JPS59114467A - Amplifier circuit for measuring dc weak current - Google Patents

Abstract

PURPOSE:To suppress an unnecessary AC component and to enable the extension of a lead wire of an input terminal side, by providing a bridge circuit for suppressing AC to an input side. CONSTITUTION:An amplifier circuit for measuring a DC weak current is constituted of a circuit A to be measured, an input circuit part B and an amplifier circuit C while the AC bridge circuit of the input circuit part B constituted of variable resistance VR1, resistances R1, R2 and condensers C1, C2. In addition, resistance R3 is connected to the condenser C2 in parallel. When the AC component in voltage induced between points a, b adjusts the variable resistance VR1 to take the balance of a bridge, voltage comes to almost zero between points c, d, and the input of an operation amplifier OP1 also comes to almost zero. As a result, the extension of the lead wire between T1-T3, T2-T4 of a measuring input terminal side is enabled.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an amplifier circuit for measuring a direct current weak current, which is suitable for a circuit for measuring a weak ion current of an ionization vacuum needle used in a vacuum device or the like.

Conventionally, weak currents of less than 1μA have been amplified and measured using operational amplifiers, etc., and the measurement circuit includes one operational amplifier.
One or a combination of two or three instrumentation amplifiers are used. These amplifier circuits have a very high common-mode rejection ratio when the input is a balanced circuit, and have the feature of being able to sufficiently remove hum and high-frequency noise that is introduced by induction from the AC power line that enters the input.

However, in many cases, the input of the measurement circuit is unbalanced, and in this case it is masked by induced hum and noise, making measurement impossible. The method used is to shield the signal, amplify it to a certain level, and then input it to the main amplifier for measurement.

When using this method to sequentially measure a large number of objects to be measured within a short period of time, preamplifiers corresponding to the number of objects to be measured are prepared, and the preamplifiers are switched at high levels to sequentially measure them. However, this method requires a large number of preamplifiers;
The entire device becomes larger and also becomes more expensive. In addition, in terms of accuracy, there are also drawbacks such as errors in the preamplifier randomly entering the measured values.

The purpose of the present invention is to improve the above-mentioned drawbacks and to provide an amplifier circuit for measuring weak DC currents, which can measure weak currents with a single measurement amplifier without the need for a preamplifier directly connected to the object to be measured. be.

In order to achieve the above object, the amplifier circuit for measuring direct current weak current according to the present invention is provided between terminals for connecting the circuit to be measured, and one path is connected to a first resistor and a second resistor (resistance on the ground side of the measuring circuit). , the other path connects an AC bridge circuit consisting of a first capacitor and a second capacitor (capacitor on the measurement circuit ground side), and connects the midpoint of one path and the midpoint of the other path to an operational amplifier, respectively. A third resistor is connected in parallel to the second capacitor, and the AC bridge circuit is balanced in an alternating current manner. By adding an input circuit that matches the DC resistance from the circuit connection terminal to the inverting input terminal of the operational amplifier and the DC resistance to the non-inverting input terminal, calculate the potential drop due to the weak current generated across the second resistor. It is configured to be a DC voltage input to the amplifier.

According to the above configuration, compared to conventional measuring circuits, it is economical, the device is small, and it is easy to handle, so that the object of the present invention is completely achieved.

Hereinafter, the present invention will be explained in more detail with reference to the drawings.

FIG. 1 is a measurement circuit diagram showing an embodiment of an amplifier circuit for measuring direct current weak current according to the present invention. Part A is the circuit under test,
Part B shows an input circuit section, and part C shows a normal amplifier circuit.

Since the present invention is characterized by the input circuit section, the explanation will focus on that part. In the figure, 'r, -'r3. ．． T2-T
, indicates the input terminal of the circuit under test. The AC bridge circuit includes a variable resistor VR, a first resistor consisting of a resistor R1, a second resistor R2, a first capacitor C1, and a second capacitor C2. Further, a third resistor R3 is connected in parallel to the capacitor C2.

The self-current component of the voltage induced between points a and b is variable resistor VR1.
When the bridge is balanced by adjusting , the value between points c and d becomes approximately 0, and the input to operational amplifier OP1 also becomes approximately 0.

Regarding the DC component, the potential at point b is applied to the non-inverting input terminal (+) of the operational amplifier op through resistor R3, and the potential at point C relative to point b, that is, resistor R2, is applied to the inverting input terminal (-). Since the voltage generated by the flowing current is equal to the applied voltage, direct current can be measured.

The circuit in Figure 1 has an output of 100 m when the input is 1 nA.
This is an example of a circuit that converts and amplifies the voltage of V, and the maximum voltage is 1.99
By connecting a 9V digital portometer DVM, it is possible to measure o, oi to 19.99nA.

Capacitor C is used to ensure DC current input in the circuit.
2 is connected in parallel with resistor R3, so in order to balance the AC bridge, reactance Xc of capacitor c2 is
It is necessary to set the relationship between X c 2 and the paper end value of resistor R3 as X c 2<<R3, and to set the reactance of capacitor c2 at 50 Hz or 60 Hz of AC power frequency to 1/1o or less of R3.

Capacitor C2 if the overall system response speed allows
The more a sufficiently large capacitance is selected for the resistor R3, the better the AC component suppression ratio will be.

In the above circuit, the balancing resistor of the operational amplifier oP1 is the sum of the resistor R2, the resistor R5 inserted between the midpoint C of the AC bridge circuit, and the inverting input terminal of the operational amplifier op, and the resistor R3. Therefore, choose so that R2+R5=R3.

Normally, the input balance resistor of an operational amplifier is selected to be around IOKΩ or more, but nA in Figure 1
In the case of measurement, R2=R3=IMΩ, and R5 is omitted and connected directly. Further, as the operational amplifier OP1, it is necessary to select an operational amplifier with a bias current of pA class.

If the measured current is on the order of 1 μ8, R2
= IKΩ, so R2+R5=R3=l
It is better to set it to OKΩ. In this case, VR, , and R are naturally selected to be around IKΩ, and the capacitor CI + C2 is also selected to be 3.3 μF or more. An example of this case is shown in FIG. Note that the resistance R4 between T3a is the capacitor C,,C
2 was inserted to prevent it from becoming a direct load on the circuit under test.

The circuits downstream of the operational amplifier ○P1 perform a predetermined amplification operation, and the operational amplifier OP2 supplies an output corresponding to the input weak current to the digital meter DVM. VDD and Vss in each operational amplifier are power supplies.

^UTOOFF 5IET each shows an automatic offset zero circuit.

In this figure, the circuit under test is configured to supply a current to the object under test 1 according to the conditions of the object to be measured from a direct current power source DCPS. In such a circuit under test, if the ground of the circuit is set to 0 point, point b becomes a pseudo ground only for the measurement circuit, but in this case, VDD, V
As long as it is floating including the SS power supply, it will not affect the circuit of the present invention.

When using the circuit according to the present invention, T1-T3 on the measurement input terminal side. There is no problem at all even if a general insulated wire is stretched several meters to several tens of meters between T2 and T4 (measurement is possible. However, in this case, stray capacitance becomes a problem. In other words, the output of the DC power supply DCPS of the gold wave measurement circuit If IOV is IOV and the circuit current is 1 nA, the circuit resistance is 10 to the 10th power Ω, and if there is a stray capacitance of 200 pF, the time constant is 2 s.
It becomes ec. This variation in stray capacitance causes a transient phenomenon that affects the measured value. Therefore, it is better to use shielded wires to stabilize stray capacitance.

Even if shielded wires are used in the unbalanced input circuit of an ordinary amplifier for the purpose of measuring weak currents, unnecessary AC bones cannot be sufficiently removed if the wires are long.

As explained in detail above, according to the present invention, in the unbalanced input direct current weak current measurement amplifier circuit, by providing a bridge circuit for suppressing alternating current on the input side, unnecessary alternating current bones can be sufficiently suppressed (40 db or more). This makes it possible to extend the lead wire on the input terminal side. Therefore, it is possible to measure a large number of objects in a short time by connecting, for example, 50 to 100 parts to be measured as in the circuit shown in FIG. 1 and switching them sequentially using switches SW to SWn.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing an example of an amplifier circuit for measuring direct current weak current according to the present invention, and FIG. 2 is a circuit diagram showing an input circuit. 11-1n...Object to be measured SW1-SWn...Switch R1-RIO...Resistance VR, ~■R5...Variable resistor 01-C4...Capacitor 01', C2°...Capacitor ( Non-polar) DCPS・
...DC power supply DVM...Digicle portometer OPI + OF2...Operation amplifier Patent applicant Art Electronics Co., Ltd. Agent Patent attorney Hisashi Inoro

Claims (1)

[Claims] An AC bridge circuit consisting of a first resistor and a second resistor on one path and a first capacitor and a second capacitor on the other path is connected between the terminals for connecting the circuit under test. connecting the midpoint of the path and the midpoint of the other path to an inverting input terminal and a non-inverting input terminal of the operational amplifier, respectively;
Further, a third resistor is connected in parallel to the second capacitor to balance the AC bridge circuit in terms of AC, and
1511) Window circuit It has an input circuit in which the DC resistance from the circuit under test connection terminal on the ground side to the inverting input terminal of the operational amplifier matches the DC resistance to the non-inverting input terminal, and An amplifier circuit for measuring direct current weak current, characterized in that the generated direct current voltage is used as a direct current voltage input.