JPH06265582A - Current detecting circuit - Google Patents

Abstract

PURPOSE:To prevent spike noise, while allowing the detection of microcurrent, when the current detection sensitivity is switched by varying the current flowing through a load circuit. CONSTITUTION:A control means 30 calculates the current flowing through a load circuit 14 based on the voltages and resistances of current detecting resistors 5, 7. When the resistance is lowered due to switching of the resistors 5, 7 through a switch means 8, voltage to be produced at the output terminal of an amplifier 3 is calculated using a previously calculated current value to vary the output voltage from an input power supply immediately before switching of resistance so that the voltage value of the load circuit 14 after switching resistance is lowered than that before switching.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current detection circuit,
In particular, the present invention relates to a current detection circuit capable of switching the amount of current flowing through a load circuit whose characteristics are to be measured by switching a resistor.

[0002]

2. Description of the Related Art FIG. 2 is a block diagram of a conventional example of a conventional current detection circuit. This circuit detects the current I flowing through the load circuit 14 when a predetermined voltage determined by the input voltage and the resistance values of the resistors 2 and 4 is generated in the load circuit 14,
This is for measuring the characteristics of the load circuit. The high gain amplifier 3 receives the input voltage Vi and outputs the voltage VA to its output terminal A. The current detection resistors 5 and 7 convert the current into a voltage so that the load circuit 1
4 are provided in parallel to detect the current flowing through the switch 4. The differential amplifier 12 detects the voltage generated in these resistors. The buffer amplifier 11 has a gain of 1,
A negative feedback loop for negatively feeding back the voltage VB generated in the load circuit 14 to the amplifier 3 is formed. The current detection resistors 5 and 7 are provided in parallel in order to switch the current detection sensitivity according to the current flowing through the load circuit 14. This switching is performed by turning on or off the switch 8 and selecting whether to connect the resistor 7 in parallel with the resistor 5. Note that the resistance values of the resistors 5 and 7 are R5 and R7, respectively, and these have a relationship of, for example, R5 = 50 · R7. The load circuit 14 actually has impedance, but for convenience sake, it is assumed that the load circuit 14 has a resistance value R14 in the following description. This is because this circuit is not for measuring the high frequency characteristics of the load circuit.

Considering the moment when the switch 8 is switched from off to on, for example, it takes a certain amount of time for the amplifier 3 to respond to this switching, and its output voltage cannot change instantaneously. Therefore, at this moment, although the output voltage VA does not change, since the resistor 7 is connected in parallel with the resistor 5, the combined resistance value thereof sharply decreases, so that the current flowing through the load circuit 14 increases. I increases, the voltage VB (voltage applied to the load circuit) at the terminal 13 rises sharply, and spike noise occurs.
Here, the output voltage V of the amplifier 3 when the switch 8 is off
When A is VAoff and the current flowing through the load circuit 14 is Ioff, it is expressed by the following formula 1.

[0004]

[Equation 1] Ioff = VAoff / (R5 + R14)

Next, when the switch 8 is switched from off to on, the combined resistance value of the resistors 5 and 7 becomes R5.R7 / (R
5 + R7). At this time, the current flowing through the load circuit 14 is Ion. At the moment of switching, the output voltage VA of the amplifier 3 cannot be changed and is held at VAoff, so that the voltage VB applied to the load circuit 14 at this time is expressed by the following mathematical expression 2.

[0006]

[Equation 2] Ion = VAoff / [{R5 · R7 / (R5 + R7)} + R14]

At this time, for example, if there is a relationship of R5 = 50.R7,

[0008]

## EQU00003 ## Ion / Ioff = (50R7 + R14) / {(50/51) R7 + R14} .apprxeq.50

Therefore, it can be seen that a large amount of current flows through the load circuit 14 at the moment when the switch 8 is turned on as compared with when it is turned off. Then, the voltage VB rises sharply, and spike noise occurs.

FIG. 3 shows a block diagram of a conventional circuit for suppressing the spike noise. Such a circuit is disclosed in, for example, Japanese Patent Publication No. 1-8310. Comparing this circuit with FIG. 2, the resistor 7
In series with a semiconductor switch, such as a junction FET, MO
SFET etc. are connected. One terminal of the sawtooth wave signal generator 9 is connected to the control terminal (gate of the FET) of the semiconductor switch 6, and the other terminal is connected to the common terminal s of the switch 10. The common terminal s is selectively connected to one of the contact a connected to the output end of the amplifier 3, the contact b connected to the output end of the buffer amplifier 11, or the contact c connected to the reference potential point. Connected.

The operation of this circuit will be described with respect to points A, B and G. The voltage at each point is VA, VB, and VG, respectively. First, the current is flowing from A to B, and the N-channel FET is used to connect the current detection resistor 7 to the current detection resistor 5
Consider the case of connecting in parallel to. In this case, the switch 10 is connected to sa. First, the voltage of the generator 9 is set to the negative maximum to turn off the FET 6, while the switch 8 is turned on.
Turn on. Next, the output voltage of the generator 9 is changed at a constant slew rate (voltage change per unit time). As the gate voltage VG of the FET 6 rises at a constant slew rate, a current gradually flows between the source and drain of the FET 6, and the voltage VB at the point B also rises. Since the voltage VB is fed back to the inverting input terminal of the amplifier 3, the rise of VB causes a negative feedback to cause VA to move in the negative direction. At this time, since the gate voltage VG also drops, the FET 6 works in the direction of turning off. Therefore,
Since the slew rate of the amplifier 3 is sufficiently higher than the slew rate of the generator 9, the negative feedback makes the gate voltage VG constant. When the FET 6 is completely turned on and the resistance value does not change, VB does not change and VA becomes constant.
In this way, the combined resistance value of the current detection resistor changes slowly, so that the voltage VB applied to the load circuit 14 does not change abruptly, and spike noise can be suppressed. When the current is flowing from B to A, the switch 10 may be set to sb or sc.

[0012]

However, in the conventional circuit, even if the slew rate of the generator 9 is set to be sufficiently smaller than the slew rate of the amplifier 3, from when the FET 6 is turned on until the amplifier 3 operates. However, the voltage VB of the load circuit changes to the desired voltage or more, though it is a short time. In addition, the use of a semiconductor switch leaks a measurement current, and is not suitable for measuring a minute current.

Therefore, an object of the present invention is to provide a current detection circuit in which there is no fear that a voltage higher than a predetermined value will be applied to a load circuit whose characteristics are to be measured. Another object of the present invention is to provide a current detection circuit that can detect even a minute current flowing through a load circuit.

[0014]

The current detecting circuit according to the present invention has a current detecting resistors 5 and 7 provided between the output terminal A of the amplifier 3 and the load circuit 14 and an input terminal of the amplifier 3. An input power source 20 that outputs an applied voltage, and a detection means 12 that detects the voltage generated in the current detection resistors 5 and 7.
And 40, switch means 8 for switching the resistance values of the current detecting resistors 5 and 7, and the voltage generated in the load circuit 14 is fed back to the input of the amplifier 3 and output to the load circuit 14 according to the input voltage of the amplifier 3. Feedback means for generating a voltage, resistance values of current detecting resistors 5 and 7 and detecting means 12 and 4
The control means 30 calculates the current flowing through the load circuit 14 from the voltage value detected by 0 and controls the voltage output from the input power supply 20. And this control means 30
When the resistance value of the current detecting resistor is switched by the switch means 8 and the resistance value is lowered, the voltage value generated in the load circuit 14 after switching the resistance value is set to the load circuit before switching the resistance value. In order to reduce the voltage value to 14 or less, the voltage value to be generated at the output terminal of the amplifier 3 is calculated by using the calculated current value, and the voltage output from the input power supply 20 is changed immediately before switching the resistance value. It is characterized by At this time, since the current flowing through the load circuit 14 before the switching of the resistance value has already been calculated, it is easy to calculate the voltage to be generated at the output terminal A of the amplifier 3 from the resistance value after the switching. If a digital / analog converter is used as the input power source 20, the control means 30
It is easy to control the input power source 20 by.

[0015]

1 is a block diagram of a current detection circuit according to the present invention. Compared to FIG. 2, the circuit of the present invention is characterized in that the control means 30 such as a microprocessor controls the switch 8, the input power supply 20, etc. to prevent spike noise. Moreover, unlike the circuit of FIG. 3, the semiconductor switch 6 is not required. As the input power source 20, a digital / analog converter (DAC) or the like is suitable. Analog-to-digital converter (ADC) 40
Converts the output voltage of the differential amplifier 12 for detecting the current flowing through the load circuit 14 into an analog-digital signal and supplies the output to the control means 30. Memory means 50
Stores the resistance values of the current detection resistors 5 and 7, and the control means 30 can calculate the current I flowing through the load circuit 14 by using these values. The memory means 50 may store a program to be processed by the control means 30. A RAM (random access memory), a ROM (read only memory) or the like is suitable for the memory means 50.

The control means 30 controls the output voltage VA of the amplifier 3.
Can also be calculated. That is, since the voltage VB applied to the load circuit 14 from the input voltage Vi and the resistors 2 and 4 is determined, the current I flowing through the load circuit 14 is determined. Since the voltage generated in the current detection resistors 5 and 7, that is, the potential difference VAB between the points A and B is obtained from this current I, the output voltage VA can be calculated by adding VAB to the voltage VB.

By the way, as described above, in the circuit shown in FIG. 2, in order to prevent the spike noise from being generated, it is sufficient to keep Ion always below Ioff or to set Ioff = Ion. Letting VAon be the voltage of VA when the switch 8 is turned on to set Ioff = Ion, the following equation 4 is established from equations 1 and 2, and VAon is obtained as in equation 5.

[0018]

## EQU00004 ## Ioff = Ion = VAon / [{R5.R7 / (R5 + R7)} + R14]

[0019]

VAon = Ioff [{R5 · R7 / (R5 + R7)} + R14]

Therefore, in order to prevent the spike noise from occurring when the switch 8 is switched from ON to OFF, the control means 30 controls the voltage of the input power source 20 and the output voltage VA of the amplifier 3 at the moment of switching. VA shown in Equation 5
It is good if it is on. The resistance value R14 of the load circuit 14 in the equation 5
Can be calculated using Equation 1, and the resistance values R5 and R7 of the resistors 5 and 7 are known, so VAon can be easily calculated.

FIG. 4 shows the voltage Vi at the input terminal 1 and the load circuit 14 when the switch 8 is switched from off to on.
2 shows changes in the voltage VB and the current I. When switching the switch 8 from OFF to ON as shown in FIG. 4A, the input voltage is switched to Vion immediately before. This Vion
Are set so that the output voltage of the amplifier 3 becomes VAon shown in the equation 5. That is, this Vion determines the voltage of the load circuit 14 from the resistors 2 and 3, and therefore the current Ion flowing through the load circuit 14 when the switch 8 is on. Therefore, this Ion is connected in parallel to the resistor 5 and The potential difference VAB between points AB obtained by applying the voltage to 7 is expressed by the following equation 6. By adding this VAB to VB, VA at that time can be set to VAon shown in Formula 5.

[0022]

[Formula 6] VAB = Ion {R5 · R7 / (R5 + R7)}

When the voltage at the input terminal 1 changes to Vion at time t1, the amplifier 3 changes its output voltage VA according to its slew rate and becomes VAon at time t2. Subsequently, when the switch 8 is switched from off to on at the time t3, the resistor 7 is connected in parallel to the resistor 5, so that the current I flowing through the load circuit 14 rapidly increases from Ion to Ioff. VB also increases sharply, but the voltage value when the switch 8 is off, that is,
It does not become spike noise because it only reaches VBoff. After time t3, the output voltage Vi of the input power supply 20 is returned to Vioff at a slew rate sufficiently lower than the slew rate of the amplifier 3. At this time, the negative feedback applied to the amplifier 3 lowers the output voltage VA of the amplifier 3, and as a result, the voltage VB applied to the load circuit 14 greatly exceeds the voltage VBoff when the switch 8 is turned off even when the switch 8 is turned on. Eventually, it will eventually stabilize at VBoff.

FIG. 5 shows the voltage Vi of the input terminal 1 and the load circuit 14 when the switch 8 is switched from on to off.
2 shows changes in the voltage VB and the current I. In this case, since the current I flowing through the load circuit 14 is in the direction of decreasing, spike noise is not likely to occur due to the switching of the switch 8. As shown in the figure, when the switch 8 is switched from ON to OFF at the time t10, the current I decreases and the voltage VB also decreases. However, since the amplifier 3 is negatively fed back, the original current and voltage will soon be output. Recover to value.

As described above, according to the present invention, the control means is provided immediately before switching the resistance value of the current detection resistor so that the same voltage as that generated in the load circuit before switching is generated in the load circuit after switching. Since the input power is controlled, there is no risk of spike noise.

[0026]

According to the present invention, the current applied to the load circuit can be switched from a very small current to a large current without generating spike noise. Therefore, an element to be measured as a load circuit, for example, a semiconductor element or the like will not be adversely affected. Furthermore, unlike the past,
Since the semiconductor switch does not control the current applied to the load circuit and the current detection resistor, no current leakage occurs and a very small current can be detected.

[Brief description of drawings]

FIG. 1 is a block diagram showing a current detection circuit of the present invention.

FIG. 2 is a block diagram showing a conventional example of a current detection circuit.

FIG. 3 is a block diagram showing another conventional example of a current detection circuit.

FIG. 4 is a diagram showing changes in voltage or current at main points when a switch is switched from off to on in the current detection circuit of the present invention.

FIG. 5 is a diagram showing changes in voltage or current at main points when the switch is switched from on to off in the current detection circuit of the present invention.

[Explanation of symbols]

 1 Input Terminal 2 Input Resistor 3 Input Amplifier 4 Input Resistor 5 Current Detection Resistor 7 Current Detection Resistor 8 Switch 11 Buffer Amplifier 12 Differential Amplifier 20 Input Power Supply 30 Control Means 40 Analog-to-Digital Converter 50 Memory Means

Claims (1)

[Claims] 1. A current detecting resistor provided between an output terminal of an amplifier and a load circuit, an input power source for outputting a voltage applied to an input terminal of the amplifier, and a current detecting resistor. Detecting means for detecting a voltage, switching means for switching the resistance value of the current detecting resistor, voltage generated in the load circuit is fed back to the input of the amplifier, and the load circuit responds to the input voltage of the amplifier. Feedback means for generating an output voltage, a current value flowing in the load circuit is calculated from the resistance value of the current detection resistor and the voltage value detected by the detection means, and the voltage output from the input power supply is controlled. The control means, after switching the resistance value, when the resistance value is lowered by switching the resistance value of the current detecting resistor by the switch means. The voltage value to be generated at the output terminal of the amplifier by using the calculated current value in order to make the voltage value generated in the load circuit of the voltage value below the voltage value generated in the load circuit before the switching of the resistance value. Is calculated, and the voltage output from the input power source is changed immediately before switching the resistance value.