US 5,317,275 A · Conductance measurement circuit with wide dynamic range

Abstract

A conductance measurement circuit to measure conductance of a solution under test with an output voltage proportional to conductance over a 5-decade range, i.e., 0.01 uS to 1000 uS or from 0.1 uS to 10,000 uS. An increase in conductance indicates growth, or multiplication, of the bacteria in the test solution. Two circuits are used each for an alternate half-cycle time periods of an alternate squarewave in order to cause alternate and opposite currents to be applied to the test solution. The output of one of the two circuits may be scaled for a different range optimum switching frequency dependent upon the solution conductance and to enable uninterrupted measurement over the complete 5-decade range. This circuitry provides two overlapping ranges of conductance which can be read simultaneously without discontinuity thereby eliminating range switching within the basic circuitry. A VCO is used to automatically change the operating frequency according to the particular value of the conductance being measured, and comparators indicate which range is valid and also facilitate computer-controlled data acquisition. A multiplexer may be used to monitor any number of solutions under test continuously.

Claims19

Dependent claims

A circuit for measuring the conductance of a solution under test which contains biological growth media where an increase in the conductance of the solution indicates growth, or multiplication, of bacteriological colonies and in which a pair of spaced-apart electrodes are placed for applying current to said solution, comprising,

first means connected to said electrodes for applying current to said electrodes and for maintaining a voltage drop at a predetermined value regardless of any change in conductance of said solution,
second means connected to said first means for adjusting the current necessary to maintain said voltage drop at said predetermined value,
third means connected to said second means responsive to said second means to provide a current output responsive to the change in voltage drop so that the output of said second means remains constant,
said second and third means including means which responds to a small change in voltage drop provides a large current output to produce a wide range of conductance measurement, and
fourth means connected to said third means for producing an output proportional to conductance of said solution.

2 more elements

means connected to said electrodes for applying current to provide a voltage drop of a predetermined value across said solution,
conductance responsive means having an output connected to one electrode and an input connected to the other electrode so that said conductive responsive means will have the same voltage drop as that of said solution,
a pair of circuits alternately connectable to said conductance responsive means and to said electrodes,
means for alternately connecting each of said pair of circuits to said conductance responsive means and to said electrodes,
each circuit having,
means for applying an alternating AC current in square wave form to said conductance responsive means and to said electrodes to provide a peak voltage across said solution,
means for maintaining the output voltage of said conductance responsive means at said predetermined value by varying the current level applied to said conductance responsive means and to said electrodes according to the change in voltage across said solution,
means responsive to said variation in said current level for producing a circuit voltage output proportional to the change in voltage determined by said conductance responsive means, and
means connected to both circuits for determining the output range of the two circuits producing a circuit output voltage representative of the change of conductance in said solution.

6 more elements

A circuit for measuring the conductance of a solution under test which contains biological growth media where an increase in the conductance of the solution indicates growth, or, multiplication, of bacteriological colonies and in which a pair of spaced-apart electrodes are placed for applying current to said solution, comprising,

an operational amplifier with its output connected to one electrode and its negative input connected to the other electrode for applying current to said electrodes and to said solution so that said operational amplifier responds to the conductance of said solution and produces an output voltage of a preselected value, and
a variable voltage source connected to said output of said amplifier and a transistor connected as a diode between said voltage source and said negative input for applying current to said negative input for maintaining the output voltage of said amplifier at said preselected value regardless of any change in conductance of said solution,
said transistor having a logarithmic output response so that a small change in voltage from said voltage source produces a large change in current through said transistor because of the logarithmic response of said transistor thus providing a wide range of current variation through said solution from a small change in the voltage from said voltage source.

A measuring circuit for measuring the conductance of a solution under test which contains biological growth media where an increase in the conductance of the solution indicates growth, or multiplication, of bacteriological colonies and in which a pair of space-apart electrodes are placed for applying current to said solution, comprising,

conductance responsive means having an output connected to one electrode and an input connected to the other electrode for applying current to said electrodes to provide a voltage drop of a first polarity and of a predetermined value across said solution, said conductance responsive means having the same voltage drop as that of said solution,
a first circuit means for maintaining the voltage drop across said solution at a constant value by maintaining the output voltage of said conductance responsive means at a constant value by varying the current level applied to said conductance responsive means and to said electrodes according to any change in voltage across said solution, comprising,
a first variable voltage source having a source output voltage of a first polarity responsive to any change in the output voltage in said conductive responsive means, and
a first variable input resistance having logarithmic characteristics connected between said variable voltage source and said conductive responsive means so that a small change in voltage from said voltage source produces a large change in current flow in one direction through said first variable input resistance because of the logarithmic response of said first variable resistance thus providing a wide range of current variation through said solution from a small change in voltage from said voltage source.

1 more element

Dependencies are inferred from the claim text.

Conductance measurement circuit with wide dynamic range

Life of this patent

Conductance measurement circuit with wide dynamic range

Life of this patent

Abstract

Claims19