JPH0690233B2 - Micro current measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is used in a device requiring a particularly wide dynamic range and quick measurement, such as a gas chromatograph mass spectrometer and an ionization vacuum gauge for high-speed use. The present invention relates to the configuration of a minute current measuring device.

[Prior Art] Conventionally, as a micro-current measuring device requiring a wide dynamic range and quick measurement, as shown in FIG. 4, conventionally, a non-inverting input terminal NT of a high input impedance operational amplifier IC is grounded, A so-called current-voltage conversion circuit in which a feedback resistor Rf is inserted between the inverting input terminal IT and the output OT and a measured current is input to the inverting input terminal IT as an input terminal is exclusively used.

Then, in order to widen the dynamic range of the measurement, as shown in FIG. 5, by using the contacts RL1 to RL3 of the relay (or by using the switching device using the field effect transistor), the resistance value is changed. A method has been adopted in which a plurality of feedback resistors Rf1 to Rf3 having a large difference are sequentially switched to perform measurement.

In addition to this, as a circuit for widening the dynamic range and for quick measurement, there is a circuit using a logarithmic amplifier whose basic circuit configuration is shown in FIG. 6, and this circuit has a maximum sensitivity of 10 ^-9. The current of unit A can be measured.

[Problems to be Solved by the Invention] However, in the above-mentioned conventional circuit, when the feedback resistor is switched by using the relay or the field effect transistor, particularly when the measurement is switched to the high sensitivity measurement, the output is stabilized. It has a drawback that it takes time and cannot measure quickly.

Further, even in a circuit using a logarithmic amplifier, for example, in the case of a mass spectrometer, the maximum sensitivity of 10 ^-9 A level is insufficient in sensitivity, so that it cannot be adopted.

As a countermeasure, a minute current measuring device in which a plurality of high-sensitivity and low-sensitivity current-voltage conversion circuits are prepared and these are switched to perform detection has been conceivable. However, two detection units are mechanically configured. Doing so made the device expensive and could not be adopted.

[Object of the Invention] An object of the present invention is to solve this problem and to provide an inexpensive micro-current measuring device having a sufficiently wide dynamic range and capable of rapid measurement.

[Means for Solving the Problems] In order to achieve the above-mentioned object, the present invention provides an input of a current to be measured, which is composed of a high input impedance operational amplifier in which an input end and an output end are connected by a high resistor. A current-voltage conversion circuit, a detection resistor inserted in series at the input end of the current-voltage conversion circuit, and a current bypass element protecting the high input impedance operational amplifier at the time of excessive input inserted in parallel at the input end. And a high input impedance differential amplifier circuit that receives the voltage across the detection resistor as an input, and outputs a high-sensitivity current detection value to the output terminal of the current-voltage conversion circuit, while simultaneously providing a high input impedance differential circuit. The configuration is such that the low-sensitivity current detection value is output to the output terminal of the amplifier circuit.

The output of the high input impedance differential amplifier circuit is used as the low sensitivity input, the output of the current-voltage conversion circuit is used as the high sensitivity input, and one of these two inputs is selected and output as the current measurement value. When providing the selection circuit, a very efficient micro-current measuring device is obtained.

[Operation] The high-sensitivity detection of the current value is obtained as the output voltage of the current-voltage conversion circuit, and the value is a value obtained by multiplying the high-sensitivity feedback resistance value by the measured current value.

The low sensitivity detection is obtained as the output voltage of the high input impedance differential amplifier when this high sensitivity detection is saturated.
That is, in this case, since the high-input impedance operational amplifier is saturated and the measured input current is shunted to the current bypass element, the presence of the current-voltage conversion circuit does not affect the low-sensitivity detection. The high-impedance differential amplifier detects and outputs the voltage appearing across both ends of the current to be measured flowing through the detection resistor as an output for low sensitivity.

Then, when the high-sensitivity detection is saturated, the low-sensitivity output is selected, and when the high-sensitivity detection is not saturated, the high-sensitivity output is selected, and this is set as the current measurement value. A minute current measuring device capable of quick measurement can be obtained.

[Embodiment] FIG. 1 is an embodiment of the present invention, and is a circuit diagram of a current detection unit of a microcurrent measuring device of the present invention, in which an output of a secondary electron multiplier of a mass spectrometer is a measured current. is there. Therefore, this current detecting section is configured to detect a negative input current to be measured.

In this figure, the negative current i input to the input terminal I is
(For low sensitivity) Inverting input terminal IT1 (higher input impedance operational amplifier IC1 via detection resistor R1) (other input terminal NT1
Reach the ground), but as long as the measured input current i is small,
Since the input impedance of IC1 is large enough not be infinite, current to the inverting input terminal IT1 from the IC1 is not flow, the current i reaches output terminal O _H of IC1 through the feedback resistor R2.

Therefore, while the measured input current i is small, the operational amplifier IC
The current-voltage conversion circuit CONV is formed by 1 and the feedback resistor R2, and the sensitivity of the measurement will be determined by the feedback resistor R2. Output obtained on the output terminal O _H is output sensitivity.

The absolute value of the measured input current i increases and the operational amplifier IC
When the output of 1 is saturated, it can no longer flow to the feedback resistor R2, and the measured current i begins to be shunted to the diode D1 used as a current bypass element. However, a current that is not influenced by the current-voltage conversion circuit CONV still flows through the detection resistor R1.

The voltage appearing across the detection resistor R1 is the resistance value of R1 multiplied by the value of the measured input current i. This voltage is sent to a high input impedance differential amplifier circuit DA having a well-known circuit configuration composed of the operational amplifiers IC2, IC3, and IC4, and a low sensitivity output is obtained at its output terminal O _L.

As described above, the high sensitivity output terminals O _H corresponding to the value of the resistor R2, low sensitivity for output corresponding to the gain of the resistor R1 a high input impedance differential amplifier circuit DA is the terminal O _L , Both outputs will be obtained at the same time.

The two outputs thus obtained at the same time can be added to various circuits at the subsequent stage to form a convenient device suitable for the application. For example, both outputs can be combined as shown in FIG. Multiplexer MUTI with analog switch ASW, A / D converter ADC, microprocessor MP
Input to the selection circuit SES consisting U, Within the selection circuit SES, the O _H side output unless the output sensitivity of the microprocessor MPU is connected to an output terminal O _H is saturated, when saturated output O _L side By outputting a select signal SL that selects the low-sensitivity output and switching the analog switch ASW in the multiplexer MUTI with that signal, the equivalent dynamic range is extremely wide and quick measurement is possible. A microcurrent measuring device with excellent performance can be obtained.

In the device of this embodiment, the time-consuming part is the A / D converter. But A / D converter AD
The conversion time of C is significantly shorter than the time required for the output to stabilize in the case of switching the relay described in FIG.
Therefore, the measurement is extremely quick.

As another example, as shown in FIG. 3, a comparator CMP using an operational amplifier is prepared for the selection circuit SES,
O _H The output sensitivity obtained, compared with the reference voltage V _F which is provided to detect the saturation of the operational amplifier, the comparator
The output of the CMP is used as the select signal SL to switch the analog switch ASW in the multiplexer MUTI so that the same select signal SL is given to the microprocessor MPU. This way, the microprocessor MPU
Can easily determine whether the data converted by the A / D converter ADC and input is the low sensitivity output of the terminal O _L or the high sensitivity output of the terminal O _H. At this time, if a relation that bit shift can be performed between the high-sensitivity output of the terminal O _{H and} the low-sensitivity output of the terminal O _L , that is, a relation of 2n times is provided, the data processing time in the microprocessor MPU becomes shorter. Needless to say.

[Effects of the Invention] According to the present invention, it is possible to obtain both the low-sensitivity output and the high-sensitivity output simultaneously and continuously with a single current detection unit. Then, by adding a circuit for automatically selecting and outputting the effective side of those outputs, a minute current measuring device capable of quickly measuring an input current to be measured having an extremely wide dynamic range can be obtained. .

Among the applications of the mass spectrometer, particularly rapid measurement is required. For example, the gas chromatograph mass spectrometer has a current value range of about 1 × 10 ⁻⁵ A to 1 × 10 ⁻¹² A, and Large and small signals are mixed and input. The microcurrent measuring device of the present invention is particularly effective for use in this case.

Conventionally, a current-voltage conversion circuit as shown in Fig. 5 was used to switch the feedback resistor with a relay, etc., so that quick measurement is impossible. For quick measurement, keep the sensitivity constant. There was no choice but to measure, and the dynamic range was sacrificed. The present invention solves that completely.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a current detector of the present invention. FIG. 2 is a block diagram of a minute current measuring device according to an embodiment of the present invention incorporating the same. FIG. 3 is a block diagram of another embodiment. FIG. 4 is a circuit diagram of a conventional minute current measuring circuit. FIG. 5 is a circuit diagram of the one whose dynamic range is widened. FIG. 6 is a circuit diagram of a conventional minute current measuring circuit using a logarithmic amplifier. IC1, IC2, IC3 ... High input impedance operational amplifier, IC4
...... operational amplifier, D1, D2 ...... diodes, R1 ...... sense resistor, R2 ...... feedback resistor, R3～R9 ...... resistor, I ...... input terminal, OT, O ...... output terminals, O _H ...... High sensitivity output terminal, O _L …… Low sensitivity output terminal, i
...... Measured input current, MUTI …… Multiplexer, ADC…
… A / D covert, MPU… Microprocessor, SL…
Select signal, SES ... Selection circuit.

Claims (2)

[Claims] 1. A current-voltage conversion circuit, which is composed of a high input impedance operational amplifier in which an input end and an output end are connected by a high resistor, and into which a measured current is input, and a serial connection to the input end of the current-voltage conversion circuit. An inserted detection resistor, a current bypass element inserted in parallel with the input end to protect the high input impedance operational amplifier at the time of over-input, and a high input impedance differential amplification inputting the voltage across the detection resistor. A high-sensitivity current detection value is output to the output terminal of the current-voltage conversion circuit, and at the same time, a low-sensitivity current detection value is output to the output terminal of the high-input impedance differential amplifier circuit. A minute current measuring device characterized in that 2. An output of the high input impedance differential amplifier circuit is used as an input for low sensitivity, an output of the current-voltage conversion circuit is used as an input for high sensitivity, and one of these two inputs is selected as a current measurement value. The microcurrent measuring device according to claim 1, further comprising a selection circuit for outputting.