JP7187953B2 - buffer circuit - Google Patents

Description

The technology disclosed in this specification relates to a buffer circuit.

Japanese Unexamined Patent Application Publication No. 2002-200010 discloses an electrocardiographic sensor including a buffer circuit with high input impedance.

JP 2011-188406 A

When a high voltage input such as static noise is input to a high input impedance buffer circuit, the buffer circuit may be charged. The output voltage of the buffer circuit drifts due to the DC offset caused by the charge, and the output waveform may be distorted, such as the upper end of the output voltage being flattened. In such a case, a waiting period is generated until the electric charge accumulated in the buffer circuit is released and the output waveform returns to normal.

One embodiment of the buffer circuit disclosed herein comprises an operational amplifier, a first resistance section, a second resistance section, a first capacitance section, a variable resistance section, and a switch section. The operational amplifier has a non-inverting input terminal, an inverting input terminal and an output terminal. An output terminal is connected to the inverting input terminal. One end of the first resistance portion is connected to the non-inverting input terminal, the other end of the first resistance portion is connected to one end of the second resistance portion at the first connection point, and the other end of the second resistance portion is connected to the reference voltage portion. It is connected. One end of the first capacitance portion is connected to the first connection point, the other end of the first capacitance portion is connected to one end of the variable resistance portion at the second connection point, and the other end of the variable resistance portion is connected to the output terminal. there is The switch section can change the resistance value of the variable resistance section, and is arranged on the connection path between the second connection point and the output terminal.

A so-called bootstrap circuit can be formed by the first resistance section, the second resistance section, the first capacitance section, and the variable resistance section. By constructing a positive feedback circuit, the input impedance of the buffer circuit can be increased. The input impedance of the buffer circuit can be changed by changing the resistance value of the variable resistance section with the switch section. For example, when there is a high voltage input such as static noise, the input impedance of the buffer circuit can be lowered by lowering the resistance value of the variable resistance section. Charged charges can be rapidly discharged to a predetermined potential. Therefore, even if the output voltage of the buffer circuit drifts due to a high voltage input, it is possible to restore the drift-free state in a short period of time.

The switch section may decrease the resistance value of the variable resistance section from the first resistance value to the second resistance value when the output voltage of the buffer circuit exceeds the first threshold value. The switch unit reduces the resistance value of the variable resistance unit from the second resistance value to the first resistance value when the output voltage falls below the first threshold value after the resistance value of the variable resistance unit is reduced to the second resistance value. may be raised to Details of the effect will be described in Examples.

The variable resistance section may include a third resistance section arranged on a connection path between the second connection point and the output terminal, and a bypass path that bypasses the third resistance section. The switch unit may be provided on the bypass path and be capable of controlling conduction and non-conduction of the bypass path. Details of the effect will be described in Examples.

An input capacitance section to which a signal is input may be connected to the non-inverting input terminal. The absolute value of the product of the resistance value of the first resistance portion and the resistance value of the second resistance portion may be smaller than the absolute value of the product of the resistance value of the variable resistance portion and the impedance value of the input capacitance portion. Details of the effect will be described in Examples.

1 is a schematic diagram of an electrocardiogram measurement system; FIG. 1 is a circuit diagram of an electrocardiographic sensor; FIG. It is a model of the measurement system of the electrocardiogram measurement system. FIG. 10 is a diagram showing an example of an electrocardiographic sensor in which the input impedance Zin of the buffer section is increased; 5 is a noise model of the circuit of FIG. 4; 3 is a noise model of the circuit of FIG. 2; It is a noise model for the offset voltage Vos. FIG. 4 is a waveform diagram showing static electricity removal operation; 4 is a graph showing absolute values and phases of a transfer function of an electrocardiographic sensor; 4 is a graph showing absolute values and phases of a transfer function of an electrocardiographic sensor;

(Configuration of electrocardiogram measurement system 1)
FIG. 1 shows a schematic diagram of an electrocardiogram measurement system 1 according to this embodiment. An electrocardiogram measurement system 1 includes a sheet 2 , a memory 8 and a display 9 . The seat 2 measures the action potential of the heart 91 of the seated person 90 . The measurement results can be recorded in the memory 8 and can be confirmed on the display 9 .

The seat 2 includes electrocardiographic sensors 11 and 12 and an electrocardiograph 5 . The electrocardiogram sensor 11 includes electrodes 3 and a buffer section 6 . Electrode 3 is an electrode embedded in the back of seat 2 . The output terminal of the electrode 3 is connected to the input terminal of the buffer section 6 via the wiring W11. An output terminal of the buffer unit 6 is connected to the electrocardiograph 5 via a wire W12. The electrocardiogram sensor 12 includes electrodes 4 and a buffer section 7 . The electrode 4 is an electrode embedded in the seat portion of the seat 2 . The output terminal of the electrode 4 is connected to the input terminal of the buffer section 7 via the wiring W21. The output terminal of the buffer section 7 is connected to the electrocardiograph 5 via the wiring W22.

The electrocardiograph 5 uses the voltage measured by the electrodes 4 as a reference potential, and measures changes in the electrocardiographic potential by electrostatic capacitive coupling between the electrodes 3 and the seated person 90 . Since a well-known circuit configuration can be used for the electrocardiograph 5, description thereof is omitted here.

(Configuration of electrocardiogram sensor 12)
FIG. 2 shows a circuit diagram of the electrocardiographic sensor 12. As shown in FIG. The electrocardiogram sensor 12 includes electrodes 4 , wires W<b>21 , and a buffer section 7 . The buffer section 7 includes an operational amplifier 30 , a first resistor R ₁ , a second resistor R ₂ , a first capacitor C _f , a variable resistance section 40 , a switch SW1 and a resistance control section 50 .

The output terminal of the electrode 4 is connected to the non-inverting input terminal of the operational amplifier 30 via the wiring W21. The output terminal of the operational amplifier 30 is connected to the inverting input terminal and also to the electrocardiograph 5 (not shown) via the wiring W22. An output voltage V _out is output from the output terminal of the operational amplifier 30 .

One end of the first resistor R ₁ is connected to the non-inverting input terminal of the operational amplifier 30 and the electrode 4 . The other end of the first resistor R1 is connected to _one end of the second resistor R2 at the first connection point N1. The other end of the second resistor R2 is connected to the ground potential _Vgnd .

One end of the first capacitor _Cf is connected to the first connection point N1. The other end of the first capacitor _Cf is connected to one end of the variable resistance section 40 at the second connection point N2. The other end of the variable resistance section 40 is connected to the output terminal of the operational amplifier 30 . The switch SW1 is arranged on the connection path between the second connection point N2 and the output terminal of the operational amplifier 30 .

The connection relationship between the variable resistance section 40 and the switch SW1 will be described more specifically. The variable resistance section 40 includes a feedback resistor _Rfh and a feedback resistor _Rfl . The other end of the first capacitor Cf is connected to one end of the feedback resistor _Rfh at the second connection point N2. The other end of feedback resistor _Rfh is connected to the output terminal of operational amplifier 30 . A bypass path is placed in parallel with the feedback resistor _Rfh . A feedback resistor _Rfl and a switch SW1 are arranged in series in the bypass path. Feedback resistor R _fl has a lower resistance than feedback resistor R _fh . For example, the resistance value of the feedback resistor R _fl may be in the range of 1/1000 to 1/100,000 of the feedback resistor R _fh .

An output voltage V _out is input to the resistance control unit 50 . A control voltage _Vg is output from the resistance control unit 50 . The resistance control unit 50 is a circuit that controls on and off of the switch SW1 according to the output voltage _Vout . In this embodiment, the resistance controller 50 is a window comparator. A control voltage _Vg output from the resistance control section 50 is input to the switch SW1. The switch SW1 is turned on when the control voltage _Vg is at low level and turned off when it is at high level. The switch SW1 can control conduction and non-conduction of the bypass path.

The variable resistance section 40 has a first resistance value when the switch SW1 is in a non-conducting state. Since the first resistance value is determined by the feedback resistor _Rfh , it has a high resistance. On the other hand, the variable resistance section 40 has a second resistance value when the switch SW1 is in a conducting state. The second resistance value is determined by the combined resistance of the parallel-connected feedback resistors _Rfh and _Rfl . Therefore, the second resistance value is lower than the first resistance value.

The configuration of the electrocardiographic sensor 11 is the same as the configuration of the electrocardiographic sensor 12 described above, so the description thereof will be omitted.

ここで、衣服を挟んだ着座者９０の体表と電極４との間の容量結合をＣ_ｂとし、インピーダンスをＺ_ｂとする。インピーダンスＺ_ｂは、「１／jωＣｂ」と表すことができる。また、バッファ部７の入力インピーダンスをＺ_ｉｎとする。 (Challenges of electrocardiographic sensors)
FIG. 3 shows a model of the measurement system of the electrocardiogram measurement system 1. As shown in FIG. In this measurement system, the relationship between the voltage change V _in of the heart 91 and the output voltage V _out of the electrocardiographic sensor 12 is expressed by Equation (1).

Here, the capacitive coupling between the electrode 4 and the body surface of the seated person 90 holding the clothes between them is _Cb , and the impedance is _Zb . The impedance Zb can be expressed as "1/ _jωCb ". Also, the input impedance of the buffer unit 7 is assumed to be _Zin .

From Equation 1, it can be seen that the output voltage _Vout is determined by the voltage ratio between the impedance _Zb and the input impedance _Zin . Therefore, _in order to increase the sensitivity of the electrocardiogram sensor 12, it is necessary to increase the input impedance Zin of the buffer section 7. FIG.

FIG. 4 shows an example of an electrocardiographic sensor 112 with an increased input impedance _Zin of the buffer section. The buffer circuit 107 of FIG. 4 achieves a high input impedance _Zin by providing a bootstrap circuit. Since the same parts are denoted by the same reference numerals in the electrocardiographic sensor 112 shown in FIG. 4 and the electrocardiographic sensor 12 shown in FIG. 2, description thereof will be omitted. One end of the first resistor R ₁ is connected to the non-inverting input terminal of the operational amplifier 30 . The other end of the first resistor R1 is connected to _one end of the _second resistor R2 at the first connection point N100. The other end of the _second resistor R2 is connected to the ground potential _Vgnd . One end of the bypass resistor _R2L is connected to the first connection point N100 via the switch SW100. The other end of bypass resistor _R2L is connected to ground potential _Vgnd . The bypass resistor _R2L has a lower resistance than the _second resistor R2. One end of the feedback resistor _Rf is connected to the first connection point N100, and the other end is connected to the output terminal of the operational amplifier 30. FIG. A bootstrap circuit is formed by the first resistor _R1 and the feedback resistor _Rf .

Also, the electrocardiogram sensor 112 is equipped with a circuit for dealing with the problem of static electricity. The problem of static electricity is a problem that when the buffer circuit 107 is charged with a high potential such as static electricity, the charged charge cannot be removed, making it difficult to measure the electrocardiographic potential. This problem arises because the RC circuit formed by the capacitive coupling _Cb of the clothing and the high input resistance of the buffer circuit 107 has a very high time constant. To solve this problem, the electrocardiogram sensor 112 includes a resistance control section 150 and a switch SW100. Since the switch SW100 is off in the normal measurement state, the input resistance of the buffer circuit 107 is in a high resistance state. Then, when the resistance control section 150 detects the generation of charge based on the output voltage V _out , the resistance control section 150 turns on the switch SW100. As a result, the input resistance of the buffer circuit 107 is switched to a low resistance state, thereby reducing the time constant of the RC circuit. Charged electric charges can be quickly discharged.

The following two problems occur in the electrocardiographic sensor 112 of FIG. A first problem is that the switch SW100 operates as a constant current source due to the leakage current of the switch SW100, which causes DC noise to be superimposed on the output voltage _Vout . For example, if the switch SW100 is an analog switch, it will operate as a constant current source because the switch is connected to the power supply through a diode. As a second problem, the offset voltage V _os of the operational amplifier 30 causes DC noise to be superimposed on the output voltage V _out . Generally, the offset voltage V _os is negligibly small. However, when a bootstrap circuit is used, the feedback amplifies the offset voltage V _os , resulting in large DC noise at the output of operational amplifier 30 .

The first and second problems will be explained using FIG. FIG. 5 is a noise model for the circuit of FIG. When the switch SW100 is off, DC noise is generated due to leakage current. Therefore, as shown in FIG. 5, the switch _SW100 can be expressed as a constant current source ISW. Also, the offset voltage V _os of the operational amplifier 30 can be expressed on the connection path between the inverting input terminal and the output terminal.

Ｉ_ＳＷの項（右辺第２項）は、直流であることを考慮すると、下式のように表せる。

数３の式で分かるように、出力電圧Ｖ_ｏｕｔにＤＣノイズが重畳してしまう。 The first problem (DC noise caused by switches) will be described using mathematical expressions. The output voltage V _out in the noise model of FIG. 5 is represented by the following formula.

Considering that the I _SW term (the second term on the right side) is a direct current, it can be expressed as the following equation.

As can be seen from Equation 3, DC noise is superimposed on the output voltage _Vout .

Ｖ_ｏｓの項（右辺第２項）に着目する。Ｖ_ｏｓが直流であることを考慮し、Ｚ_ｂを含む項が無限大に大きくなることを用いると、Ｖ_ｏｓの項は下式のように簡略化できる。

ここで、高入力インピーダンスな設計を前提とすると、「Ｒ２＞Ｒｆ」の関係が成立する。すると数５の式から、オフセット電圧Ｖ_ｏｓが増幅されて出力電圧Ｖ_ｏｕｔに重畳してしまうことが分かる。 The second problem (DC noise caused by the offset voltage of the operational amplifier) will be described using mathematical expressions. The relational expression of the input and output in the noise model of FIG. 5 is shown below.

Focus on the V _os term (the second term on the right side). Considering that V _os is DC, and using the infinitely large term involving Z _b , the V _os term can be simplified as:

Here, assuming a high input impedance design, the relationship "R2>Rf" is established. Then, from the equation (5), it can be seen that the offset voltage V _os is amplified and superimposed on the output voltage V _out .

(Effect of electrocardiogram sensor 12)
The electrocardiogram sensor 12 (see FIG. 2) according to the present embodiment is a circuit having the effect of solving the first and second problems described above. Also, the electrocardiographic sensor 12 is a circuit having the effect of being able to calculate stable operating conditions. The effects are described in detail below.

ここでＺ_ｆは、第１キャパシタＣ_ｆのインピーダンスであり、「１／jωＣｆ」で表すことができる。定電流源Ｉ_ＳＷがＤＣであることを考慮した場合、数６の式においてＺ_ｆとＺ_ｂは共に無限大に近づくため、Ｉ_ＳＷの項（右辺第２項）は０とみなせる。よって、スイッチＳＷ１によるＤＣノイズを無くすことができることが分かる。 The fact that the first problem (DC noise caused by the switch) can be solved will be described using mathematical formulas. FIG. 6 shows a noise model for the constant current source _ISW . FIG. 6 is a noise model of the circuit of FIG. DC noise does not occur when the switch SW1 is on. FIG. 6 is therefore a noise model for when the switch SW1 is off. The output of the circuit is expressed by the following equation.

Here, Zf _is the impedance of the first capacitor Cf and can be expressed as "1/ _jωCf ". Considering that the constant current source I _SW is DC, both Z _f and Z _b in Equation 6 approach _infinity . Therefore, it can be seen that the DC noise caused by the switch SW1 can be eliminated.

That is, by locating the first capacitor _Cf on the connection path between the switch SW1 and the non-inverting input terminal of the operational amplifier 30, the DC component due to the leak current of the switch SW1 can be cut. In other words, when the switch SW1 is arranged on a positive feedback loop from the output terminal of the operational amplifier 30 to the non-inverting input terminal, the first capacitor Cf is arranged so as to cut DC between the switch SW1 and the non-inverting input terminal. is inserted. As a result, the DC noise generated by the switch SW1 is not superimposed on the output voltage _Vout . The first problem can be solved.

Also, the switch SW100 of the buffer circuit 107 of FIG. 4 and the switch SW1 of the buffer section 7 of FIG. 2 are arranged at different positions. However, the buffer section 7 in FIG. 2 also has the function of removing static electricity described in the buffer circuit 107 in FIG. This is because the input resistance of the operational amplifier 30 can be switched even when the switch SW1 is placed at the position shown in FIG. The details of the static electricity removal function of the buffer unit 7 in FIG. 2 will be described later.

ここで、スイッチＳＷ１がオンのときは、「Ｒｆ＝Ｒ_ｆｈ・Ｒ_ｆｌ／（Ｒ_ｆｈ＋Ｒ_ｆｌ）」となる。一方、スイッチＳＷ１がオフのときは、「Ｒｆ＝Ｒ_ｆｈ」となる。オフセット電圧Ｖ_ｏｓがＤＣであることを考慮した場合、数７の式においてＺ_ｆとＺ_ｂは共に無限大に近づくため、Ｖ_ｏｓの項（右辺第２項）は下式のように変形できる。

よって、オペアンプ３０のオフセット電圧によるＤＣノイズは増幅されないことから、無視できることが分かる。 The fact that the second problem (DC noise caused by the offset voltage of the operational amplifier) can be solved will be described using mathematical formulas. FIG. 7 shows a noise model for the offset voltage _Vos . FIG. 7 is a noise model of the circuit of FIG. The output of the circuit is expressed by the following equation.

Here, when the switch SW1 is on, "Rf=R _fh ·R _fl /(R _fh +R _fl )". On the other hand, when the switch SW1 is off, "Rf=R _fh ". Considering that the offset voltage V _os is DC, both Z _f and Z _b in Equation 7 approach infinity, so the V _os term (the second term on the right side) can be transformed as follows: .

Therefore, it can be seen that the DC noise caused by the offset voltage of the operational amplifier 30 is negligible because it is not amplified.

That is, by placing the first capacitor _Cf on the positive feedback loop, it is possible to block the feedback of the offset voltage V _os which is DC. The offset voltage _Vos is not amplified. It is possible to eliminate the influence of DC noise superimposed on the output voltage _Vout by the offset voltage _Vos . The second problem can be solved.

A stable operation condition (oscillation condition) of the electrocardiographic sensor 12 according to this embodiment will be described. In Equation 6 and Equation 7, the Vin term (the first term on the right side) appears _in the same equation. Oscillation conditions can be obtained from this _Vin term. As for the constant current source I _SW and the offset voltage V _os , the effects have been eliminated by the countermeasures described above, so they are ignored here. The transfer function of the electrocardiographic sensor 12 of FIG. 2 is shown below.

数１０の式を変形すると、下式となる。

In general, when the absolute value of the transfer function is 0 dB or more, oscillation will occur if the phase is +180 degrees or -180 degrees. Based on this oscillation condition, the stable operation condition of the electrocardiogram sensor 12 of FIG. 2 is derived below. First, focusing on the denominator of Expression 9, since " _{R1R2+R1Rf + R2Rf " and} " _ZfZb " are in _opposite phase, they cancel each other out at _a specific frequency. Also, the magnitude relationship _between " _{R1R2+R1Rf + R2Rf " and} " _ZfZb " _changes at this specific frequency, and the phase of the transfer function is reversed. Therefore, if the gain is smaller than ₁ when " _{R1R2+R1Rf+R2Rf+ZfZb = 0 " , no oscillation} occurs. This condition is shown in the following formula.

Transformation of the formula 10 yields the following formula.

数１２の式で示されるように、「第１抵抗器Ｒ_１の抵抗値（Ｒ_１）と第２抵抗器Ｒ_２の抵抗値（Ｒ_２）との積の絶対値が、可変抵抗部４０の帰還抵抗器Ｒ_ｆｈまたは帰還抵抗器Ｒ_ｆｌの抵抗値（Ｒ_ｆ）と容量結合Ｃ_ｂのインピーダンス値（Ｚ_ｂ＝１／ｊωＣｂ）との積の絶対値よりも小さい」ことが、安定動作条件となる。この安定動作条件を満たすように、心電センサ１２の回路設計を行えばよい。 Here, the magnitude relationship of the circuit constants when actually designing the circuit is taken into consideration. That is, in actual design, circuit constants are determined so that the buffer has a high input impedance, so that |R ₁ |>>|R _f | and |R ₂ |>>|R _f | are established. Then, on the left side of Equation 11, it can be seen that the terms "R ₁ R _f " and "R ₂ R _f " are negligibly small compared to the term "R ₁ R ₂ ". Therefore, Equation 11 can be simplified as shown below.

12, the absolute value of the product of the resistance value (R ₁ ) of the first resistor R ₁ and the resistance value (R ₂ ) of the second resistor R ₂ is the variable resistance unit 40 is less than the absolute value of the product of the resistance value (R _f ) of the feedback resistor R _fh or the feedback resistor R _fl and the impedance value (Z _b =1/jωCb) of the capacitive coupling C _b ”, the stable operation be a condition. The circuit design of the electrocardiographic sensor 12 may be performed so as to satisfy this stable operation condition.

(Operation of electrocardiogram sensor 12)
The operation of removing static electricity in the electrocardiographic sensor 12 according to this embodiment will be described with reference to the waveform diagrams of FIGS. The horizontal axis of FIG. 8A indicates time. The vertical axis in FIG. 8A indicates the output voltage V _out output from the electrocardiographic sensor 12. FIG. The electrocardiographic sensor 12 outputs a positive output voltage V _out and a negative output voltage V _out with respect to a reference voltage of 0V. First thresholds Vth11 and Vth21 and second thresholds Vth12 and Vth22 are thresholds for resistance control unit 50 to perform a window comparator operation with hysteresis. The horizontal axis of FIG. 8B indicates time. The vertical axis of FIG. 8(B) indicates the control voltage _Vg output from the resistance control section 50 .

A case where a high positive charge due to static electricity is input at time t0 in FIG. 8 will be described. When the wiring W21 is charged with positive charges, the output voltage _Vout drifts to the positive side due to the DC offset. At time t1, when the output voltage _Vout exceeds the first threshold value Vth11, the resistance control section 50 changes the control voltage _Vg from high level to low level. The switch SW1 transitions from the non-conducting state to the conducting state. Therefore, the resistance value of the variable resistance section 40 can be decreased from the first resistance value to the second resistance value. Since the time constant of the RC circuit composed of the capacitive coupling Cb of the clothes and the high input resistance of the buffer can be reduced, the electric charges stored in the wiring W21 can be rapidly discharged. Therefore, the output voltage _Vout drops from the first threshold Vth11.

At time t2, when the output voltage Vout falls below the first threshold Vth11 and the second threshold _Vth12 , the resistance control section 50 returns the control voltage Vg from low level to high level. The switch SW1 returns from the conducting state to the non-conducting state. Therefore, the resistance value of the variable resistance section 40 can be increased from the second resistance value to the first resistance value. As a result, the discharge through the _second resistor R2 is terminated, and the sensitivity of the electrocardiographic sensor 12 can be returned to its original high state. This makes it possible to shorten the waiting period during which the electrocardiographic potential cannot be measured.

(Example of circuit design and transfer function)
3 shows a circuit design example of the electrocardiographic sensor 12 of FIG. 2. FIG. The capacitive coupling Cb is _40pF , the first resistor _R1 is 1GΩ, the _second resistor R2 is 1MΩ, the first capacitor Cf is _660μF , the feedback resistor _Rfh is 100MΩ, the feedback resistor _Rfl is 20kΩ, and so on. did.

9 and 10 show the absolute values and phases of the transfer function of the electrocardiographic sensor 12 in the above circuit design example. 9 and 10, the horizontal axis is the frequency, the left vertical axis is the absolute value of the transfer function, and the right vertical axis is the phase of the transfer function. FIG. 9 is a graph when the switch SW1 is in the OFF state (that is, when the variable resistance section 40 is in the high resistance state and the electrocardiographic potential can be measured). FIG. 10 is a graph when the switch SW1 is in the ON state (that is, the variable resistance section 40 is in the low resistance state and static electricity is being discharged).

As shown in FIG. 9, in the cardiac potential measurement state in which the switch SW1 is off, the absolute value (that is, the gain) of the transfer function is "1" in the cardiac potential frequency band of several tens to several hundred Hz. , the phase is "0". From this, it can be seen that the electrocardiogram sensor 12 stably functions as a voltage follower without divergence. Further, as shown in FIG. 10, it can be seen that the band in which the electrocardiographic sensor 12 stably performs the voltage follower operation can be expanded further downward when the switch SW1 is on and the electrostatic discharge is in progress.

Although specific examples of the present invention have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above. The technical elements described in this specification or in the drawings exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims as of the filing. In addition, the techniques exemplified in this specification or drawings can simultaneously achieve a plurality of purposes, and achieving one of them has technical utility in itself.

(Modification)
The mode of changing the resistance value of the variable resistance section 40 is not limited to the bypass mode shown in FIG. 2, and may be various. For example, it may be a mode in which contacts with a plurality of resistors are switched.

The measurement target of the electrocardiographic sensor 12 is not limited to electrocardiographic potential. Different signals with different frequency bands can be measured. In this case, as long as the stable operation condition expressed by Equation 12 is satisfied, any circuit design can be made in accordance with the characteristics of the signal to be measured.

Although the static electricity removing function has been described as an example of the function that can be realized by the switch SW1 and the variable resistance section 40, the function is not limited to this function. For example, a function of optimizing the frequency band of the electrocardiographic sensor 12 according to the frequency of the signal to be measured can also be realized. A specific description will be given. The frequency of the output voltage V _out is monitored by the resistor controller 50 . When it is detected that the frequency of the output voltage V _out is out of the frequency band currently used by the electrocardiographic sensor 12, the resistance value of the variable resistance section 40 is changed. Thereby, the frequency band of the electrocardiographic sensor 12 can be changed as described with reference to FIGS. 9 and 10. FIG. At this time, the resistance value of the variable resistance section 40 may be changed so that the frequency of the output voltage _Vout is included in the changed frequency band of the electrocardiographic sensor 12 .

The number of resistance values switchable in the variable resistance section 40 is not limited to two, and may be three or more.

The switch SW1 is not limited to an analog switch. A MOS transistor or a CMOS switch circuit may be used.

The number of electrocardiographic sensors provided on the seat 2 is not limited to two. It may be one or three or more.

Capacitive coupling _Cb is an example of an input capacitance section.

2: sheet 3 and 4: electrodes 5: electrocardiograph 6 and 7: buffer unit 11 and 12: electrocardiogram sensor 30: buffer circuit 40: variable resistance unit 50: resistance control unit SW1: switch _R1 : first resistor R ₂ : Second resistor C _f : First capacitor R _fh , R _fl : Feedback resistor

Claims (2)

A buffer circuit comprising an operational amplifier, a first resistance section, a second resistance section, a first capacitance section, a variable resistance section, and a switch section,
The operational amplifier has a non-inverting input terminal, an inverting input terminal and an output terminal,
The output terminal is connected to the inverting input terminal,
One end of the first resistance portion is connected to the non-inverting input terminal, the other end of the first resistance portion is connected to one end of the second resistance portion at a first connection point, and the other end of the second resistance portion is connected to the reference voltage part,
One end of the first capacitance section is connected to the first connection point, the other end of the first capacitance section is connected to one end of the variable resistance section at a second connection point, and the other end of the variable resistance section is connected to the connected to the output terminal,
The variable resistance section is
a third resistance section arranged on a connection path between the second connection point and the output terminal;
a bypass path that bypasses the third resistor;
and
The switch unit is provided on the bypass path and can control conduction and non-conduction of the bypass path,
The switch section
decreasing the resistance value of the variable resistance unit from a first resistance value to a second resistance value when the output voltage of the buffer circuit exceeds a first threshold;
When the output voltage falls below the first threshold value after the resistance value of the variable resistance portion is decreased to the second resistance value, the resistance value of the variable resistance portion is decreased from the second resistance value to the second resistance value. increase to 1 resistance,
buffer circuit. An input capacitance section to which a signal is input is connected to the non-inverting input terminal,
The absolute value of the product of the resistance value of the first resistance unit and the resistance value of the second resistance unit is smaller than the absolute value of the product of the resistance value of the variable resistance unit and the impedance value of the input capacitance unit. 2. A buffer circuit as claimed in claim 1 .

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