JP3640817B2 - Analog signal switching circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an analog signal switching circuit, and more particularly to an analog signal switching circuit capable of stabilizing an input / output voltage in a switching circuit such as an analog switch.
[0002]
[Prior art]
Conventionally, an analog switch is used to selectively output a plurality of analog signals. Figure 8 shows an example of the switching circuit 10 of the conventional analog signal using an analog switch.
[0003]
In FIG. 8, 2 is an analog switch with 4 inputs and 1 output, 4 is a buffer connected to the output side of the analog switch 2, 5 is an AD converter for converting an analog signal output from the buffer 4 into a digital signal, and 6 is The CPU outputs a control signal Sc for controlling switching of the analog switch 2 and inputs a digital signal from the AD converter 5. The CPU 6 can sequentially input the analog signals S _{1 to} S ₄ after AD conversion by periodically switching the internal switch Sw of the analog switch 2 by the control signal Sc.
[0004]
That is, by using the analog switch 2 as the analog signal switching circuit 10 as described above, the four analog signals S _{1 to} S ₄ are converted into digital signals using one relatively expensive AD converter 5. Can do. The analog signals S _{1 to} S ₄ are connected to the channels CH 1 to CH ₄ of the analog switch 2 through low-pass filters with resistors R _{1 to} R ₄ and capacitors C _{1 to} C ₄ , respectively. Noise components are removed in advance so that the instructions of the analog signals S _{1 to} S ₄ are stabilized when digital conversion is performed.
[0005]
[Problems to be solved by the invention]
However, in general, some capacitance component C ₀ exists on the output side of the analog switch 2. In the above-described conventional analog signal switching circuit 10, the influence of this capacitance component C _{0 is} caused by the internal switch Sw. It sometimes appeared remarkably when switching. That is, as shown in FIG. 9, for example, a voltage of the analog signals S ₁ inputted to the channel CH1 is greater than the voltage of the analog signal S ₂ to be input to the channel CH2, the internal switch Sw from the channel CH1 When switching to the channel CH2, a transient voltage fluctuation occurred at the output point A.
[0006]
Situation in which the waveform shown in FIG. 9 is output during the first time T ₀ through T _1, the charge while the internal switch Sw is selecting channel CH1, the capacitor C ₀ charge corresponding to the analog signal S1 Is done. Next, switch the internal switch Sw to the channel CH2 at time T _1, the charge stored in the capacitor C ₀ is charged in the capacitor C ₂ flows into the capacitor C _2.
[0007]
The voltage output to the output point A changes transiently for a time t ₁ determined by the time constant C ₂ R ₂ of the resistor R ₂ and the capacitor C ₂ constituting the low-pass filter of the analog signal S _2. . In other words, between time T ₁ switching the internal switch Sw for a period of time t _1, is different from the voltage of the analog signal S ₂ the voltage at the output point A is inputted to the channel CH2, which appears as an error.
[0008]
For this reason, the lower the cut-off frequency by the low-pass filter, the longer the time t _1. For example, in the case of a low-pass filter attached to a gas analyzer sensor such as an automobile exhaust gas measuring device, the time t ₁ is about 100 μs. There was a transient phenomenon. Further, since the charge stored in the capacitor C ₀ flows back to the input side capacitor C ₂ , it is not desirable in terms of the stability of the circuit on the input side of the analog signals S _{1 to} S ₄ .
[0009]
In general, as a characteristic of the analog switch 2, when the voltage of the analog signal switched by switching the switch Sw becomes large, there is a characteristic that can quickly follow the change. That is, it is conceivable that an internal circuit that can be equivalently represented by not only the capacitor C ₀ but also the diode D ₀ and the resistor R ₀ exists on the output side of the analog switch 2.
[0010]
However, since the magnitudes of the analog signals S _{1 to} S ₄ input to the analog switch 2 are not known in advance, in any case, AD converted data should be used during the period t _{1 in which} the transient phenomenon occurs. could not. Due to this problem, the switching speed of the analog switch 2 cannot be increased beyond a certain speed, and this can follow the input of analog signals S _{1 to} S ₄ such as measured values that change at high speed in accordance with the rotation of the engine. It was the cause that disappeared. Further, when the analog switch 2 is switched at a high speed, there is a disadvantage that the accuracy is deteriorated depending on the magnitude of the voltage difference between the input analog signals S _{1 to} S ₄ .
[0011]
The present invention has been made in consideration of the above-described matters, and provides an analog signal switching circuit that can achieve stabilization and high accuracy of the input / output voltage even in an analog switch that can be switched at high speed. It is aimed.
[0012]
[Means for Solving the Problems]
To achieve the above object, the switching circuit of the analog signal according to the present invention has an input channel of the plurality of analog signals, and analog switches for switching selectively connected to the output side of one of said input channels, this A discharge circuit for discharging the charge accumulated on the output side of the analog switch by connecting the output point of the analog switch to a discharge potential point when the analog switch is switched, and a CPU for outputting a channel switching control signal to the analog switch In the analog signal switching circuit, the control signal for switching the internal switch of the discharging circuit to the closed state in response to the input of the channel switching control signal output from the CPU , and the closed state of the internal switch of the discharging circuit The output point of the analog switch for any channel is longer than that It is characterized in that the output disenable period state not-option is provided control circuitry configured to generate a control signal of the analog switch inserted.
In the analog signal switching circuit according to the second aspect of the present invention, the discharging circuit is formed by connecting one vacant input channel of the analog switch to a discharge potential point. .
[0013]
Therefore, by using the analog signal switching circuit of the present invention, when the analog switch is switched, the charge stored on the output side of the analog switch is discharged after disconnecting from the old channel and before switching to the new channel. Since it is discharged through the circuit, there is no influence of the analog signal in the state before switching at the time of switching of the analog signal. In general, an analog switch has a characteristic capable of responding at a high speed to the rising of the input voltage, so that high-speed switching is possible.
[0014]
In particular, as in claim 2 , when one vacant input channel of the analog switch is connected to a discharge potential point, and this vacant input channel is formed in the discharge circuit, the discharge circuit is Therefore, the production cost of the analog signal switching circuit can be reduced.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Figure 1 is a diagram showing an example of the switching of the circuit 1 configured of an analog signal in accordance with the present invention. As shown in FIG. 1, an analog signal switching circuit 1 of this example includes, for example, an analog switch 2 having four inputs and one output, and an output point A (connection side) of the analog switch 2 as a discharge potential point (hereinafter referred to as a common). It comprises an analog switch (discharge circuit) 3 connected to G. Further, the output point A of the analog switch 3 is connected to the AD converter 5 via, for example, a buffer 4. A CPU 6 outputs a channel switching control signal Sc of the analog switch 2 and inputs a digital signal output from the AD converter 5. Reference numeral 7 denotes a control signal Sc ₁ for controlling the analog switches 2 and 3 in response to the input of a channel switching control signal Sc output from the CPU 6. , Sc ₂ And control signals Sc ₁ , Sc ₂ Is a control circuit that outputs.
[0016]
In the following description, members denoted by the same reference numerals as those shown in FIGS. 8 and 9 are the same or equivalent members, and thus detailed description thereof is omitted to avoid redundant description.
[0017]
FIG. 2 is a diagram showing the output waveform at the output point A of the analog signal switching circuit 1 shown in FIG. 1 and the states of the control signals Sc ₁ and Sc ₂ . That is, first, during a period of time T _{0 to} T ₁ , a channel switching control signal Sc output from the CPU 6 is input to the control circuit 7 , and the internal switch Sw 1 is set to the channel CH 1 from the control circuit 7 to the analog switch 2. A control signal Sc ₁ to be connected is output, and a control signal Sc ₂ for opening the internal switch Sw ₂ is output to the analog switch 3. At this time, a charge corresponding to the voltage of the analog signal S ₁ is charged in the capacitor C ₀ .
[0018]
Next, during the period of time T _{1 to} T ₂ , the CPU 6 first sets the analog switch 2 to the output disable state (described as “OD” in FIG. 2), and any channel is selected as the output point A. After outputting the control signal Sc ₁ to be in the non-existing state, the control signal Sc ₂ for closing the internal switch Sw2 of the analog switch 3 is output. At this time, the electric charge charged in the capacitor C ₀ is discharged to the common G through the analog switch 3, and the output voltage at the output point A becomes 0V (the same potential as the discharge potential point). It should be noted that the time t ₂ when the analog switch 3 is in the closed state may be a time required for the potential at the connection point A to be 0 V, and may be about 1 μs in the case of the normal analog switches 2 and 3.
[0019]
Next, at time T ₂ CPU 6 in after outputted a control signal Sc ₂ for switching the analog switch 3 in an open state, and outputs a control signal Sc ₁ to switch the internal switch Sw1 of the analog switch 2 to the channel CH2. At this time, the potential at the connection point A, the analog signal S ₂ to be input to quickly channel CH2 at the same potential.
[0020]
That is, according to the present invention, when the internal switch Sw1 of the analog switch 2 is switched, after the old channel CH1 is disconnected and before the new channel CH2 is switched, the charge stored in the capacitor C ₀ (the analog switch 2 By adding a discharge circuit 3 that once discharges the charge accumulated on the output side and then switches to the new channel 2, it is not affected by the analog signal of the old channel when switching channels. Can be switched to.
[0021]
In the case of this example, the number μs after the time T ₁ in which the channel switching can output a stable analog signal S2, allow for channel switching at a speed of several tens of times as compared with the conventional it can. In addition, since the previous channel is not affected at all, the accuracy can be improved accordingly. That is, even when the engine is required to be high speed like the switching of the analog signals S _{1 to} S ₄ of the exhaust gas measuring device, the switching of the signal can be reliably performed by using the analog signal switching circuit of the present invention. Is possible.
[0022]
In the present invention, as in the example described above, instead of the control signal Sc ₂ of analog switch 3 constituting the control signal Sc ₁ and the discharge circuit of the analog switch 2 may given directly from CPU 6, an enlarged view of FIG. 3 as shown Te, enter the channel switching control signal Sc output from the CPU6 in the control circuit 7, configured and to generate a control signal Sc _1, Sc ₂ analog switches 2 and 3 the control circuit 7 Yes.
[0023]
FIG. 4 is a diagram for explaining a method of generating the control signals Sc ₁ and Sc ₂ using the channel switching control signal Sc output from the CPU 6 . As shown in FIG. 4, the control circuit 7 generates a control signal Sc ₂ for switching the analog switch 3 to the closed state only for a short period t ₂ when the channel CH1 to CH4 is switched by the control signal Sc. The control signal Sc ₁ is generated by inserting an output disable period OD having a slightly longer period including the period t ₂ with respect to the control signal Sc. By including such a control circuit 7 in the analog signal switching circuit 1, the analog signal switching circuit 1 can be easily handled.
[0024]
In each example described above, an example of forming a capacitor C ₀ discharging circuit discharges the charge accumulated in the 3 'present at the output side of the analog switch 2 by the analog switch 3, the present invention Is not limited to this, and can be variously modified.
[0025]
FIG. 5 shows an example in which a transistor 8 is provided as a discharging circuit instead of the analog switch 3. Further, modifications such as using FETs instead of the transistors 8 can be easily performed. That is, by using these transistors 8 (including FETs) instead of the analog switch 3, the configuration of the analog signal switching circuit 1 can be made simpler and faster, and the production cost can be reduced accordingly. be able to. Further, instead of the transistor 8 (including FET), a photo MOS relay or an electromagnetic relay can be used.
[0026]
6 and 7 are diagrams showing another modification of the present invention. As shown in FIG. 6, the analog signal switching circuit 1 of this example uses a free input channel CH5 of the analog switch 2 without using a separate analog switch 3 or other switching elements 8 as a discharging circuit. A discharging circuit is formed. That is, by connecting the input channel CH5 to the discharge potential point G as shown in FIG. 7, when the channels CH1 to CH4 are switched, the control signal Sc ′ to be connected to the channel CH5 is output to the analog switch 2. By doing so, the electric charge accumulated in the capacitor C ₀ can be discharged. That is, the empty channel CH5 of the analog switch 2 constitutes the discharging circuit 3 ''.
[0027]
To configure the case as in this embodiment can be further simplified switching of circuit 1 configuration of the analog signal, it is possible to reduce the manufacturing cost as much as possible. Also in this example, the internal switch Sw1 of the analog switch 2 that also serves as the internal switch of the discharging circuit 3 '' is sequentially assigned to the channels CH1 to CH5 in accordance with the input of the channel switching control signal Sc output from the CPU 6. A control circuit 7 that generates and outputs a control signal Sc ′ that is controlled to be switched is provided.
[0028]
In each example described above, by entering the analog signal S ₁ to S ₄ which is selected to the AD converter 5, although this to be converted to a digital signal, but the present invention is not limited thereto Absent. That is, it can also be used when the analog signals S _{1 to} S ₄ that are selectively connected are input to various analog arithmetic circuits at high speed. Needless to say, the number of analog signals input to the analog switch 2 is not limited.
[0029]
Further, in each of the above examples, an example of the analog switch 2 that operates with a single power supply is shown, but the present invention is not limited to this, and can be implemented even when operated with a power supply of ± 12 V, for example. is there. In this case, the discharge potential point to which the discharge circuit is connected is not the common G but the negative power supply -12V.
[0030]
In addition, in the above description, the equivalent circuit of the internal circuit on the output side of the analog switch 2 shown in the drawing is considered in view of its characteristics, and therefore the actual IC has such a configuration. It does not indicate. Therefore, in the claims of the present invention, the charge stored in the capacitor C ₀ in the equivalent circuit is expressed as “charge stored on the output side of the analog switch”.
[0031]
【The invention's effect】
As described above, according to the present invention , it is possible to respond to switching at high speed without being affected by the analog signal in the state before switching when switching the analog switch. Further, as in claim 2, when one input channel of the analog switch is connected to a discharge potential point and this input channel is used as the discharge circuit, the circuit can be simplified, and only , Can reduce the production cost.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of an example of an analog signal switching circuit according to the present invention.
FIG. 2 is a diagram illustrating an operation of the analog signal switching circuit;
FIG. 3 is a diagram showing a modification of the analog signal switching circuit;
4 is a diagram illustrating a method for controlling the analog signal switching circuit shown in FIG. 3; FIG.
FIG. 5 is a diagram showing another example of the analog signal switching circuit;
FIG. 6 is a diagram showing a configuration of a modification of the analog signal switching circuit of the present invention .
7 is a diagram for explaining a method of controlling the analog signal switching circuit shown in FIG. 6;
FIG. 8 is a diagram illustrating a configuration of a conventional analog signal switching unit;
FIG. 9 is a diagram illustrating an operation of the analog signal switching unit.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Analog signal switching circuit, 2 ... Analog switch, 3, 3 ', 3''... Discharge circuit, 7 ... Control circuit, A ... Output point, CH1-CH4 ... Input channel, CH5 ... Empty input channel , G: discharge potential point, S _{1 to} S ₄ : analog signal.

Claims (2)

An analog switch having a plurality of analog signal input channels, and selectively switching one of the input channels to the output side, and connecting the output point of the analog switch to the discharge potential point when the analog switch is switched. the a discharging circuit for discharging the charges accumulated in the output side of the analog switch in the switching circuit of the analog signal and a CPU for outputting a channel switching control signal to said analog switch, the channel change output from the CPU Te Along with the control signal input, the control signal for switching the internal switch of the discharging circuit to the closed state and the closed state time of the internal switch of the discharging circuit for a longer period, the output point of the analog switch is in any channel. An unselected output disable period is inserted. Switching circuit of the analog signal, characterized in that a control circuit configured to generate a control signal of the analog switches. 2. The analog signal switching circuit according to claim 1, wherein the discharging circuit is formed by connecting one vacant input channel of the analog switch to a discharging potential point .

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