Isolation amplifier

There is disclosed an isolation amplifier having an input amplifier, an output amplifier, a transformer having primary and secondary windings, an alternator connected to the secondary winding of the transformer for alternately presenting a voltage and a high impedance to the secondary winding, a first switching device connected between the primary winding of the transformer and the output of the input amplifier for connecting the output of the input amplifier to the primary winding when the alternator presents a high impedance to the secondary winding and for disconnecting the output of the input amplifier from the primary winding when the alternator presents a voltage to the secondary winding and a second switching device connected between the secondary winding and the input to the output amplifier therefore connecting the input of the output amplifier to the secondary winding when the alternator presents a high impedance to the secondary winding and disconnecting the input of the output amplifier from the secondary winding when the alternator presents a voltage to the secondary winding. There is further disclosed a coupling circuit between the primary of the transformer and the input to the input amplifier and a rectifier circuit connected to the input of the input amplifier for causing the input amplifier to go into lock-up when the input to the input amplifier is disconnected from a source of signal.

The present invention relates to isolation amplifiers and more particularly 
to transformer isolated instrumentation amplifiers for electrocardiogram 
and other bio-potential measurements. 
In the fields of data acquisition systems, process control measurements, 
medical and bio-potential measurements, and vital power system parameter 
measurement, it has been the practice to employ isolation amplifiers for 
accurate signal recovery along with excellent common mode noise and 
voltage rejection and high electrical isolation between the signal source 
and the processing system. The need for isolation arises when it is 
necessary to measure low level signals in the presence of high common mode 
voltages to eliminate measurement errors caused by disturbances on a 
common ground network, to avoid ground loops and inherent pick-up 
problems, to protect processing circuitry from damage from large common 
mode voltage levels at both input and output and provide a 
patient-interface when making measurements on human subjects. 
In the field of isolation amplifiers it has been the practice to employ 
transformer coupled amplifiers and optically coupled amplifiers to obtain 
the required isolation between input and output terminals. The transformer 
coupled isolation amplifier is commonly termed the carrier isolation 
amplifier and generally employs at least two transformers as shown in U.S. 
Pat. No. 3,754,193. One transformer is used to couple a carrier signal to 
an isolated input modulator and to provide power for the isolated input 
amplifier and the other transformer is used to couple the modulated signal 
from the isolated input to a synchronous demodulator in the output portion 
of the isolated amplifier. 
The optically coupled amplifiers generally employ a signal controlled light 
source, such as a light-emitting diode, (L.E.D.) and a pair of matched 
photo-detecting devices, such as photo-diodes. The accuracy of the signal 
transfer between the input and the output of the optically coupled 
amplifier is dependant upon the matching of the photo-diodes and the 
spectral emission of the light emitting diode to the spectral response of 
each of the photo-diodes. Since the optically coupled isolation amplifier 
has no carrier frequency coupled to the isolated input, as in the 
transformer coupled isolated amplifier, a DC to DC converter is required 
in order to provide power to the isolated front end. The DC to DC 
converter employs a transformer for isolating the DC voltages applied to 
the floating input from the DC voltage applied to the converter input. 
Although such dual transformer isolated amplifiers and optically coupled 
amplifiers have served the purpose they have not proven entirely 
satisfactory under all conditions of service for the reasons that 
considerable difficulty has been experienced in reducing the size, 
complexity and weight of the amplifier's circuitry and electrical 
components and difficulties encountered in reducing the cost of the 
amplifier and in improving the reliability of the amplifier's operation. 
Those concerned with the development of isolation amplifiers have long 
recognized the need for a simple, low cost circuit with a minimum number 
of components and with the same isolation accuracy and precision provided 
heretofor. The present invention fulfills this need. 
One of the most critical problems confronting designers of isolated 
amplifiers has been to design a coupling circuit between the input and 
output of the isolation amplifier comprising a single electrical 
component. This problem is overcome by the present invention. 
The general purpose of the invention is to provide an isolation amplifier 
which embraces all the advantages of similarly employed isolated 
amplifiers and possesses none of the aforedescribed disadvantages. To 
obtain this the present invention contemplates a unique common mode 
isolation and signal coupling circuit employing a single transformer 
having only two windings and an alternator or relaxation oscillator and 
switching arrangement whereby power supply energy and signal information 
are coupled through a single dual winding transformer and large physical 
size, high complexity and cost, and multiplicity of electrical coupling 
elements are avoided. 
An object of the present invention is the provision of isolation in an 
isolated amplifier by a single electrical coupling element. 
Another object is to provide an isolated amplifier having a single 
transformer which couples both power and signal information. 
A further object of the invention is the provision of an isolated amplifier 
which time shares a single coupling transformer for power and signal 
transmission. 
Still another object is to provide a loose electrode indication in an 
isolated amplifier having a single electrical coupling element.

Referring now to the drawings wherein like reference characters designate 
like or corresponding parts throughout the several figures, there is shown 
in FIG. 1 (which illustrates a preferred embodiment) an amplifier 5 having 
input terminals 7 and 9, positive and negative power supply terminals 11 
and 13, respectively, a common terminal 15 connected to an input reference 
ground 16 and an output terminal 17. A capacitor 18 is connected between 
output terminal 17 and input reference ground 16 and a diode 19 has the 
cathode thereof connected to output terminal 17 and the anode thereof 
connected to terminal 21 of the primary winding of a transformer 25. The 
other end of the primary winding is designated as terminal 27 and the 
center tap of primary winding is designated as terminal 29 which in turn 
is connected to the reference input ground 16. Terminal 27 is connected to 
the anode of a diode 31, the cathode of which is connected to a capacitor 
33 which in turn is connected to input reference ground 16. The junction 
between diode 31 and capacitor 33 is designated as terminal 35 and the 
source of power supply voltage +V.sub.f. 
Terminal 21 is further connected to the cathode of a diode 37 which in turn 
has the anode thereof connected to a capacitor 39 which in turn is 
connected to input reference ground 16. The junction between diode 37 and 
capacitor 39 is designated as terminal 41 and the source of power supply 
voltage -V.sub.f. The power supply source voltages +V.sub.f and -V.sub.f 
provide power to power supply terminals 11 and 13, respectively, of 
amplifier 5. 
Transformer 25 has a secondary winding connected between terminals 43 and 
45 and a center tap designated as terminal 47 which in turn is connected 
to an output reference ground 65. The polarity of the secondary winding 
with respect to the primary winding of transformer 25 is such that when a 
current enters into the primary winding at terminal 21, a current will 
leave the secondary winding at terminal 43. 
Terminal 43 is connected to an oscillator 51 which has a power supply 
terminal 53 connected to a source of direct current voltage -V. Oscillator 
51 in turn is connected to terminal 45 of the secondary winding of 
transformer 25. Terminal 45 is further connected to the cathode of a diode 
57 which has the anode thereof connected to the positive input terminal 59 
of an amplifier 61. Terminal 59 further has a capacitor 63 connected 
therefrom to output reference ground 65. Amplifier 61 has a power supply 
terminal 67 to which is connected a source of direct current voltage +V, 
and a power supply terminal 69 which is connected to a source of direct 
current voltage -V along with the negative input terminal. Amplifier 61 
has a pair of output terminals 71 and 73 of which terminal 73 is connected 
to output reference ground 65. 
Turning now to FIG. 2, there is further illustrated a coupling circuit 75 
which is connected to the primary winding of transformer 25. Within 
coupling circuit 75 a resistor 77 is connected in series with a capacitor 
79 which in turn is connected to terminal 27 of the primary winding of 
transformer 25. Similarly a resistor 81 is connected in series with a 
capacitor 83 which in turn is connected to terminal 21 of the primary 
winding of transformer 25. The other end of resistor 77 is connected to 
input terminal 9 of amplifier 5 and the other end of resistor 81 is 
connected to input terminal 7 of amplifier 5. A neon lamp 85 is connected 
between the junction of resistor 77 and capacitor 79 and the junction of 
resistor 81 and capacitor 83. 
Within amplifier 5 a resistor 87 is connected from input terminal 7 to a 
resistor 89 which in turn is connected to positive input terminal 90 of a 
differential amplifier 91. A resistor 92 is connected from input terminal 
9 to a resistor 93 which in turn is connected to negative input terminal 
94 of amplifier 91. A neon lamp 95 is connected between the junctions of 
resistors 87 and 89 and 92 and 93, respectively. A capacitor 97 is 
connected from the junction of resistors 87 and 89 to the anode of a diode 
99 which in turn has the cathode thereof connected to the cathode of a 
diode 101. The anode of diode 101 is connected to a capacitor 103 which in 
turn is connected to the junction of resistors 92 and 93. The junction of 
diode 101 and capacitor 103 is connected to a resistor 105 which in turn 
is connected to a resistor 107 being connected in turn to the junction of 
capacitor 97 and diode 99. The junction of resistors 105 and 107 is 
connected to a resistor 109 which in turn is connected to a source of 
direct current supply voltage +V.sub.f designated as terminal 11 of 
amplifier 5. A resistor 111 also is connected between the junction of 
resistors 105 and 107 and a source of direct current supply voltage 
-V.sub.f designated as terminal 13 of amplifier 5. 
A resistor 113 is connected from input terminal 90 of amplifier 91 to a 
resistor 115 which in turn is connected to input reference ground 16. The 
junction of resistors 113 and 115 is connected to resistor 117 which in 
turn is connected to output terminal 17 of amplifier 5. An npn transistor 
119 has the collector thereof connected to output terminal 17, the emitter 
connected to input reference ground 16 and the base connected through a 
resistor 121 to the output of amplifier 91. 
Oscillator 51 includes an npn transistor 123 which has the collector 
thereof connected to terminal 43, the base connected to the junction of a 
capacitor 125 and a resistor 127, and the emitter connected to the other 
end of capacitor 125 the junction being designated as terminal 53. The 
other end of resistor 127 is connected to the junction of a capacitor 128 
and a resistor 129, the other end of resistor 129 being connected to 
terminal 43 and the other end of capacitor 128 being connected to terminal 
48 of transformer 25. 
In FIG. 2 center tap terminal 47 of the secondary winding of transformer 25 
is illustrated as being connected to a source of direct current voltage +V 
instead of output reference ground shown in FIG. 1. Similarly terminal 53 
of oscillator 51 is shown in as being connected to output reference ground 
65. The difference in connections between FIG. 2 and FIG. 1 illustrates 
the choice to the designer for the selection of reference and power supply 
voltage points within the circuit. It should be further noted that a 
negative voltage may be used by appropriately changing diode polarity and 
transistor type, well known to electronic circuit designers. 
The circuit of FIG. 2 further has a resistor 130 connected across diode 57, 
the function of which will be explained hereinbelow. 
Amplifier 61 includes a resistor 131 which is connected from terminal 69 to 
a negative input 133 of a differential amplifier 134. A resistor 135 may 
be connected from terminal 69 to a source direct current voltage +V 
designated as a terminal 67. Input terminal 133 of amplifier 134 is 
further connected to a resistor 137 which in turn is connected to the 
output of amplifier 134 and to output terminal 71. Positive input terminal 
138 of amplifier 134 is connected to output reference ground 65 which in 
turn is connected to output terminal 73. The positive and negative power 
supply terminals 67 and 69, respectively, of amplifier 134 are connected 
to +V and -V power sources, respectively. 
The signal and power coupling circuits of the invention can best be 
described by reference to FIG. 1. Tranformer 25 is readily observed to be 
the sole electrical element coupling signals from input amplifier 5 to 
output amplifier 61 and isolating input reference ground 16 from output 
reference ground 65. 
Oscillator 51 functionally can be described as an alternator which 
alternately presents or connects a source of direct current voltage and a 
high impedance or open circuit to the secondary winding of the transformer 
25. During the time oscillator 51 presents a voltage to the secondary 
winding, -V is connected to terminal 43 of the secondary winding and, 
therefore, a voltage -V is applied from terminal 43 to center tap terminal 
47 of the secondary winding. In response, a voltage of +V appears at 
terminal 45 with respect to center tap terminal 47. The +V voltage at 
terminal 45 backbiases diode 57 and, therefore, prevents any conduction of 
current to capacitor 63. Therefore a voltage of 2V appears across the 
secondary winding and is coupled through transformer 25 to produce a 
negative voltage at terminal 21 with respect to center tap terminal 29 of 
the primary transformer 25 and a positive voltage between terminal 27 and 
center tap terminal 29 of transformer 25. The negative voltage on terminal 
21 and the positive voltage on terminal 27 with respect to center tap 
terminal 29 causes diodes 31 and 37 to conduct to charge capacitors 33 and 
39, respectively. The charging of capacitors 33 and 39 develops +V.sub.f 
and -V.sub.f power supply voltages to power amplifier 5. The negative 
voltage on terminal 21 of the primary winding of transformer 25 causes 
diode 19 to be backbiased and, therefore, capacitor 18 is disconnected 
from terminal 21. 
When oscillator 51 presents a high impedance to secondary winding of 
transformer 25 or substantially creates an open circuit between -V and 
terminal 43, the energy stored in the transformer cannot change 
instantaneously and a voltage is developed across the windings of the 
transformer of opposite polarity to that previously established, which 
reversed polarity tends to try to keep the current flowing in the windings 
in the same direction as before. Therefore, terminal 45 becomes negative 
with respect to center tap terminal 47 causing diode 57 to conduct and 
similarly terminal 21 of the primary winding of transformer 25 becomes 
positive with respect to center tap terminal 29 causing diode 19 to 
conduct. The conduction of diode 19 clamps terminal 21 of primary winding 
of transformer 25 to the voltage across capacitor 18 which is the signal 
voltage amplified by amplifier 5. Consequently, the signal voltage across 
capacitor 18 is transmitted through the transformer to the secondary 
winding and a negative voltage directly proportional thereto is induced 
between terminal 45 and center tap 47 of the secondary winding. This 
negative voltage is coupled through diode 57 to capacitor 63 which in turn 
holds the negative voltage coupled thereto while diode 57 is back-biased. 
Oscillator 51 then applies a -V voltage between terminal 43 and center tap 
47 to start the cycle again. 
The frequency of oscillation of oscillator 51 is chosen to be significantly 
larger than the highest frequency of the signal being transmitted between 
the input and the output of the isolation amplifier such that during the 
times that the signal is coupled from capacitor 18 through transformer 25 
to capacitor 63, there is substantially negligible signal variation. 
It should be pointed out that amplifier 5 can be an operational amplifier 
well known in the field of electrical measurement and does not necessarily 
have to be a differential amplifier as illustrated. Input terminal 9 could 
be connected to input reference ground 16 and the input applied between 
terminal 7 and input reference ground 16. 
However, for medical applications input reference ground is generally 
connected to some reference point on a living subject and input terminals 
7 and 9 would be connected to a specific area of the living subject to 
measure a bio-potential therebetween. In such an application amplifier 5 
would be a differential amplifier. Similarly, amplifier 61 can be an 
operational amplifier. 
Transformer 25 can be a toroidal transformer in which the primary winding 
and the secondary winding contain the same number of turns. The higher the 
oscillator frequency, the smaller the core and the number of turns 
required for transformer 25. A typical frequency for the oscillator is 
approximately 1 megaHz. 
Directing the discussion of operation now to FIG. 2, oscillator 51 is shown 
as a simple single transistor relaxation oscillator. When power is applied 
to the center tap 47, the transistor initially turns on clamping terminal 
43 to terminal 53 and, therefore, applying +V voltage between center tap 
47 and terminal 43 of transformer 25. This results in 2 times +V volts 
appearing at terminal 45 maintaining diode 57 in a backbiased condition. 
However, resistor 130 across diode 57 allows a current to flow into 
capacitor 63 so that capacitor 63 can follow the positive going signals 
applied thereacross. The time constant established by resistor 130 and 
capacitor 63 is sufficient to allow capacitor 63 to follow the signal 
voltage. The ratio of resistor 137 to 131 determines the gain of amplifier 
134. 
Coupling circuit 75 couples through capacitors 83 and 79 and resistors 81 
and 77, respectively, the alternating signal across the primary of 
transformer 25 to the input terminal 7 and 9. Capacitors 79 and 83 couple 
the alternating signal but prevent any DC from being coupled therethrough. 
Neon lamp 85 provides a form of limiting or clamping device to limit the 
voltage appearing thereacross from external sources. Resistors 81 and 77 
are sufficiently large in magnitude so as to provide a small AC current 
across a low impedance that may be connected between terminals 7 and 9. 
When a measuring electrode becomes disengaged from terminals 7 and 9 the 
current flowing through resistors 81 and 77 is then directed through 
resistors 87 and 92 and alternately through capacitors 103 and 97 through 
diodes 99 and 101, to cause input terminal 90 of amplifier 91 to be biased 
into a positive direction lock-up, thereby providing a positive full-scale 
lock-up as an electrode disconnect indication. Resistors 107 and 105 are 
connected to a voltage at the junction of resistors 109 and 111 so as to 
allow capacitors 97 and 103 to discharge between cycles and thereby allow 
sufficient current through diodes 101 and 99 to maintain amplifier 91 in 
lock-up. Neon lamp 85 provides a voltage limitation or break down element 
to prevent the voltage between terminals 9 and 7 from developing to an 
excessive amount. 
It should be noted that the input circuitry between terminals 7 and 9 and 
terminals 90 and 94 of amplifier 91 is alternating current linear, thereby 
preventing direct polarization of biopotential electrodes which may be 
connected to input terminal 7 and 9. If the electrodes were utilized for 
electrocardiogram monitoring, direct current polarization would cause loss 
of this monitoring ability until depolarization occurs. Therefore, if 
there is a large magnitude of radio frequency from diathermy apparatus or 
other medical treating apparatus, this high level radio frequency would 
not create any direct current in the input circuitry of the isolation 
amplifier nor polarize electrodes. 
Amplifier 91 and resistors 89 and 113, and 93 and 96 (which resistors are 
all equal precision resistors) form a unity gain differential amplifier. 
Resistors 98 and 100 set the reference bias of the amplifier such that 
with a zero input signal, transistor 119 clamps the transformer primary 
winding through diode 19 to an amplitude set to be approximately +V.sub.f 
arbitrarily. R17 and R15 set the gain of the amplifier. 
It now should be apparent that the present invention provides a circuit 
arrangement which may be employed in conjunction with an isolated 
amplifier for coupling both power and signal information through a single 
isolation transformer in addition to providing an open electrode 
indication for the input terminals. 
Although particular components, etc., have been discussed in connection 
with specific embodiments of an isolated amplifier constructed in 
accordance with the teachings of the present invention, others may be 
utilized. Furthermore, it will be understood that although an exemplary 
embodiment of the present invention has been disclosed and discussed, 
other applications and circuit arrangements are possible and that the 
embodiments disclosed may be subjected to various changes, modifications 
and substitutions without necessarily departing from the spirit of the 
invention.