Apparatus and method for detecting metabolic activity

An apparatus and method for detecting metabolic activity in a medium which includes an electrical circuit suitable for passing electrical current through a medium which contains a suspected metabolic agent. The circuit includes means for generating a reference voltage. When metabolic activity occurs in the medium the voltage drop thereacross changes. To balance the changing voltage developed across the medium with the generated reference voltage, an adjustment of current has to be performed. By monitoring such adjustments, made electrically and automatically, there is a correlation made between those adjustments and the metabolic activity in the medium. Use of this apparatus and method allows large quantities of sample media to be tested for detection of metabolic activity rapidly, accurately and reliably.

BACKGROUND OF THE INVENTION 
This invention relates to a method and apparatus for detecting changes in 
voltage potential developed in a medium. More particularly, this invention 
pertains to a method and apparatus for detecting metabolic activity in a 
growth medium based upon changes of electrical characteristics, such as 
the impedance, of that medium. 
As a means of making rapid and accurate measurements of various media to 
determine whether metabolic activity is occurring, and sometimes 
identifying and enumerating the particular microorganisms involved, it has 
become known to correlate changes of electrical impedance with such 
activity. By establishing a relationship between metabolic activity and 
change of an electrical characteristic, such as impedance, of the growth 
medium, tremendous savings in time to detect bacteria presence or other 
microorganism and/or cellular activity have been achieved along with 
greater accuracy and more reliable test results than in the known 
turbidity analysis tests or radiometric methods. Well conceived equipment 
such as described in U.S. Pat. No. 3,984,766 to Thornton now makes it 
possible to automatically measure impedance ratios of a pair of cells 
containing a selected medium, one medium of which contains a suspected 
microorganism contaminant. The changes of impedance ratios of the media 
are directly related to growth of a microorganism therein. By using a 
ratio of impedance it is possible to eliminate all the variables affecting 
impedance changes except organism growth; these influential variables 
include temperature fluctuations, gradual corrosion of electrodes, aging 
of the medium, medium changes due to absorption of gases, etc. Moreover, 
the Thornton equipment is capable of handling the testing of many samples 
rapidly, accurately and automatically. 
Other devices have been described, for instance, by Ur in British 
Specification No. 1,299,363 and U.S. Pat. No. 3,699,437, but such devices 
neither have the capacity nor the automation and rapidity of measurement 
as does the Thornton equipment. 
As this technique of measuring reactions of an electrical characteristic of 
a medium to indicate metabolic activity occurring therein becomes more 
acceptable to users and potential users, additional improvements in these 
type devices are being sought. These sought-after improvements include the 
capability to test hundreds, even thousands, of samples in one system 
while keeping space constraints under consideration; greater accuracy in 
characterizing not only the presence of microorganisms growing in a 
medium, but their identification, levels of concentration and 
susceptibility to antibiotics; the ability to lower the threshold at which 
to detect the presence of microorganisms; and a computer system to monitor 
a very large number of test samples to analyze various data inputs from 
each and provide the user with a variety of results including specific 
identification of microorganisms in the shortest time span. 
One of the shortcomings of the Thornton system as described in U.S. Pat. 
No. 3,984,766, and other impedance measuring systems related to 
microorganism growth, is in the lack of ability to handle very large 
numbers of samples during one test. While it is explained in the Thornton 
patent that large numbers of cell pairs can be measured in that apparatus, 
a number of factors combine to realistically limit the actual number of 
cell samples which can be measured during one test: the electrical 
circuit, in which the source of electrical energy, the oscillator, is a 
constant voltage source across which each pair of cells is serially 
connected; the fact that each sample or specimen cell has a reference cell 
physically proximate thereto and electrically connected in the circuit; 
and as a number of pairs of cells accumulate, their physical presence 
removes them further from the source of electrical energy. In practice, 
the oscillator provides a given voltage to all of the pairs of cells in 
parallel with each other. It can be appreciated that with a fixed voltage 
souuce there will be differences of voltage "seen" across each cell pair, 
especially at the cells which are furthest in distance from the voltage 
source or as cells are added to be tested in the fixed voltage system. 
Consequently, true voltage readings are not attained across the cell pairs 
at increased distance from the source since the lengths of electrical 
leads provides a transmission loading effect. Proper voltage requirements 
are critical in this type of system because extremely small voltage 
changes in the cells are the indicative means of pinpointing microorganism 
growth. Clearly, with this type system accuracy and reliability of 
measurements are sacrificed when the number of samples in the system are 
sufficient to produce variations in or unequal voltages applied across 
those samples. Of course, closely packaged samples can reduce the effect 
of transmission loading, but at some point the space physically occupied 
by a large number of samples will produce a line length loading problem. 
Moreover, it must be kept in mind that the Thornton system has a reference 
cell for every sample cell. Thus, double the space and electrical wiring 
is needed for every additional sample to be tested. While this system 
provides excellent results for a moderately large number of test samples, 
there is indeed room for improvement in many aspects of such a system in 
order to accommodate a very large number of samples, such as in the 
hundreds, to be monitored during a single test. 
SUMMARY OF THE INVENTION 
Many of the deficiencies noted above have been overcome by the new 
apparatus and method of present invention. In addition, the improvements 
achieved by the present invention lend themselves to automation so that 
rapid and accurate results can be produced and reproduced. Besides 
providing for accurate and time saving results, a significant advantage in 
this new invention is its ability to continuously monitor a very large 
number of test samples. This ability to handle such an amount of samples 
is accomplished without sacrificing the accuracy of electrical energy 
which is delivered to each test sample such as occurs in some prior art 
devices. In other words, the electrical energy delivered to each sample in 
the array of samples is consistent with that delivered to the others 
regardless of electrical line length and the transmission loading effect. 
Furthermore, since state of the art technology in this field continues to 
advance, the electrical devices and circuitry to eliminate the 
transmission loading effect can be assembled relatively inexpensively. 
One of the benefits of having a system in which a very large number of 
samples can be tested at once is the reduction of time inefficiencies and 
periodic changes of samples to be tested. In addition, better control is 
achieved since, for example, the environmental changes (humidity, 
temperature, etc.) which may be prevalent when testing samples of a given 
batch in separate tests no longer have to be considered in a system where 
the entire batch can be accommodated in one complete test. Also, since 
electrical line loading effects have been eliminated, there is no need to 
crowd together all the test samples if the room is available to spread 
them out. 
Besides the above advantages, the present invention provides for a computer 
interface feature so that various results can be monitored during and 
after the test, and many pieces of information can be gathered on the test 
samples based upon that which the programmer provides in coordination with 
the type of test being conducted. Some of these computer features shall be 
discussed hereinafter. 
It can be seen that these advantages are highly desirable in the search for 
metabolic activity such as growth of microorganisms which may cause health 
and medical problems. Systems which allow a very large number of blood, 
urine and related clinical cultures to be tested can produce results in a 
very short time span; rapid detection assists in medical diagnosis of a 
problem and may lead to life-saving treatment that needs initiation as 
soon as possible. Besides clinical or hospital applications, the 
apparatuses of the present invention are available to users such as the 
food industry for performing assurance tests for quality and healthiness 
of products headed for the consumer market. A wide range of applications 
is foreseen for the apparatus of the present invention which includes 
those improvements noted above among others which the users thereof may 
find. 
In accordance with the principles of the present invention the new 
apparatus detects changes in voltage drop across a medium. Included in 
this apparatus is a source of electrical energy and means for passing that 
electrical energy through a sample medium and developing a voltage across 
that medium. This medium is susceptible to changes in its voltage drop at 
a given current flow. In addition, this apparatus comprises means for 
generating an electrical reference signal in the form of a voltage. Means 
to measure the voltage developed across the sample and to compare the same 
with the reference voltage are also included. When the voltage drop across 
the sample changes to an amount different from the reference voltage, 
adjusting means alters the voltage developed across the sample media to 
closely approximate or balance same with the reference voltage. There is 
also means to detect and indicate the amount of adjustment of the voltage 
drop across the sample medium necessary to balance the voltage drop 
thereacross with the voltage of the reference signal. 
This type apparatus is especially useful in detecting growth of one or more 
microorganisms in a growth medium. The sample medium contains a suspected 
metabolic agent such as a microorganism contaminant; the adjusting means 
alters the voltage developed across the sample growth medium, which 
changes in response to microorganisms growing in that medium, to closely 
approximate or balance the voltage drop across the sample with the 
reference voltage which remains substantially constant. Monitoring means 
correlates the amount of adjustment of the voltage drop across the sample 
with metabolic activity in that sample. 
In the preferred embodiment of the present invention in which the apparatus 
detects growth of microorganisms in a medium, the means to adjust the 
voltage drop across the sample includes an electrical circuit with a 
variable current source which changes sufficiently to allow the voltage 
drop across the sample medium to closely approximate the reference 
voltage. Use of binary input information from a successive approximation 
register in conjunction with the variable current source provides the 
necessary voltage drop across the sample to closely approximate the 
reference voltage. In this embodiment it is also preferable to test a 
plurality of samples in succession or in random sequence to determine 
whether there is metabolic activity occurring in any of the samples. In 
addition, this preferred apparatus includes a computer in the electrical 
circuit which is programmed to indicate when microorganism growth in any 
sample has occurred amongst other significant pieces of data. 
The present invention further consists of the methods of detecting changes 
in voltage drop across a medium, and, particularly, of detecting metabolic 
activity in a growth medium. In the preferred method of this invention the 
steps include applying electrical energy from a source through a sample of 
growth medium containing a suspected metabolic agent whereby a voltage is 
developed across such medium. An electrical reference signal which is 
proportional to the oscillator voltage is also generated. Measuring the 
voltage developed across the medium and comparing such voltage and the 
reference voltage to each other are sequentially accomplished. This aspect 
of the invention also comprises adjusting the voltage developed across the 
sample medium, which changes in response to metabolic activity in that 
medium, with the reference voltage which remains substantially constant. 
Monitoring the adjustment of the voltage developed across the sample to 
correlate that adjustment with a detection of metabolic activity is the 
final part of this novel method.

While the invention will be described in connection with a preferred 
embodiment, it is understood that it is not intended to limit the 
invention to that embodiment. On the contrary, it is intended to cover all 
alternatives, modifications and equivalents as may be included within the 
spirit and scope of the described invention. 
DETAILED DESCRIPTION OF THE INVENTION 
Adverting to the drawings in which a preferred embodiment is illustrated 
there is shown in FIG. 1 a block diagram containing the basic elements of 
the apparatus for detecting changes in voltage drop across a medium. This 
basic diagram is essentially an electrical circuit 10 in which an 
alternating current electrical energy source 11, such as an oscillator, 
delivers current to a selected cell 12 thereby developing a voltage 
thereacross. A cell 12 is selected by an analog multiplexer 13 which 
includes a drive current multiplexer 13A and a voltage feedback 
multiplexer 13B. The cell 12 designated R is a reference cell while the 
cells 12 designated S.sub.1 -S.sub.N are sample cells. 
The selected sample cell S.sub.1 -S.sub.N may be a container holding a 
medium which is suitable for growing of microorganisms therein. Although 
the apparatus preferably monitors growth of microorganisms in a growth 
medium, it is intended to monitor and detect metabolic activity in general 
in addition to microorganism growth. These activities include, but are not 
limited to, cell growth, whether animal or plant, monitoring protozoa, 
metazoa and the like, and monitoring suspensions of cellular enzymes. For 
the sake of clarity and by way of example only, the description of the 
preferred embodiment of this invention is directed to detection of 
microorganism growth in a growth medium. The current is delivered to the 
selected sample medium 12 by means of a pair of electrodes placed in the 
medium for galvanic contact, the electrodes being part of the circuit. In 
the sample medium 12 is placed, generally before the test begins, a 
suspected metabolic agent such as a microorganism contaminant, which is to 
be tested for a determination of whether microorganisms are indeed 
present, and possibly their identities, levels of concentration, 
antibiotic susceptibility, among other determinations. One convenient 
carrier for conducting these types of tests is what is known as a "blood 
bottle" for monitoring low concentrations of organisms, and which is 
depicted in FIG. 2. A "module" may be used to monitor large concentrations 
of organisms where a small volume of media is satisfactory. Sample medium 
12, a selected fluid, is placed in a bottle 14 so that electrodes 15 are 
placed in the fluid. To receive the electrical current in the medium, the 
electrodes 15 are placed in the electrical circuit 10 for connection 
ultimately to the source 11. This type bottle is normally stoppered, but 
has a small opening or is puncturable so that a suspected contaminant may 
be inoculated into the medium in the bottle. It can be appreciated that 
many bottles comprising sample cells S.sub.1 -S.sub.N or other media 
carriers may be connected in the electrical circuit 10 via multiplexer 13 
so that a plurality of samples may be tested. Furthermore, a bottle 
carrying the fluid medium may be used as the reference medium 12 
designated R for a plurality of bottles by withholding any inoculants; the 
reference medium R may be inserted into the circuit and compared with any 
given sample medium S.sub.1 -S.sub.N for the purpose of eliminating all 
variables which affect the voltage changes of the media except 
microorganism growth. In addition, for purposes of testing, one sample may 
consist of more than one bottle or well appropriately wired, in series for 
example, to include activity in a multiplicity of wells as indicative of 
one sample under test. Proper computer software may be used to compare a 
reference with a plurality of samples which can be employed. 
Returning to FIG. 1, it is noted that before the A.C. source 11 develops a 
voltage potential across cells 12, a programmable current driver 24 has 
been interposed in the electrical circuit 10. Current driver 24 receives 
current from the oscillator 11, but its current output to a selected cell 
12 is controlled by information received from the digital control section 
25. Thus, the current driver 24 is a programmable or variable current 
source which is controlled to deliver current to any selected sample in 
variable amounts depending upon input data received from the digital 
control section 25. 
Digital control section 25, as will be more fully described, controls 
programmable current driver 24 which is a multiplier of the current from 
A.C. source 11, and, therefore, determines the amount of current sent to a 
selected cell 12 via multiplexer 13. Current driver 24 includes an R-2R 
ladder network having a plurality of resistors which are selectively 
coupled to the source 11 as determined by 16 bit digital control words 
generated by section 25. The determination of the appropriate binary 
information generated by section 25 to control the current driver 24 for 
delivery of current to the sample is illustrated by referring now to FIG. 
3, taken in conjunction with FIG. 1. 
In FIG. 3, which is a simplified flow diagram of the measuring and control 
sequence, the initial step is to electrically connect the selected sample 
12 to the programmable current driver 24 via multiplexer 13. At this stage 
a sixteen bit successive approximation process is initiated in digital 
control section 25 which will ultimately determine how much current must 
be delivered to sample 12 to develop a voltage drop thereacross sufficient 
to balance with the fixed reference output from the A.C. source 11. In the 
first step of this successive approximation process the digital control 
section 25 sends a signal to programmable current driver 24 which causes 
the latter to deliver to sample 12 a current corresponding to the most 
significant bit (MSB) value of the sixteen bit successive approximation 
word. 
With this amount of current delivered to sample 12, a certain voltage drop 
will result across that sample. By the appropriate circuitry, as 
hereinafter described more thoroughly in conjunction with FIG. 4, the 
resistive component (E.sub.R) and the reactive component (E.sub.X) of the 
voltage drop across the sample are measured. The resistive component of 
voltage is related to the electrical resistance of the sample medium as 
the current passes therethrough; the reactive component of voltage is 
related to the capacitive and inductive values of the sample medium. 
Since, however, inductance is not known to play any significant role in 
biological-electrical characteristic measurements, it is assumed that the 
reactive component is due solely to capacitive considerations. 
At this stage, the average voltage of the resistive and reactive components 
is automatically computed with the resulting value being compared with the 
fixed reference voltage from A.C. source 11. While the preferred apparatus 
and method, as exemplified herein, uses this average voltage to make a 
comparison with a fixed reference voltage, to be subsequently discussed, 
other approaches may be used. For instance, either the resistive or the 
reactive components, independently, may be compared with a reference 
voltage; or a complex entity similar to impedance, composed of both 
resistive and reactive components, may be employed. However, a phase shift 
must be reckoned with in designing the proper circuitry. It has been found 
that using the average voltage of resistive and reactive components for 
carrying out the comparison steps of this invention provides satisfactory 
results. 
Before continuing with the flow chart of FIG. 3, attention is directed to 
the block diagram of FIG. 1. It is noted that from the sample, the 
electrical signal is broken down into two components E.sub.R, the 
resistive, and E.sub.X, the reactive after amplification and filtering. An 
in-phase filter 16 and a quadrature filter 18 include appropriate 
switching devices which separate the resistive and the reactive 
components, respectively, which are then fed to an averaging network 19. 
As indicated by its name, filter 16 separates that component of the 
voltage that is in phase with the current, represented by E.sub.R, whereas 
the quadrature filter 18 separates that component that is 90.degree. out 
of phase with the current, represented by E.sub.X. Averaging network 19 is 
the electrical circuitry which is employed to average the values of 
resistive and reactive components of voltage of the sample at any given 
time during the test. A voltage amplifier 20 is provided in the circuit 10 
to amplify the signal before it reaches filters 16 and 18 which require a 
higher amount of voltage for optimum accuracy in the circuit to be 
described more fully in conjunction with FIG. 4. 
As seen in FIG. 1, the A.C. source 11, the oscillator, also generates a 
reference output signal. This reference signal is a substantially constant 
level of voltage and is proportional to the voltage of the oscillator. 
Both the reference voltage and the average of the voltage components are 
directed to the balance detector circuitry 21 which comprises a comparator 
and low pass filter. The function of the balance detector 21 is best 
illustrated by referring to the flow chart of FIG. 3. 
After the average voltage 
##EQU1## 
is computed, it is compared with the fixed reference voltage (E.sub.REF) 
generated by the oscillator. By appropriate digital circuitry, the average 
voltage is compared with the reference voltage to determine whether the 
average voltage is greater than the reference voltage. If the average 
voltage exceeds the reference voltage, the MSB of the successive 
approximation circuitry 25A is reset causing the MSB increment of current 
fro current driver 24 to be removed since the MSB increment is too much 
and only less significant bit increments are needed to achieve balance. 
If, on the other hand, the average voltage does not exceed the reference 
voltage, the MSB is retained since this increment of current will be 
needed to achieve balance. 
At this point, the digital control section 25 sends a signal to the 
programmable current source 24 which causes the second most significant 
bit value (one half of the MSB value) of current to be added to the 
current, if any, in sample 12. The resulting new values of E.sub.R and 
E.sub.X are then measured and 
##EQU2## 
is compared with (E.sub.REF) in the same manner as previously described to 
determine whether this increment of current is too much or too little to 
achieve balance. This cycle continues until all sixteen bits have been 
tested as determined by the digital control section 25. 
When all the bits have been tested, the current driver 24 delivers a 
sufficient amount of current to the selected sample so that the previously 
defined average voltage thereacross either balances with or closely 
approximates the reference voltage generated by the A.C. source. This 
entire aproximation process is done automatically in the electrical 
circuit, and can be completed in about two seconds or less. After one 
approximation process is accomplished for one sample, multiplexer 13 
selects another sample cell 12 for another approximation process similar 
to that already described. 
It has been known for some time that the electrical characteristics such as 
resistance and reactance of a medium change perceptibly by the growth of 
microbiological organisms which may be present therein. For instance, 
ionized waste products from microorganisms tend to change the electrical 
impedance of the medium into which they are discharged. This change in 
electrical impedance will produce a change of voltage when the current is 
constant in tests where the medium is connected in a circuit with an 
appropriate power source. Generally, the growth of microorganisms in a 
medium causes a decrease in the voltage drop across the medium. These 
changes become more pronounced as the metabolic activity and concentration 
levels of microorganisms increase in the medium. 
In the present invention, when the sample medium contains a microorganism 
contaminant, the electrical characteristics of the medium change as the 
microorganisms increase in number. If the electrical current remained at a 
constant level, the voltage across the sample would change, most likely 
decrease. However, the new apparatus of the present invention is concerned 
with keeping the voltage drop across the sample as closely balanced as 
possible with the fixed reference voltage. Accordingly, when 
microorganisms start growing in the sample, the voltage drop across the 
sample is adjusted, periodically, to always closely approximate the 
reference voltage. This adjustment is accomplished by supplying more or 
less electrical current to the sample as explained above. The amount of 
current required to make such a voltage adjustment to achieve balance with 
a reference represents the output of the electrical circuit 10 as seen in 
FIGS. 1 and 3. 
The digital control section includes successive approximation and timing 
logic 25A, whose digital output is the 16 bit word controlling current 
driver 24, a digital multiplexer 25B, a serial interface 25C, address 
logic 25D and a command decoder 25E. Each time there is a complete run 
through 16 bits for a selected sample, the output of logic 25A is 
monitored. This output is fed through digital multiplexer 25B and serial 
interface 25C to a computer 26 which controls CRT display terminal 27 or a 
hard copy printer terminal 28. Alternatively, the output from interface 
25C and be fed directly to a chart recorder (not shown) which becomes a 
source of collective data relating to the growth of microorganisms in a 
medium, and is a ready reference used to make a rapid determination that 
metabolic activity is occurring. 
With the desirable decrease in time for making determinations relating to 
the presence of microorganisms, it is preferred to interface the computer 
26 with the output of the circuit 25 for performing a multiplicity of 
tasks. Some of the functions which can feasibly be programmed into the 
computer in conjunction with the instant invention include, but are no 
means limited to, the following: a correlation of data with various sample 
numbers when many samples are being tested; constant monitoring of 
hardware and look at critical components and variables under which the 
tests are being performed; a selective or random monitoring of individual 
samples when called upon in addition to the standard successive 
monitoring; a detection of errors or indication of false readings; 
ignoring samples which are out of line with standard or expected results; 
possible detection algorithms, i.e., whether there is growth, time to 
reach a threshold concentration level, and possibly an enumeration per 
unit volume of the microorganisms; possible identification of the type or 
types of microorganisms present, or the percentage probabilities of 
various species of microorganisms present; types of organisms which prove 
susceptible to anti-metabolites, and the like. The computer 26 also can be 
connected to appropriate circuitry built into the system to check the 
reliability and proper functioning of the same, during each cycle or 
periodically, according to choice of the designer. Depending upon the 
computer capability desired, the interface circuitry can be designed 
accordingly as is known in that art. 
The address logic 25D functions to send address information to multiplexer 
13 to switch in one of the samples 12. This address information is also 
fed to multiplexer 25B where it is multiplexed with the digital 
information from logic 25A to inform the computer to which sample 12 the 
digital information applies. The command decoder 25E functions to decode 
commands from the computer 26 through interface 25C so as to, for example, 
start the successive approximation technique via logic 25A and generate 
address information via logic 25D. 
One of the significant features of the present invention is its ability to 
handle a large plurality of samples during one test. As illustrated in 
FIG. 3, and as already indicated, after the 16 bit control word has all 
its bits tested, the output of the digital control section 25 is 
monitored, and the next sample is automatically connected to the current 
driver 24. This automatic connection is accomplished by means of address 
logic 25D and multiplex switches 13, which allow each sample to be 
sequentially tested until the complete array is exhausted, whereupon the 
process of testing each sample repeats itself. Thus, with the comparing 
and balancing features of the present invention, along with multiplexer 
switching capabilities, true and accurate readings of voltage across a 
large number of samples are achieved, with a sensitive and accurate 
monitoring of changes in those voltage levels to indicate growth of 
microorganisms in a sample medium. 
An area in which large numbers of samples of media are to be tested is the 
culturing of blood. A system wherein over a hundred samples in bottles 
(similar to the embodiment of FIG. 2) or modules known in the art can be 
monitored during one test is readily devisable using the apparatus and 
techniques described above. Such system, according to choice of design, 
may accommodate various types of medium carriers, such as bottles, sealed 
measurement cells (such as described in U.S. Pat. Nos. 3,743,581 and 
3,890,301), modules comprising a unitized array of chambers for media, and 
the like. In addition, for control over temperature and other 
environmental conditions, and to produce a more rapid determination of 
microorganism presence in the medium, the apparatus of this invention may 
include one or more incubators. By heating the medium uniformly, the 
metabolic activity of the microorganisms is accelerated with an 
opportunity to make a much earlier detection. 
While the selection of the electrical circuitry required to perform the 
functions indicated in FIG. 3 hereof in combination with the outline of 
FIG. 1 is not, in itself, critical, nor the essence of this invention, 
FIGS. 4 and 5 depict a schematic diagram of one specific circuit which 
satisfies the aims and features of this invention. 
As shown in FIG. 4, the A.C. source 11 is a quadrature oscillator of a type 
well known in the art and including two operational amplifiers 30 and 32 
with the output of the former coupled to the input of the latter. The 
various resistors R' and capacitors C' are frequency determining elements 
which enable the oscillator to oscillate at a predetermined fixed rate. 
Oscillator 11 provides a sinusoidal output on lines 34 and 36 to 
programmable current driver 24, and to the filters 16,18. Oscillator 11 
provides another output on line 38 to the filters 16,18, which is 
90.degree. phase shifted in relation to the signals on lines 34 and 36. 
The programmable current driver 24 comprises a resistive ladder network 40 
having resistors R.sub.1 -R.sub.32 connected as shown. The sinusoidal 
output on line 34 can be coupled to the odd-numbered resistors R.sub.1 
-R.sub.15 through the energization of eight relay coils K.sub.1 -K.sub.8 
which control, respectively, switches SW.sub.1 -SW.sub.8. As illustrated 
in FIG. 4, these switches SW.sub.1 -SW.sub.8 are in their low state 
connected to ground via line 42. In this position of the switches SW.sub.1 
-SW.sub.8, the output on line 34 is not coupled to any one of the 
odd-numbered resistors R.sub.1 -R.sub.15. However, when one of the relays, 
for example, K.sub.3, is energized, then switch SW.sub.3 will change state 
to couple the signal on line 34 through this switch to resistor R.sub.5 
and then through resistors R.sub.4 and R.sub.2 to the output of the ladder 
40 on line 44. In a similar manner, it may be seen that when any one of 
the relay coils K.sub.1 -K.sub.8 is energized, the corresponding switches 
SW.sub.1 -SW.sub.8 will change state to couple the signal on line 34 
through any one of the corresponding odd-numbered resistors R.sub.1 
-R.sub.15 and ultimately out to output line 44. 
Current driver 24 also includes four integrated circuits 46,48,50,52 for 
coupling the signal on line 34 to odd-numbered resistors R.sub.17 
-R.sub.31 of the ladder network 40. Each integrated circuit includes, 
respectively, solid state switching devices which may be MOSFET's. Thus, 
circuit 46 includes field effect transistor switches (FET's) 46a,46b and 
46c, circuit 48 includes FET's 48a,48b and 48c, circuit 50 includes FET's 
50a,50b and 50c, while circuit 52 includes FET's 52a,52b and 52c. Eight 
lines 54,56,58,60,62,64,66 and 68 are coupled, as shown, to the control 
inputs for these FET's so that when a signal on one of these lines is 
applied to a respective control input, the corresponding FET changes 
state. 
As shown in FIG. 4, the FET's on circuits 46,48,50 and 52 are in their 
"low" state and connected to ground through line 42. When, for example, a 
gating signal is applied on line 54, the FET's 46a,46b,46c and 48a will be 
turned to their "high" state, i.e., the "contacts" shown for these 
transistors will connect to A.C. source 11 through line 34. In this 
condition, the signal on line 34 will be coupled to resistor R.sub.17 
through respective FET's 46a-46c which connect a line 70 to a line 72, and 
then to line 44 via even-numbered resistors R.sub.16 -R.sub.2. In a 
similar manner, FET 48a couples the signal on line 34 from line 70 to the 
line 72 and then to resistor R.sub.17. If, as another example, a gating 
signal were applied on line 64, FET 52a will be switched so as to couple 
the signal on line 34 from line 70 to a line 74 and thereby to resistor 
R.sub.27. Based on the above discussion and the illustration of FIG. 4, it 
will be readily apparent that if a gating signal is applied to one of the 
control inputs, the corresponding FET's will couple the signal on line 34 
through line 70 to the various odd-numbered resistors R.sub.17 -R.sub.31 
of the ladder 40. As can be appreciated, therefore, the current on line 44 
will depend on which of switches SW.sub.1 -SW.sub.8 and the FET's are 
"high" or "low". The manner in which a gating signal is applied to one of 
the lines 54,56,58,60,62,64,66 or 68, or in which one of the relay coils 
K.sub.1 -K.sub.8 is energized, is controlled by the digital control 
section 25 which will be described in connection with FIG. 5. 
The current signal on line 44 is fed to an operational amplifier 76 which 
maintains its inverting input at virtual ground while forcing the current 
on line 44 through sample 12 via line 78, multiplexer 13 and line 80. The 
two resistors R.sub.33, together with capacitor C, shown between line 78 
and line 80 coupled to amplifier 76, provide a D.C. feedback path with a 
very high impedance to the A.C. portion of the signal on line 78 so that 
only the A.C. signal is fed to the multiplexer and test cells 12,13. 
The voltage developed across one of the multiplexed cells by the current 
signal on line 78 is then fed back over lines 82 to the voltage amplifier 
20, which is of a type well known in the art. The amplified voltage is 
then fed from amplifier 20 over line 84 to filters 16,18. The signal on 
line 84 is fed over line 86 as an input to a phase inverter 88 whose 
output is produced on a line 90. The signals on lines 84 and 90 are fed 
into a switching network 92 which functions to provide signals E.sub.R, 
E.sub.X and the reference voltage from source 11. 
Other signals fed to the switching network 92 include the output of a phase 
inverter 94 whose one input is the signal on line 36, a non-inverted 
signal from line 36 on line 96, and the output of a comparator 98 on line 
100. Comparator 98 receives the sinusoidal signal on line 36 and converts 
this signal to a square wave output on line 100. The comparator 98 
functions in a standard manner to provide a square wave transition each 
time the sinusoidal input goes through a zero crossing, this pulse 
corresponding to the polarity of the sinusoidal input. 
Switching network 92 also receives the output of another comparator 102 on 
line 104, the input to this comparator being the 90.degree. phase shifted 
sinusoidal signal on line 38. As with comparator 98, comparator 102 
provides a square wave output, but this output is 90.degree. phase shifted 
in relation to the square wave output on line 100. 
The switching network 92 includes three switches 92a, 92b and 92c which 
also may be MOSFET's. Network 92 also includes an output line 106 
connected to switch 92a and which supplies the reference voltage, an 
output line 108 connected to switch 92b and which supplies the signal 
E.sub.R, and an output line 110 connected to switch 92c and which supplies 
the signal E.sub.X. As indicated in FIG. 4, switch 92a is caused to change 
its state by the signal on line 100, while switches 92b and 92c change 
their state in response to the signals on lines 100 and 104 respectively. 
Switching network 92 operates in the following manner. To provide the 
reference voltage output on line 106, when the output on line 100 is 
negative, corresponding to the negative half cycle of the sinusoidal input 
on line 36, the contacts of switch 92a are in the position shown. 
Accordingly, a negative sinusoidal output is applied from line 96, through 
switch 92a, to line 106. On the next half cycle, the output on line 100 
will be positive to cause the contacts on switch 92a to switch state to 
connect the output of phase inverter 94 to line 106. At this time of the 
next half cycle, the positive cycle of the sinusoidal wave on line 36 will 
be phase inverted by inverter 94, thereby providing a negative half cycle 
on line 106. On the next negative half cycle of the input to comparator 
98, switch 92a again will change state to the state shown in FIG. 4 so 
that another negative half cycle will be output on line 106. This 
switching of switch 92a continues with each half cycle of the input signal 
on line 36 so that it can be seen that the output on line 106 is a 
rectified negative signal comprising the reference voltage. It should be 
noted that in FIG. 1 the reference voltage is shown as coming from the 
source 11 while in FIG. 4 it is shown on line 106 as being output from the 
filters 16,18. The former was shown merely for ease of explanation of the 
invention, while FIG. 4 shows the actual source of this reference signal. 
To obtain the signal on line 108, switch 92b constantly changes state with 
each zero crossing of the signal on line 36 in response to the gating 
signal on line 100. With switch 92b in the state shown in FIG. 4, the 
inverted signal on line 90 is fed through switch 92b to line 108. On the 
next half cycle of the signal on line 36, switch 92b changes state to 
couple the non-inverted signal on line 84 to output line 108. The 
switching of switch 92b is in phase with the signal on line 36 and hence, 
the signal on line 34, so that the component of the voltage from amplifier 
20 which is in phase with the current on line 34 is separated out. This 
component voltage is the resistive component E.sub.R of the cell 12 being 
tested. 
To obtain the signal on line 110, switch 92c constantly changes state 
90.degree. phase shifted from each zero crossing of the signal on line 34 
in response to the signal on line 104. With the switch 92c in the state 
shown in FIG. 4, the non-inverted signal on line 84 is fed through switch 
92c to line 110. On the next half cycle of the signal on line 38, switch 
92c changes state to couple the phase inverted signal on line 90 to output 
line 110. The switching of switch 92c, being 90.degree. phase shifted from 
the signal on line 34, separates out the component of the voltage from 
amplifier 20 that is at quadrature with the signal on line 34. Hence, this 
voltage component E.sub.X on line 110 is the voltage across the capacitive 
component of the cell 12 being tested. 
The positive outputs on lines 108 and 110 are fed through respective 
resistors R.sub.34 and R.sub.35 which function as the averaging network 19 
while the rectified negative output on line 106 is fed through a resistor 
R.sub.36. These three resistors R.sub.34, R.sub.35 and R.sub.36 are 
connected to a summing junction 112 which forms the balance detector 21. 
The signal on line 114 will be 0 when the average of E.sub.R and E.sub.X 
is balanced with the reference voltage. The signal on line 114 is filtered 
by a low pass filter 116 which provides an output to a comparator 118. 
When the signals are not balanced, comparator 118 produces an output on 
line 120 that is then fed to the digital control section 25 which causes a 
change of state in one of the switches SW.sub.1 -SW.sub.8 or one of the 
FET's in switching circuits 46,48,50 or 52. 
As an example of the operation of the structure of FIG. 4, assume that the 
output on line 120 indicates an above balance condition (i.e. 
##EQU3## 
is greater than E.sub.REF), and that in the previous step switch SW.sub.1 
was closed to couple the output on line 34 to line 44. The digital control 
section 25 will produce a signal to energize relay K.sub.2 via the +5 volt 
supply and deenergize relay K.sub.1. Accordingly, switch SW.sub.1 will 
return to the state shown in FIG. 4 and switch SW.sub.2 will switch states 
to couple the signal on line 34 through this switch, then through 
resistors R.sub.3 and R.sub.2 to the output on line 44. The current on 
line 44, as determined by the ladder network 40, will then be fed to a 
selected sample cell via line 78 to develop a voltage across this cell. 
This voltage will then be fed back over lines 82, amplified by amplifier 
20 and fed to switching network 92 to develop signals E.sub.R and E.sub.X 
as already described. These two signals will then be averaged via 
resistors R.sub.35 and R.sub.36 and compared to the reference voltage 
developed on line 36. If a below balance condition exists (i.e., 
##EQU4## 
is less than E.sub.REF), the signal on line 120 causes K.sub.2 to remain 
energized and in either event the same process will be repeated for each 
of the remaining steps of the successive approximation sequence. 
In reference to FIG. 5, timing logic 25A includes a successive 
approximation register 122 comprising two eight bit shift registers 122A 
and 122B. The register 122 is initially activated by a signal on line 124 
from a start signal generator 126 which may be turned on by a signal from 
the decoder 25E. The registers 122A and 122B receive the signal from 
balance detector 21 over line 120 as shown. The output of the successive 
approximation register 122 is fed over four lines 128 to a driver 130, 
with each line 128 representing four bits. Shift register 122B also 
provides an output on line 132 after 16 bits for a selected cell 12 have 
been tested. This signal on line 132 is fed to two latches 134 and 136 
which latch the information in the successive approximation register 122 
after this 16 bit testing. Driver 130 includes transistor-transistor logic 
(TTL) having a plurality of transistors which, when energized, close a 
circuit through respective relays K.sub.1 -K.sub.8 and the MOSFET's on 
circuits 46,48,50 and 52. 
The digital multiplexer 25B includes a multiplexer 138 which selects the 
data in latches 134 and 136 via lines 140. A counter 142 which counts the 
output of an oscillator 144 activates the multiplexer 138 over lines 146 
to select the latched data at preset times. 
The address logic 25D includes an address counter 148 which provides a 
seven bit address identifying any one of 128 sample cells to analog 
multiplexer 13 via line 150. The address in counter 148 is stored in a 
latch 152 and then incremented by one after the signal on line 132 is 
produced. The data in latch 152 is also fed to the multiplexer 138 via 
line 154. 
In operation, upon testing of the first cell 12, start generator 126 is 
enabled and a signal shifted over line 124 into the first stage of 
register 122A. This signal is then fed to driver 130, so that transistors 
in the driver 130 are energized to close a circuit through relay K.sub.1. 
After the balance testing operation is performed for the MSB, as described 
above, the signal on line 120 is fed into the first stage of register 122A 
with the signal from generator 126 formerly in the first stage shifted 
down to the second stage. The data in the successive approximation 
register 122 is then fed to driver 130 which activates suitable 
transistors to close a circuit through relay K.sub.2. After the balance 
testing operation again is performed, the register 122 again shifts down 
one stage to receive any information on line 120. Again, the data now 
stored in register 122 is fed to driver 130 to activate other transistors 
for energizing relay K.sub.3. This process continues for the 16 bits 
stored in register 122 and, at the end of this testing of the 16 bits, 
register 122B provides a signal over line 132. As a result, the 16 bit 
data stored in register 122 for the particular cell being sampled, is 
stored in latch 134 and 136 and then fed via multiplexer 138 to the serial 
interface 25C and then to computer 26 which processes this data for such 
cell. At the same time, the data in the address counter 148 is latched in 
latch 152 and fed over line 154 through multiplexer 138 to the serial 
interface 25C and ultimately to the computer 26, this address information 
defining the particular cell being sampled. Also, address counter 148 is 
incremented by one to address a new cell 12, this address data being fed 
to the analog multiplexer 13 to close the switches in drive current 
multiplexer 13A and voltage feedback multiplexer 13B for another 16 bit 
testing cycle for a new cell. As may be appreciated, by updating counter 
148 through 128 addresses and sequentially testing the 16 bits in register 
122, the voltage feedback information for each cell can be obtained to 
detect microbial growth in each sample.