U.S. patent number 5,515,001 [Application Number 08/189,344] was granted by the patent office on 1996-05-07 for current-measuring operational amplifier circuits.
This patent grant is currently assigned to The United States of America as represented by the United States. Invention is credited to John M. Vranish.
United States Patent |
5,515,001 |
Vranish |
May 7, 1996 |
Current-measuring operational amplifier circuits
Abstract
An operational amplifier connected in series with a load and a
current-measuring impedance and having at least two input ports and
an output port, the output port of the operational amplifier
further being directly connected to current-measuring impedance,
with one of the input ports being coupled to the current input
source to be measured, and the other input port being connected to
a feedback loop coupled from the output side of the
current-measuring impedance. The voltage across the
current-measuring impedance which comprises a resistor is
thereafter sensed, amplified and conditioned to provide a voltage
which is proportional to the load current and accordingly a measure
of the current input from the current source being measured. Such a
current-measuring circuit can be used in connection with an
inverter, a current-measuring amplifier, a current-measuring
precision diode, a current integrator, and a current-measuring
bridge.
Inventors: |
Vranish; John M. (Crofton,
MD) |
Assignee: |
The United States of America as
represented by the United States (Washington, DC)
|
Family
ID: |
22696907 |
Appl.
No.: |
08/189,344 |
Filed: |
January 31, 1994 |
Current U.S.
Class: |
330/69; 324/123C;
330/75 |
Current CPC
Class: |
G01R
1/30 (20130101); G01R 19/0023 (20130101); G01R
19/0092 (20130101); H03F 1/34 (20130101); H03F
3/45475 (20130101); G01R 1/203 (20130101); G01R
15/146 (20130101); H03F 2200/261 (20130101); H03F
2203/45138 (20130101); H03F 2203/45528 (20130101) |
Current International
Class: |
G01R
1/30 (20060101); G01R 19/00 (20060101); G01R
1/00 (20060101); G01R 15/14 (20060101); G01R
1/20 (20060101); H03F 003/45 (); H03F 001/34 ();
G01R 001/30 () |
Field of
Search: |
;324/123C,123R,124
;330/69,75,110,146 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Mottola; Steven
Attorney, Agent or Firm: Marchant; Robert D.
Government Interests
ORIGIN OF THE INVENTION
This invention may be made and used by or for the Government for
governmental purposes without the payment of any royalties thereon
or therefor.
Claims
I claim:
1. An electrical circuit coupleable to a load impedance and adapted
to measure current comprising:
an amplifier circuit having an output port and at least two input
ports;
means for coupling an input signal to one of said input ports;
a current-measuring impedance coupled between said output port and
said load impedance;
feedback circuit means coupled from a circuit node between said
current-measuring impedance and said load impedance to the other of
said two input ports; and
current-measuring means coupled across said current-measuring
impedance and being responsive to a voltage across said
current-measuring impedance for generating an output voltage
proportional to said current through said current-measuring
impedance.
2. The electrical circuit according to claim 1 wherein said means
coupled across the current-measuring impedance includes amplifier
means.
3. The electrical circuit according to claim 1 wherein said means
coupled across the current-measuring impedance includes signal
conditioning means.
4. The electrical circuit according to claim 1 wherein said
amplifier circuit comprises an operational amplifier and wherein
said at least two input ports comprise relatively high impedance
ports and said output port comprises a relatively low impedance
port.
5. The electrical circuit according to claim 1 wherein said
amplifier circuit comprises an operational amplifier, said at least
two input ports include a non inverting (+) input port and an
inverting (-) input port, and wherein said input signal is coupled
to the (+) input port and said feedback circuit means is coupled
from said circuit node to the (-) input port.
6. The electrical circuit according to claim 5 wherein said
feedback circuit means comprises a direct connection between said
circuit node and the (-) input port.
7. An electrical circuit coupleable to a load impedance and adapted
to measure current comprising:
an operational amplifier with an output port and at least two input
ports including a noninverting input port and an inverting input
port;
means for coupling an input signal to said noninverting input
port;
a current-measuring impedance coupled between said output port and
said load impedance; and
feedback circuit means coupled from a circuit node between said
current-measuring impedance and said load impedance to said
inverting input port, said feedback circuit means further including
a first voltage divider network and additionally including a second
voltage divider network coupling said input signal to said
noninverting input port,
whereby a voltage generated across said current-measuring impedance
is proportional to said current through said current-measuring
impedance.
8. The electrical circuit according to claim 7 wherein said first
voltage divider network includes first impedance means coupling the
(-) input port to a reference potential and second impedance means
coupling said circuit node back to ground potential.
9. The electrical circuit according to claim 8 wherein the
impedance value of the second impedance means is greater than the
impedance value of the first impedance means.
10. The electrical circuit according to claim 9 wherein said first
and second impedance means include first and second resistive
impedances.
11. The electrical circuit according to claim 8 wherein said second
voltage divider network comprises third impedance means coupling
the input signal to the (+) input port and fourth impedance means
coupling the (+) input port to a bias potential.
12. The electrical circuit according to claim 11 wherein said third
and forth impedance means includes third and fourth resistive
impedances.
13. The electrical circuit according to claim 1 wherein said
amplifier circuit comprises an operational amplifier, said at least
two input ports include a non inverting (+) input port and an
inverting (-) input port, and wherein said (+) port is coupled to a
reference potential and said current input signal and said feedback
circuit means are coupled to the (-) input port.
14. The electrical circuit according to claim 13 and additionally
including first impedance means coupling said input signal to the
(-) input port and wherein said feedback circuit means coupled to
the (-) input port includes second impedance means.
15. The electrical circuit according to claim 14 said first and
second impedance means include first and second resistive
impedances.
16. The electrical circuit according to claim 15 wherein the
magnitude of said first and second resistive impedances are
substantially equal.
17. The electrical circuit according to claim 15 wherein the
magnitude of said second resistive impedance is greater than the
magnitude of said first resistive impedance.
18. An electrical circuit coupleable to a load impedance and
adapted to measure current comprising:
an operational amplifier having an output port and at least two
input ports including a noninverting input port and an inverting
input port;
means for coupling an input signal to said noninverting input
port;
a current measuring impedance coupled between said output port and
a diode, said diode being coupled between said current measuring
impedance and said load impedance; and
feedback circuit means coupled from a circuit node between said
diode and said load impedance to said inverting input port,
whereby a voltage generated across said current-measuring impedance
is proportional to said current through said current-measuring
impedance.
19. The electrical circuit according to claim 18 wherein said
feedback circuit means comprises a direct connection between said
circuit node and the (-) input port.
20. An electrical circuit coupleable to a load impedance and
adapted to measure current comprising:
an operational amplifier having an output port and at least two
input ports including a noninverting input port and an inverting
input port;
integrator circuit means coupleable between an input signal and
said noninverting input port of said operational amplifier;
a current-measuring impedance coupled between said output port and
said load impedance; and
a feedback circuit including a direct connection coupled from a
circuit node between said current-measuring impedance and said load
impedance to said inverting input port of said operational
amplifier,
whereby a voltage generated across said current-measuring impedance
is proportional to said current through said current-measuring
impedance.
21. The circuit according to claim 20 wherein said integrator
circuit means includes a second operational amplifier having at
least two input ports and an output port, one of said two input
ports being coupled to said input signal and the other of said two
input ports being coupled to a reference potential, and capacitive
feedback means coupled between the output and said one input
port.
22. The circuit according to claim 21 and additionally including
impedance means coupling said input signal to said one input
port.
23. The circuit according to claim 22 wherein said impedance means
comprises resistive impedance means.
24. An electrical bridge measuring circuit, comprising:
a first and a second leg of a bridge circuit commonly coupled to a
first circuit node;
a third and fourth leg of the bridge circuit commonly coupled to a
second circuit node;
said first and third leg commonly coupled to a third circuit node
and said second and fourth leg commonly coupled to a fourth circuit
node;
a voltage source coupled across said first and second circuit
nodes;
wherein said third and fourth legs of the bridge circuit
respectively comprise impedance means; and
wherein said first and second legs of the bridge circuit comprise
respective amplifier circuits, each including an amplifier having
an output port and at least two input ports, and wherein one input
port of said two input ports of both said amplifiers are commonly
connected to said first circuit node, impedance means coupled from
said output port to said third and fourth circuit nodes,
respectively, and respective feedback circuit means coupled from
said third and fourth circuit node to the other input port of said
two input ports of said amplifiers; and
means coupled between the respective output ports of said
amplifiers for detecting a voltage difference therebetween.
25. The bridge measuring circuit according to claim 24 wherein each
said amplifier comprises an operational amplifier, said at least
two input ports include a non inverting (+) input port and an
inverting (-) input port and wherein said first circuit node is
commonly coupled to the (+) input ports and said feedback means are
coupled to the (-) input ports.
26. The bridge measuring circuit according to claim 25 wherein said
feedback means comprise a direct circuit connection between the
respective (-) input ports of said operational amplifiers and said
third and fourth circuit nodes.
27. The bridge measuring circuit according to claim 25 wherein said
impedance means coupled from said output ports of said amplifiers
comprise resistive impedance means.
28. The circuit according to claim 7 wherein a current-measuring
means is coupled across said current-measuring impedance, said
current-measuring means being responsive to said voltage across
said current-measuring impedance and generating an output voltage
proportional to said current through said current-measuring
impedance.
29. The circuit according to claim 18 wherein a current-measuring
means is coupled across said current-measuring impedance, said
current-measuring means being responsive to said voltage across
said current-measuring impedance and generating an output voltage
proportional to said current through said current-measuring
impedance.
30. The circuit according to claim 20 wherein a current-measuring
means is coupled across said current-measuring impedance, said
current-measuring means being responsive to said voltage across
said current-measuring impedance and generating an output voltage
proportional to said current through said current-measuring
impedance.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to instrumentation electronics and
more particularly to electronic circuitry which provides a measure
of current flow in a circuit.
2. Description of the Prior Art
Techniques for measuring electrical current are well known and
cover a wide spectrum of devices and circuits including classical
current meters and circuits which add known current-measuring
resistance in series with a load, whereupon the voltage drop across
the resistor is measured, thus yielding a measure of that current.
However, the act of measuring the current often affects the results
in that there is a voltage drop across the resistor as well as
across the load so that the voltage across the load in effect
"droops". If the load is reactive or partially reactive, the
measurement also introduces a phase shift in the current. Thus to
keep these effects to a minimum, the resistance must be kept
relatively small which ultimately reduces the accuracy of the
measurement. While high gain instrumentation amplifiers can
somewhat compensate for this effect, they also have a tendency to
introduce noise which is undesirable and cannot be tolerated in
certain applications, such as capacitance type proximity sensors.
Accordingly, there is a need to provide current-measuring circuitry
which will permit such proximity sensors to be operated relatively
close to one another without being affected by cross-talk between
them.
SUMMARY
Accordingly, it is an object of the present invention, therefore,
to provide an improvement in current-measuring apparatus.
It is another object of the invention to provide a means for
providing current measurement in a straight forward, simple,
precise and compact manner.
It is a further object of the invention to provide an improvement
in current-measuring circuitry which can be utilized to measure
relatively small currents.
It is yet another object of the invention to provide an electronic
circuit which will permit sensors to be operated in close proximity
to each other without cross talk by means of locking each of the
other in voltage phase and frequency.
The foregoing and other objects of the invention are achieved in
its simplest form by means of an operational amplifier connected in
series with a load and a current-measuring impedance and having at
least two input ports and an output port, the output port of the
operational amplifier further being directly connected to
current-measuring impedance, with one of the input ports being
coupled to the current input source, and the other input port being
connected to a feedback loop coupled from the output side of the
current-measuring impedance. The voltage across the
current-measuring impedance which comprises a resistor is
thereafter sensed, amplified and conditioned to provide a voltage
which is proportional to the load current and accordingly a measure
of the current output from the current source being measured. Such
a current-measuring circuit can be used in connection with an
inverter, a current-measuring amplifier, a current-measuring
precision diode, a current integrator, and a current-measuring
bridge.
BRIEF DESCRIPTION OF THE DRAWINGS
The following detailed description of the invention will be more
readily understood when considered together with the accompanying
drawings in which:
FIG. 1 is a schematic diagram of a conventional operational
amplifier type voltage follower circuit;
FIG. 2 is an electrical schematic diagram of a current-measuring
voltage follower circuit in accordance with the subject
invention;
FIG. 3 is an electrical equivalent circuit diagram of the circuit
shown in FIG. 2;
FIG. 4 is an electrical schematic diagram of a typical application
for the current-measuring voltage follower shown in FIG. 2;
FIG. 5 is an electrical schematic diagram illustrative of a
current-measuring inverter circuit in accordance with the subject
invention;
FIG. 6 is an electrical schematic diagram of a current-measuring
amplifier in accordance with the subject invention;
FIG. 7 is an electrical schematic diagram illustrative of a
current-measuring precision diode circuit in accordance with the
subject invention;
FIG. 8 is an electrical schematic diagram illustrative of an
inverting type of current-measuring integrator in accordance with
the subject invention;
FIG. 9 is an electrical schematic diagram illustrative of a
conventional Wheatstone bridge circuit; and
FIG. 10 is an electrical schematic diagram of a bridge circuit
configured in accordance with the subject invention.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to the drawings wherein like reference numerals refer
to like parts throughout, attention is directed first to FIG. 1
wherein there is shown a conventional operational amplifier circuit
10 configured as a voltage follower. Reference numeral 11 denotes
an operational amplifier (op-amp) having a non-inverting(+) input
port 12, an inverting(-) input port 14, and an output port 16. The
output port 16 is coupled from circuit node 17 back to the
inverting(-) input port 14 by way of feedback loop 15 and also to a
load impedance 18 which is shown returned to ground. A terminal 19
for the application of an input voltage V.sub.in is directly
connected to the non-inverting (+) port 12.
The circuit configuration 10 is, in effect, a precision high gain
servo system where the voltage applied to the (+) input port 12 is
servoed through the op-amp 11 to its (-) input port. Both of the
input (+) and (-) input ports 12 and 14 comprise high impedance
inputs; however, the device so configured becomes a current source
having a low impedance output at the output port 16.
If a standard voltage follower circuit 10 as shown in FIG. 1 were
to be inserted between a voltage source and capacitive sensor, for
example, the required current would be supplied thereby but there
would be no measurement of the current. Since there are instances
where one needs to know the amount of current being supplied to the
sensor, the subject invention in its simplest form involves a
current-measuring voltage follower circuit 20 as shown in FIG. 2.
The current 20 of FIG. 2 is derived from the voltage follower
circuit 10 of FIG. 1 by the inclusion of a current measurement
resistive type impedance element 22, typically a fixed resistor of
a predetermined value or magnitude, connected in series between the
output port 16 and the circuit node 17 which is common to the load
impedance 18 and the feedback loop 15.
If the feedback loop 15 is such that the magnitude of the feedback
current I.sub.fb is negligible or substantially equal to zero, i.e.
I.sub.fb .apprxeq.0, then the output current out of the operational
amplifier 11 is equal to the load current I.sub.L at the circuit
node 17. By connecting an amplifying and conditioning circuit 28
across the current measurement impedance 22, a voltage V.sub.m
=I.sub.L .times.Z.sub.m is generated across the impedance 22 which
is proportional to current I.sub.L and, further, when applied to
the amplifying and conditioning circuit 28, there is provided an
output voltage V.sub.0 which is also proportional to the current
I.sub.L.
An equivalent circuit of the current-measuring voltage follower 10
is shown in FIG. 3 and is helpful in understanding the operation of
the current-measuring voltage follower shown in FIG. 2. As shown in
FIG. 3, the operational amplifier 11 includes an input resistive
impedance R.sub.i to which the (+) and (-) input ports 12 and 14
are connected to opposite ends thereof. The operational amplifier
11 is further shown providing a current open loop current gain of
G.sub.m and having an output resistive impedance R.sub.0 which is
shown as a series resistor coupled to the output port 16.
As shown, the output current I.sub.L from the operational amplifier
11 is expressed as:
The voltage at terminal 16 can be expressed as:
Since the voltage at circuit node 17 corresponds to V.sub.in (-),
then the voltage V.sub.m across the current sensing resistor 22
is:
Again, the amplifying and conditioning circuit 28 is shown coupled
across circuit nodes 16 and 17, providing an output voltage V.sub.0
which is proportional to the current I.sub.L.
The current-measuring voltage follower circuit 20 shown in FIG. 2
is particularly applicable where a plurality of proximity sensors,
shown in FIG. 4 as a plurality of relatively closely spaced
capacitive type proximity sensors 30.sub.1, 30.sub.2 . . .
30.sub.n, are utilized in connection with the end effector and arm
of a robotic device, not shown, and where the sensors are all
commonly energized from a single source such as a precision crystal
controlled oscillator 32 so as to facilitate noise rejection and
filtering. Each of the sensors 30.sub.1, 30.sub.2 . . . 30.sub.n
couple to a respective input terminal 19.sub.1, 19.sub.2 . . .
19.sub.n of the current-measuring voltage followers 20.sub.1,
20.sub.2 . . . 20.sub.n to which provide individual current
indicating outputs of V.sub.01, V.sub.02 . . . V.sub.on.
The current-measuring voltage follower concept of FIG. 2 can be
extended to other types of circuits which are also capable of
providing a current-measuring function as will now be
described.
Referring now to FIG. 5, shown thereat is a current-measuring
inverting amplifier circuit 34 which has the non-inverting or (+)
input port 12 of the op-amp 11 returned to ground. The input signal
is also now applied to the inverting or (-) input port 14 via input
signal terminal 36 and a resistive impedance R.sub.1. Also instead
of a direct circuit type feedback loop 15 (FIG. 2) coupled between
circuit node 17 and the (-) input port 14, it now comprises a
feedback loop 15' which includes impedance means comprising a
resistive impedance R.sub.2. The impedance values of R.sub.1 and
R.sub.2 are chosen such that R.sub.2 =R.sub.1, in which case a
simple signal inverter results; however, if R.sub.2 >R.sub.1,
then an inverting amplifier is provided. In both instances, the
voltage across the current sensing impedance 22 is equal to I.sub.L
Z.sub.m. With the amplifying and conditioning circuit 28 being
coupled across circuit nodes 16 and 17, an output voltage V.sub.0
is still generated which is proportional to I.sub.L.
Referring now to FIG. 6, shown thereat is a current-measuring
non-inverting amplifier configuration 36. As shown, the input
signal V.sub.in is again applied to the non-inverting (+) input
port 12, as before, but now it is applied via a voltage divider
consisting of resistive impedances R.sub.3 and R.sub.4. R.sub.3 is
connected to input signal terminal 19, while R.sub.4 is connected
to terminal 38, to which is applied a bias voltage V from a source,
not shown. With respect to the inverting (-) input port 14, the
feedback loop 15' includes a voltage divider comprising resistive
impedances R.sub.1 and R.sub.2 ; however, resistive impedance
R.sub.1 is now returned to ground. For proper operation, R.sub.2
>R.sub.1 and R.sub.4 >R.sub.3. As before, the voltage across
the current-measuring resistive impedance (Z.sub.m) 22 is coupled
to an amplifying and conditioning circuit 28 which provides an
output voltage V.sub.0 which is proportional to I.sub.1.
In the configuration of FIG. 6, the current-measuring feature does
not change the amplification of the circuit, but limits the current
I.sub.L op-amp 11 can produce because the voltage in front of the
current-measuring impedance Z.sub.m will increase with device
output current and can in certain instances, cause cut off of the
device.
Another current-measuring circuit derived from the
current-measuring voltage follower concept comprises a
current-measuring precision diode circuit configuration 40 which is
shown in FIG. 7. This embodiment comprises a circuit substantially
identical to that shown in FIG. 2, with the exception that a
precision diode 42 is connected in series between the
current-measuring impedance 22 and the load impedance 18. Also, the
feedback loop 15 comprising a direct connection now is taken from
circuit node 44 which is on the output side of the diode 42 rather
than circuit node 17.
Yet another circuit configuration 46 is shown in FIG. 8 and differs
from the previous circuit configurations shown in FIGS. 2 through 7
in that the current-measuring voltage follower circuit shown in
FIG. 2 is now coupled to the output of a conventional inverting
integrator circuit including a second operational amplifier 48. As
shown in FIG. 8, an input signal is coupled to the inverting (-)
input port 50 of the op-amp 48 by way of an input terminal 30 and a
resistive impedance 54. The non-inverting (+) input port 56 of the
op-amp 48 is connected directly to ground. The output port 58 of
the operational amplifier 48 is coupled back to the (-) input port
50 by means of a capacitive impedance shown by reference numeral
60.
In such a circuit, adding a measurement impedance Z.sub.m to the
feedback loop of the operational amplifier 48 would drastically
alter its performance; however, adding a current-measuring voltage
follower (FIG. 2) to the output will not affect performance.
The circuits of FIGS. 2-6 are superior to their non
current-measuring counterparts in circumstances where the current
output must be known.
Referring now to FIG. 9, shown thereat is a conventional four arm
Wheatstone bridge 62 comprised of four impedances Z.sub.1, Z.sub.2,
Z.sub.3 and Z.sub.4, with Z.sub.2 being an adjustable impedance.
The impedances Z.sub.1 and Z.sub.2 form adjacent arms of the bridge
and terminate in a common circuit node 64 and which is coupled to
one side of an AC voltage source 65 which generates a voltage
V.sub.s. The other two impedances Z.sub.3 and Z.sub.4 form a second
pair of adjacent arms opposite the impedances Z.sub.1 and Z.sub.2
and terminate in a common circuit node 66 which is returned to
ground as is the opposite side of the voltage source 64.
As is well known, if the impedances Z.sub.3 and Z.sub.4 are equal
valued, the currents I.sub.L1 and I.sub.L2 will be equal when
Z.sub.1 =Z.sub.2 and the voltage difference between circuit nodes
68 and 70 will be equal to zero.. This and any bridge unbalance can
be detected when a voltage measuring device 72 is coupled between
circuit nodes 68 and 70.
If the impedance Z.sub.4 comprises a sensor element, the voltage
difference between circuit nodes 68 and 70 changes and the voltage
measuring device 72 between these two points reflects such a
change. However, if the impedances Z.sub.3 and Z.sub.4 are
comprised of two sensor elements or a sensor element and a
reference element, the difference in potential between circuit
nodes 68 and 70 indicates leakage or cross-talk. This can be a
significant problem which increases as bridge imbalance increases,
for example at relatively high sensor readings.
This now leads to the configuration shown in FIG. 10 wherein an
active current bridge 74 is configured from a pair of
current-metering voltage follower circuits 20'.sub.1 and 20'.sub.2
which form the upper left and right legs of the bridge and being
connected at the common circuit node 64, while the left and right
lower leg portions comprise the impedances Z.sub.3 and Z.sub.4 of
FIG. 9 which are connected at the common circuit node 66.
Impedances Z.sub.1 and Z.sub.2 which are shown comprising legs of
the conventional bridge circuit 62, shown in FIG. 9, are now
included in the voltage follower circuits 20'.sub.1 and 20'.sub.2,
including respective op-amps 11.sub.1 and 11.sub.2 having their
respective non-inverting (+) input ports 12.sub.1 and 12.sub.2
commonly connected to circuit node 64. Direct connection feedback
loops 15.sub.1 and 15.sub.2 couple back from circuit nodes 17.sub.1
and 17.sub.2 to the respective inverting(-) input ports 14.sub.1
and 14.sub.2. The circuit nodes 17.sub.1 and 17.sub.2 correspond to
the bridge circuit nodes 68 and 70 shown in FIG. 9.
The left leg current I.sub.L1 is generated by the current-measuring
voltage follower 20'.sub.1 while the right leg current I.sub.L2 is
generated by the current-measuring voltage follower 20'.sub.2. The
left leg current I.sub.L1 passes through the impedance Z.sub.1
which corresponds to the current-measuring impedance Z.sub.m of the
current-measuring voltage follower shown in FIG. 2. From circuit
node 68 the current I.sub.L1 travels through impedance Z.sub.3 to
ground. In a like manner, the right leg current I.sub.L2 travels
through the impedance Z.sub.2 to circuit node 70 and through
impedance Z.sub.4 to ground.
In the circuit configuration of FIG. 10, the voltages at circuit
nodes 68 and 70 are identical and equal to the source voltage
V.sub.s. Assuming that the impedance Z.sub.4 comprises an unknown
impedance, the right leg current I.sub.L2 is adjusted by the right
leg op-amp 11.sub.2 as Z.sub.4 changes to maintain the voltage drop
across Z.sub.4 locked to V.sub.s. However, this changes the voltage
across the impedance Z.sub.2 and accordingly a voltage differential
occurs between output ports 16.sub.1 and 16.sub.2 due to the
respective voltage drop across Z.sub.2 and Z.sub.1. Accordingly,
bridge measurement is now made across 16.sub.1 and 16.sub.2. Thus,
the differential voltage measurement is made in front of the
impedance Z.sub.1 and Z.sub.2 in the active current bridge of FIG.
10 rather than behind the impedances Z.sub.1 and Z.sub.2 of the
classical Wheatstone bridge arrangement shown in FIG. 9.
In the active current bridge circuit 74 shown in FIG. 10, the
current I.sub.s drawn from the voltage source 65 is approximately
zero, whereas in the conventional Wheatstone bridge circuit 62
shown in FIG. 9, the current I.sub.s is equal to the sum of the
currents I.sub.L1 and I.sub.L2.
Thus what has been shown and described is a new family of
operational amplifier circuitry which is particularly able to keep
instrumentation sensors locked at the same voltage potentials so
that they do not exhibit cross-talk between them and yet are able
to precisely measure the current that passes through each one of
them individually as they function as sensors. Also, these circuits
can be used to create simple, stand alone precision current meters
for relatively small currents. Furthermore, the active current
bridge shown and described can perform many functions that
classical Wheatstone bridges are incapable of performing.
Having thus shown and described what is at present considered to be
the preferred embodiments of the invention, it should be noted that
the same has been made by way of illustration and not limitation.
Accordingly, all modifications, alterations and changes coming
within the spirit and scope of the invention are as set forth in
the appended claims.
* * * * *