CN205786852U

CN205786852U - Highly sensitive anti-interference digital electrical conductivity instrument

Info

Publication number: CN205786852U
Application number: CN201620041271.6U
Authority: CN
Inventors: 冯越
Original assignee: Yancheng Teachers University
Current assignee: Yancheng Teachers University
Priority date: 2016-01-12
Filing date: 2016-01-12
Publication date: 2016-12-07
Anticipated expiration: 2026-01-12

Abstract

This utility model provides a kind of highly sensitive anti-interference digital electrical conductivity instrument, including push-pull oscillator circuit, measure electrode anode, measure Electrode Negative, selective circuit of measuring range, high impedance input circuit, rejector circuit, bandwidth-limited circuit, linear rectification circuit, direct current follow circuit, analog digital conversion and drive circuit, light-emitting diode display, instrument is provided with variable resistance and takes advantage of switch again, the Q-value of adjustable rejector circuit frequency characteristic and the amplification quantity of bandwidth-limited circuit, conductivity meter is provided with three grades of amplifications, highly sensitive, capacity of resisting disturbance is strong, environment 50Hz Hz noise can be effectively filtered out, eliminate the measurement error produced by 50Hz Hz noise, the minimum flow number of passes magnitude of instrument is little, device measures is to 10^‑12S/cm～10^‑14The faint electrical conductivity of S/cm, it is mainly used in the measurement of some solid-phase reaction system electrical conductivity in Solid state coordination research field it can also be used to pure water measurement, sewage disposal, environmental monitoring, petrochemical industry, biological medicine, mine smelt, universities and colleges and R&D institution.

Description

Highly sensitive anti-interference digital electrical conductivity instrument

Technical field

The present invention relates to a kind of conductivity meter, particularly relate to a kind of highly sensitive anti-interference conductivity meter, belong to electronic measuring instrument technical field.

Background technology

Conductivity meter is widely used in pure water and measures, sewage disposal, environmental monitoring, commercial production, petrochemical industry, biological medicine, mine smelts, universities and colleges and R&D institution, conventional conductivity meter is broadly divided into pointer-type, digital and intelligent three kinds, existing commercial conductivity meter is the most pointer or digital, 50Hz power frequency component filtering circuit is not all contained inside it, it is easily subject to environment 50Hz Hz noise, it is easily subject to external influence when measuring small electric conductance by lower range, the reliability of interferometry data, in addition, existing commercial conductivity meter sensitivity is relatively low, range ability is limited, range ability is generally 10^-2～10^-9S/cm, minimum range is 10^-9S/cm, but, in Solid state coordination research field, the electrical conductivity of solid-phase reaction system is mostly 10^-5～10^-9The S/cm order of magnitude, minority is 10^-10～10^-14The S/cm order of magnitude, existing conductivity meter can not meet the measurement requirement in this field.

Utility model content

The purpose of this utility model is to provide a kind of highly sensitive, anti-interference, digital conductivity meter, to meet the measurement requirement of some solid-phase reaction system electrical conductivity in Solid state coordination research field.

The technical problem to be solved is achieved through the following technical solutions: highly sensitive anti-interference digital electrical conductivity instrument includes push-pull oscillator circuit (1), measure electrode anode (2), measure Electrode Negative (3), selective circuit of measuring range (4), high impedance input circuit (5), rejector circuit (6), bandwidth-limited circuit (7), linear rectification circuit (8), direct current follow circuit (9), analog digital conversion and drive circuit (10), light-emitting diode display (11) and electric source filter circuit (12)；Transistor T₁, transistor T₂, resistance R₁, resistance R₂, resistance R₃, resistance R₄, resistance R₅, resistance R₇, variable resistance VR₁, variable resistance VR₂, electric capacity C₁, electric capacity C₂, electric capacity C₃, electric capacity C₄, electric capacity C₅, frequency selective switch S₁, correction switch S₂With transformator B composition push-pull oscillator circuit (1), resistance R₈, resistance R₉, resistance R₁₀, resistance R₁₁, resistance R₁₂, resistance R₁₃, resistance R₁₄With range switch S₃Composition selective circuit of measuring range (4), amplifier A₁With resistance R₁₄Composition high impedance input circuit (5), amplifier A₄, resistance R₂₆, resistance R₂₇, resistance R₂₈, resistance R₂₉, resistance R₃₀, variable resistance VR₄, electric capacity C₁₃, electric capacity C₁₄, electric capacity C₁₅, electric capacity C₁₆, electric capacity C₁₇, electric capacity C₁₈, diode D₁With diode D₂Composition linear rectification circuit (8), field effect transistor T₃, resistance R₃₁, resistance R₃₂, resistance R₃₃, resistance R₃₄, resistance R₃₅With variable resistance VR₅Composition direct current follow circuit (9), IC L, resistance R₃₇, resistance R₃₈, resistance R₃₉, variable resistance VR₆, electric capacity C₂₀, electric capacity C₂₁, electric capacity C₂₂, electric capacity C₂₃With electric capacity C₂₄Composition analog digital conversion and drive circuit (10), charactron LED composition light-emitting diode display (11), resistance R₆, resistance R₃₆, electric capacity C₆, electric capacity C₇With electric capacity C₁₉Composition electric source filter circuit (12)；

nullThe order of connection of each circuit of described conductivity meter is followed successively by push-pull oscillator circuit (1)、Measure electrode anode (2)、Conductance cell (G)、Measure Electrode Negative (3)、Selective circuit of measuring range (4)、High impedance input circuit (5)、Rejector circuit (6)、Bandwidth-limited circuit (7)、Linear rectification circuit (8)、Analog digital conversion and drive circuit (10) and light-emitting diode display (11)，The linear rectification circuit of the signal (8) that bandwidth-limited circuit (7) exports amplifies and becomes after rectification direct current signal point two-way output，One road signal carries out following through direct current follow circuit (9) and is exported and external function recording instrument by OUT terminal after impedance transformation，Another road signal promotes light-emitting diode display (11) work and demonstrate the numerical value of electrical conductivity after analog digital conversion and the conversion of drive circuit (10) and driving，Described conductivity meter is provided with rejector circuit (6)，Rejector circuit (6) is by amplifier A₂, resistance R₁₅, resistance R₁₆, resistance R₁₇, resistance R₁₈, resistance R₁₉, variable resistance VR₃, electric capacity C₈, electric capacity C₉With electric capacity C₁₀Composition, wherein, resistance R₁₅, resistance R₁₇With electric capacity C₉Constituting T-shaped low pass filter, the upper limiting frequency of low pass filter is set to 50Hz, i.e. the frequency signal less than 50Hz can pass through low pass filter, electric capacity C₈, electric capacity C₁₀With resistance R₁₉Constitute T-shaped high pass filter, the lower frequency limit of high pass filter is set to 50Hz, the i.e. frequency signal higher than 50Hz can pass through high pass filter, band elimination filter is become after T-shaped low pass filter and T-shaped high pass filter parallel connection, frequency can be transferred through band elimination filter less than 50Hz or the signal higher than 50Hz, and the frequency signal equal to 50Hz can not pass through band elimination filter；

Described conductivity meter is provided with bandwidth-limited circuit (7), and bandwidth-limited circuit (7) is by amplifier A₃, resistance R₂₀, resistance R₂₁, resistance R₂₂, resistance R₂₃, resistance R₂₄, resistance R₂₅, electric capacity C₁₁, electric capacity C₁₂Take advantage of switch S again₄Composition, wherein, resistance R₂₀With electric capacity C₁₂Constituting low pass filter, the upper limiting frequency of low pass filter is set to 230Hz, i.e. the frequency signal less than 230Hz can pass through low pass filter, electric capacity C₁₁With resistance R₂₄Constitute high pass filter, the lower frequency limit of high pass filter is set to 140Hz, the i.e. frequency signal higher than 140Hz can pass through high pass filter, band filter is become after low pass filter and high pass filter series connection, frequency range is that the signal of 140Hz～230Hz can pass through band filter, and the frequency signal equal to 50Hz can not pass through band filter；

After the measurement signal produced by push-pull oscillator circuit (1) passes sequentially through measurement electrode anode (2), conductance cell (G), measurement Electrode Negative (3), selective circuit of measuring range (4), high impedance input circuit (5), rejector circuit (6), bandwidth-limited circuit (7), linear rectification circuit (8), analog digital conversion and drive circuit (10), arriving light-emitting diode display (11), described conductivity meter is provided with variable resistance VR₃Take advantage of switch S again₄, the Q-value of variable resistance VR3 adjustable rejector circuit (6) frequency characteristic, take advantage of the amplification quantity of switch adjustable bands bandpass filter circuit (7) again, described conductivity meter is provided with three grades of amplifications, respectively amplifier A₂, amplifier A₃With amplifier A₄, the minimum range of conductivity meter is finally reached 10^-12S/cm, resolution is 10^-14S/cm。

Owing to using technique scheme, advantage for present invention and good effect be: instrument highly sensitive, and minimum flow number of passes magnitude is little, can measure 10^-12S/cm～10^-14The faint electrical conductivity of S/cm, can meet the measurement requirement of some solid-phase reaction system electrical conductivity in Solid state coordination research field, and the capacity of resisting disturbance of instrument is strong, can effectively filter out environment 50Hz Hz noise, eliminates the measurement error produced by 50Hz Hz noise.

Accompanying drawing explanation

The present invention is further described with embodiment below in conjunction with the accompanying drawings, and the present invention has following 3 width accompanying drawings:

Fig. 1 is circuit structure block diagram of the present utility model,

Fig. 2 is circuit structure diagram of the present utility model,

Fig. 3 is circuit theory diagrams of the present utility model.

Marked each numeral is expressed as follows respectively:

1. push-pull oscillator circuit, 2. measures electrode anode, 3. measures Electrode Negative, 4. selective circuit of measuring range, 5. high impedance input circuit, 6. rejector circuit, 7. bandwidth-limited circuit, 8. linear rectification circuit, 9. direct current follow circuit, 10. analog digital conversion and drive circuit, 11.LED display, 12. electric source filter circuits, A₁. amplifier, A₂. amplifier, A₃. amplifier, A₄. amplifier, B. transformator, C₁. electric capacity, C₂. electric capacity, C₃. electric capacity, C₄. electric capacity, C₅. electric capacity, C₆. electric capacity, C₇. electric capacity, C₈. electric capacity, C₉. electric capacity, C₁₀. electric capacity, C₁₁. electric capacity, C₁₂. electric capacity, C₁₃. electric capacity, C₁₄. electric capacity, C₁₅. electric capacity, C₁₆. electric capacity, C₁₇. electric capacity, C₁₈. electric capacity, C₁₉. electric capacity, C₂₀. electric capacity, C₂₁. electric capacity, C₂₂. electric capacity, C₂₃. electric capacity, C₂₄. electric capacity, D₁. diode, D₂. diode, G. conductance cell, OUT. signal output part, R₁. resistance, R₂. resistance, R₃. resistance, R₄. resistance, R₅. resistance, R₆. resistance, R₇. resistance, R₈. resistance, R₉. resistance, R₁₀. resistance, R₁₁. resistance, R₁₂. resistance, R₁₃. resistance, R₁₄. resistance, R₁₅. resistance, R₁₆. resistance, R₁₇. resistance, R₁₈. resistance, R₁₉. resistance, R₂₀. resistance, R₂₁. resistance, R₂₁. resistance, R₂₂. resistance, R₂₃. resistance, R₂₄. resistance, R₂₅. resistance, R₂₆. resistance, R₂₇. resistance, R₂₈. resistance, R₂₉. resistance, R₃₀. resistance, R₃₁. resistance, R₃₂. resistance, R₃₃. resistance, R₃₄. resistance, R₃₅. resistance, R₃₆. resistance, R₃₇. resistance, R₃₈. resistance, R₃₉. resistance, S₁. frequency selective switch, S₂. correction switch, S₃. range switch, S₄. take advantage of switch, T again₁. transistor, T₂. transistor, T₃. field effect transistor, V+. positive source, V-. power cathode, VR₁. variable resistance, VR₂. variable resistance, VR₃. variable resistance, VR₄. variable resistance, VR₅. variable resistance, VR₆. variable resistance, LED. charactron, ICL. integrated circuit.

Detailed description of the invention

1., according to Fig. 1 to Fig. 3, highly sensitive anti-interference digital electrical conductivity instrument includes push-pull oscillator circuit (1), measures electrode anode (2), measurement Electrode Negative (3), selective circuit of measuring range (4), high impedance input circuit (5), rejector circuit (6), bandwidth-limited circuit (7), linear rectification circuit (8), direct current follow circuit (9), analog digital conversion and drive circuit (10), light-emitting diode display (11) and electric source filter circuit (12)；Transistor T₁, transistor T₂, resistance R₁, resistance R₂, resistance R₃, resistance R₄, resistance R₅, resistance R₇, variable resistance VR₁, variable resistance VR₂, electric capacity C₁, electric capacity C₂, electric capacity C₃, electric capacity C₄, electric capacity C₅, frequency selective switch S₁, correction switch S₂With transformator B composition push-pull oscillator circuit (1), resistance R₈, resistance R₉, resistance R₁₀, resistance R₁₁, resistance R₁₂, resistance R₁₃, resistance R₁₄With range switch S₃Composition selective circuit of measuring range (4), amplifier A₁With resistance R₁₄Composition high impedance input circuit (5), amplifier A₄, resistance R₂₆, resistance R₂₇, resistance R₂₈, resistance R₂₉, resistance R₃₀, variable resistance VR₄, electric capacity C₁₃, electric capacity C₁₄, electric capacity C₁₅, electric capacity C₁₆, electric capacity C₁₇, electric capacity C₁₈, diode D₁With diode D₂Composition linear rectification circuit (8), field effect transistor T₃, resistance R₃₁, resistance R₃₂, resistance R₃₃, resistance R₃₄, resistance R₃₅With variable resistance VR₅Composition direct current follow circuit (9), IC L, resistance R₃₇, resistance R₃₈, resistance R₃₉, variable resistance VR₆, electric capacity C₂₀, electric capacity C₂₁, electric capacity C₂₂, electric capacity C₂₃With electric capacity C₂₄Composition analog digital conversion and drive circuit (10), charactron LED composition light-emitting diode display (11), resistance R₆, resistance R₃₆, electric capacity C₆, electric capacity C₇With electric capacity C₁₉Composition electric source filter circuit (12).

2. the conductivity meter described in is provided with rejector circuit (6), and rejector circuit (6) is by amplifier A₂, resistance R₁₅, resistance R₁₆, resistance R₁₇, resistance R₁₈, resistance R₁₉, variable resistance VR₃, electric capacity C₈, electric capacity C₉With electric capacity C₁₀Composition, wherein, resistance R₁₅, resistance R₁₇With electric capacity C₉Constituting T-shaped low pass filter, the upper limiting frequency of low pass filter is set to 50Hz, i.e. the frequency signal less than 50Hz can pass through low pass filter, electric capacity C₈, electric capacity C₁₀With resistance R₁₉Constitute T-shaped high pass filter, the lower frequency limit of high pass filter is set to 50Hz, the i.e. frequency signal higher than 50Hz can pass through high pass filter, band elimination filter is become after low pass filter and high pass filter parallel connection, frequency can be transferred through band elimination filter less than 50Hz or the signal higher than 50Hz, and the frequency signal equal to 50Hz can not pass through band elimination filter.

3. the conductivity meter described in is provided with bandwidth-limited circuit (7), and bandwidth-limited circuit (7) is by amplifier A₃, resistance R₂₀, resistance R₂₁, resistance R₂₂, resistance R₂₃, resistance R₂₄, resistance R₂₅, electric capacity C₁₁, electric capacity C₁₂Take advantage of switch S again₄Composition, wherein, resistance R₂₀With electric capacity C₁₂Constituting low pass filter, the upper limiting frequency of low pass filter is set to 230Hz, i.e. the frequency signal less than 230Hz can pass through low pass filter, electric capacity C₁₁With resistance R₂₄Constitute high pass filter, the lower frequency limit of high pass filter is set to 140Hz, the i.e. frequency signal higher than 140Hz can pass through high pass filter, band filter is become after low pass filter and high pass filter series connection, frequency range is that the signal of 140Hz～230Hz can pass through band filter, and the frequency signal equal to 50Hz can not pass through band filter.

null4. the workflow of instrument is: push-pull oscillator circuit (1) produces measures signal，Measure signal to be transported to measure electrode anode (2) from the lead-in wire extraction of B level of transformator，Measure measurement signal loading on electrode anode (2) to conductance cell (G)，Signal is after the electric resistance partial pressure in conductance cell (G) with selective circuit of measuring range (4)，By measurement Electrode Negative (3), signal is transported to high impedance input circuit (5) carry out following and impedance transformation，Then signal is amplified through rejector circuit (6) and bandwidth-limited circuit (7) and filters，Filter 50Hz Hz noise，Amplify through linear rectification circuit (8) after signal and become direct current signal after rectification，Direct current signal divides two-way to export，One road signal carries out following with impedance transformation through direct current follow circuit (9) and is exported by OUT terminal，So as external function recording instrument；One road signal promotes light-emitting diode display (11) work after analog digital conversion and the conversion of drive circuit (10) and driving, demonstrates the numerical value of electrical conductivity.

5. the measurement signal produced by push-pull oscillator circuit (1) passes sequentially through measurement electrode anode (2), conductance cell (G), measure Electrode Negative (3), selective circuit of measuring range (4), high impedance input circuit (5), rejector circuit (6), bandwidth-limited circuit (7), linear rectification circuit (8), after analog digital conversion and drive circuit (10), arrive light-emitting diode display (11), described conductivity meter is provided with variable resistance VR3 and takes advantage of switch S4 again, the Q-value of variable resistance VR3 adjustable rejector circuit (6) frequency characteristic, take advantage of the amplification quantity of switch adjustable bands bandpass filter circuit (7) again, described conductivity meter is provided with three grades of amplifications, it is respectively amplifier A₂, amplifier A₃With amplifier A₄, the increase of amplification quantity makes the minimum range of conductivity meter be finally reached 10^-12S/cm, resolution is 10^-14S/cm。

6. the signal that push-pull oscillator circuit (1) produces is AC signal, and virtual value is 2 volts, and frequency of oscillation is by the primary electrical sensibility reciprocal of transformator B and electric capacity C₂And electric capacity C₃Determine, frequency selective switch S₁Frequency of oscillation can be changed, be low week 140Hz during Guan Bi, be high week 230Hz during open circuit；Resistance R₁, resistance R₂With variable resistance VR₂Composition bleeder circuit, variable resistance VR₂With correction switch S₂Calibrate for instrument full scale, for calibration when upwards dialling, for measuring in time setting aside；Variable resistance VR₁For the capacitance compensation of solid phase conductance cell (G), electric capacity C₁For every directly, resistance R₃For regulated output voltage.

7. variable resistance VR₃The Q-value of rejector circuit (6) frequency characteristic can be changed, take advantage of switch S again₄Can change the amplification quantity of bandwidth-limited circuit (7), during Guan Bi, sensitivity is low, highly sensitive during open circuit, and range is expansible × and 10, variable resistance VR₅Scalable DC balance, it is ensured that during without measuring signal, the direct current signal of the OUT outfan of instrument is zero.

8. the model of IC L is ICL7107, resistance R₃₇With electric capacity C₂₀For the RC network of clock oscillator, determine clock frequency；Resistance R₃₆With variable resistance VR₆On the basis of the bleeder circuit of voltage, for adjusting the sensitivity of numeral table；Electric capacity C₂₁On the basis of electric capacity；Electric capacity C₂₂With resistance R₃₈Constitute filter circuit together, capacity of resisting disturbance can be improved；Electric capacity C₂₃For automatic zero set electric capacity, resistance R₃₉With electric capacity C₂₄Being respectively integrating resistor and integrating capacitor, IC L includes drive circuit, can directly drive light-emitting diode display (11) to work.

9. the frequency characteristic of wave filter can use formula f=1/ (2 π RC) to calculate, and f mono-timing, RC can have multiple combination, and in the present embodiment, the adsorption frequency of rejector circuit (6) is set to 50Hz, low pass filter resistance R₁₅=R₁₇=47K Ω, electric capacity C₉=136nF, the electric capacity C of high pass filter₈=C₁₀=68nF, resistance R₁₉=23.5K Ω；The frequency range of bandwidth-limited circuit (7) is set to 140Hz～230Hz, the resistance R of low pass filter₂₀=6.8K Ω, electric capacity C₁₂=100nF, the electric capacity C of high pass filter₁₁=100nF, resistance R₂₄=12K Ω.

Claims

1. a highly sensitive anti-interference digital electrical conductivity instrument, including push-pull oscillator circuit (1), measure electrode anode (2), measurement Electrode Negative (3), selective circuit of measuring range (4), high impedance input circuit (5), rejector circuit (6), bandwidth-limited circuit (7), linear rectification circuit (8), direct current follow circuit (9), analog digital conversion and drive circuit (10), light-emitting diode display (11) and electric source filter circuit (12), transistor T₁, transistor T₂, resistance R₁, resistance R₂, resistance R₃, resistance R₄, resistance R₅, resistance R₇, variable resistance VR₁, variable resistance VR₂, electric capacity C₁, electric capacity C₂, electric capacity C₃, electric capacity C₄, electric capacity C₅, frequency selective switch S₁, correction switch S₂With transformator B composition push-pull oscillator circuit (1), resistance R₈, resistance R₉, resistance R₁₀, resistance R₁₁, resistance R₁₂, resistance R₁₃, resistance R₁₄With range switch S₃Composition selective circuit of measuring range (4), amplifier A₁With resistance R₁₄Composition high impedance input circuit (5), amplifier A₄, resistance R₂₆, resistance R₂₇, resistance R₂₈, resistance R₂₉, resistance R₃₀, variable resistance VR₄, electric capacity C₁₃, electric capacity C₁₄, electric capacity C₁₅, electric capacity C₁₆, electric capacity C₁₇, electric capacity C₁₈, diode D₁With diode D₂Composition linear rectification circuit (8), field effect transistor T₃, resistance R₃₁, resistance R₃₂, resistance R₃₃, resistance R₃₄, resistance R₃₅With variable resistance VR₅Composition direct current follow circuit (9), IC L, resistance R₃₇, resistance R₃₈, resistance R₃₉, variable resistance VR₆, electric capacity C₂₀, electric capacity C₂₁, electric capacity C₂₂, electric capacity C₂₃With electric capacity C₂₄Composition analog digital conversion and drive circuit (10), charactron LED composition light-emitting diode display (11), resistance R₆, resistance R₃₆, electric capacity C₆, electric capacity C₇With electric capacity C₁₉Composition electric source filter circuit (12)；

nullIt is characterized in that: the order of connection of each circuit of described conductivity meter is followed successively by push-pull oscillator circuit (1)、Measure electrode anode (2)、Conductance cell (G)、Measure Electrode Negative (3)、Selective circuit of measuring range (4)、High impedance input circuit (5)、Rejector circuit (6)、Bandwidth-limited circuit (7)、Linear rectification circuit (8)、Analog digital conversion and drive circuit (10) and light-emitting diode display (11)，The linear rectification circuit of the signal (8) that bandwidth-limited circuit (7) exports amplifies and becomes after rectification direct current signal point two-way output，One road signal carries out following through direct current follow circuit (9) and is exported and external function recording instrument by OUT terminal after impedance transformation，Another road signal promotes light-emitting diode display (11) work and demonstrate the numerical value of electrical conductivity after analog digital conversion and the conversion of drive circuit (10) and driving，Described conductivity meter is provided with rejector circuit (6)，Rejector circuit (6) is by amplifier A₂, resistance R₁₅, resistance R₁₆, resistance R₁₇, resistance R₁₈, resistance R₁₉, variable resistance VR₃, electric capacity C₈, electric capacity C₉With electric capacity C₁₀Composition, wherein, resistance R₁₅, resistance R₁₇With electric capacity C₉Constituting T-shaped low pass filter, the upper limiting frequency of low pass filter is set to 50Hz, i.e. the frequency signal less than 50Hz can pass through low pass filter, electric capacity C₈, electric capacity C₁₀With resistance R₁₉Constitute T-shaped high pass filter, the lower frequency limit of high pass filter is set to 50Hz, the i.e. frequency signal higher than 50Hz can pass through high pass filter, band elimination filter is become after T-shaped low pass filter and T-shaped high pass filter parallel connection, frequency can be transferred through band elimination filter less than 50Hz or the signal higher than 50Hz, and the frequency signal equal to 50Hz can not pass through band elimination filter.

Highly sensitive anti-interference digital electrical conductivity instrument the most according to claim 1, it is characterised in that: described conductivity meter is provided with bandwidth-limited circuit (7), and bandwidth-limited circuit (7) is by amplifier A₃, resistance R₂₀, resistance R₂₁, resistance R₂₂, resistance R₂₃, resistance R₂₄, resistance R₂₅, electric capacity C₁₁, electric capacity C₁₂Take advantage of switch S again₄Composition, wherein, resistance R₂₀With electric capacity C₁₂Constituting low pass filter, the upper limiting frequency of low pass filter is set to 230Hz, i.e. the frequency signal less than 230Hz can pass through low pass filter, electric capacity C₁₁With resistance R₂₄Constitute high pass filter, the lower frequency limit of high pass filter is set to 140Hz, the i.e. frequency signal higher than 140Hz can pass through high pass filter, band filter is become after low pass filter and high pass filter series connection, frequency range is that the signal of 140Hz～230Hz can pass through band filter, and the frequency signal equal to 50Hz can not pass through band filter.

Highly sensitive anti-interference digital electrical conductivity instrument the most according to claim 1, it is characterized in that: after the measurement signal produced by push-pull oscillator circuit (1) passes sequentially through measurement electrode anode (2), conductance cell (G), measurement Electrode Negative (3), selective circuit of measuring range (4), high impedance input circuit (5), rejector circuit (6), bandwidth-limited circuit (7), linear rectification circuit (8), analog digital conversion and drive circuit (10), arriving light-emitting diode display (11), described conductivity meter is provided with variable resistance VR₃Take advantage of switch S again₄, variable resistance VR₃The Q-value of adjustable rejector circuit (6) frequency characteristic, takes advantage of the amplification quantity of switch adjustable bands bandpass filter circuit (7) again, is provided with three grades of amplifications, respectively amplifier A in described conductivity meter₂, amplifier A₃With amplifier A₄, the minimum range of conductivity meter is finally reached 10^-12S/cm, resolution is 10^-14S/cm。