-12 Smc Corp Capacitance measuring circuit
US20080068029A1 (en) * 2006-09-15 2008-03-20 Agilent Technologies, Inc. Capacitance measuring apparatus and capacitance measuring method
CN101958090A (en) * 2009-07-21 2011-01-26 上海天马微电子有限公司 Touch detection circuit and method
CN102193033A (en) * 2010-03-08 2011-09-21 上海海栎创微电子有限公司 Self-capacitance change measuring circuit with quick response

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN110418970A (en) * 2017-03-08 2019-11-05 赛普拉斯半导体公司 Ratio formula mutual capacitance-code converter
CN110418970B (en) * 2017-03-08 2020-10-13 赛普拉斯半导体公司 Ratio type mutual capacitance-code converter
CN108169658A (en) * 2017-12-14 2018-06-15 上海华虹宏力半导体制造有限公司 The output current test circuit and test method of device blocks
CN109709613A (en) * 2018-12-24 2019-05-03 福建联迪商用设备有限公司 A kind of method, terminal and circuit whether detection wearable device is dressed

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Application publication date: 20130918

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