A bio-isolated dc operational amplifier is described for use in making bioelectrical measurements of a patient while providing isolation of the patient from electrical shocks. The circuit contains a first operational amplifier coupled to the patient with its output coupled in a forward loop through a first optic coupler to a second operational amplifier. The output of the second operational amplifier is coupled to suitable monitoring circuitry via a feedback circuit including a second optic coupler to the input of the first operational amplifier

Lee, R. D.

English

NASA Technical Reports Server

REPLY TO
ATTN OF: GP
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON, D.C. 20546
MAY 2 4 1974
TO: KSI/Scientific & Technical Information Division
Attn: Miss Winnie M. Morgan
FROM: GP/Office of Assistant General
Counsel for Patent Matters
SUBJECT: Announcement of NASA-Owned U.S. Patents in STAR
In accordance with the procedures agreed upon by Code GP
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The following information is provided:
U.S. Patent No. :
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Source (if applicable)
NASA Patent case No.
U-S-
NOTE - If this patent covers an invention made by a corporate
employee of a NASA Contractor, the following is applicable:
/-y NO /~y/
Pursuant to Section 305 (a) of the National Aeronautics and
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an invention of ..."
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Bonnie L. Woerner
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https://ntrs.nasa.gov/search.jsp?R=19740013738 2020-03-17T14:51:52+00:00Z
United States Patent
Lee
[in 3,811,094
145] May 14, 1974
[54] BIO-ISOLATED DC OPERATIONAL
AMPLIFIER
[75] Inventor: Robert D. Lee, San Mateo, Calif.
[73] Assignee: The United States of America as
represented by the Administration of
the National Aeronautics and Space
Administration, Washington, D.C.
[22] Filed: June 30, 1972
[21] Appl. No.: 267,862
[52] U.S. Cl 330/59, 330/28
[51] Int. Cl H03f 17/00
[58] Field of Search 330/59; 250/217 S
[56] References Cited
UNITED STATES PATENTS
3,699,468 10/1972 Harmgardt.... 330/59 X
2,904,696 9/1959 Elliot et al 250/213
1,855.863 4/1932 McCreary 330/59
Primary Examiner—Nathan Kaufman
Attorney, Agent, or Firm—Darrell G. Brekke; Armand
G. Morin, Sr.; John R. Manning
[57] ABSTRACT
A bio-isolated DC operational amplifier is provided
for use in making bio-electrical measurements of a pa-
tient while providing isolation of the patient from elec-
trical shocks and the like, the circuit comprising a first
operational amplifier coupled to' the patient and hav-
ing its output coupled in a forward loop through a first
optic coupler to a second operational amplifier, the
output of the second operational amplifier being cou-
pled to suitable monitoring circuitry. The output of
the second operational amplifier is coupled via a feed-
back circuit including a second optic coupler to the
input of the first operational amplifier.
5 Claims, 1 Drawing Figure
(NASA-Case-ABC-10596-1) BIO-ISOLATED dc
OPERATIONAL AMPLIFIER Patent (NASA)
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BIO-ISOLATED DC OPERATIONAL AMPLIFIER e rational amplifier and associated circuitry is provided
with a separate battery power supply.
The invention described herein was made by an em- Thus the first operational amplifier circuitry coupled
ployee of the United States Government and may be to the patient is electrically isolated from the second
manufactured and used by or for the Government for 5 operational amplifier circuitry and subsequent elec-
governmental purposes without -the payment of any tronic stages of the system by a first optic coupler in the
royalties thereon or therefor. forward loop between the two operational amplifiers
BACKGROUND OF THE ,NVENT,ON
Biomedical instrumentation is widely used in hospi- 10 The operational amplifiers may be coupled to the
tals and medical clinics for bio-electrical measurements light emitters of the associated optic couplers by cur-
such as ECG, EEC and EMG and for providing electri- rent amplifiers, if desired.
cal analog signal outputs indicative of organ dimen- The resultant circuit is capable of directly isolating
sions and the like. Sensors or transducers attached to DC and AC and amplifying the same with the superior
the patient provide inputs to the electrical instrumenta- IS operating characteristic typical of a simple directly
tion where the information signals which are in the connected operational amplifier. Thus direct transmis-
form of DC levels or low frequency AC, e.g. up to sion of DC as well as AC signals is accomplished with
1 ,000 Hz, are amplified and converted to suitable read- DC drift and gain performance improved by negative
outs. Such known instruments provide a substantial ha- feedback by the utilization of the two optic couplers
zard to the patient; for example an electrical shock to 20 with total DC and AC isolation for the patient against
a catherized patient produced by ground leakage cur- electrical hazards.
time. The current path for the leakage generally in- The drawing is a schematic diagram of a preferred
volves the patient ground traditionally coupled to a 25 embodiment of a bio-isolated DC operational amplifier
local electrical power line or to earth ground. It is very circuit of the present invention.
desirable, therefore, to isolate the patient from this nFSrRIPTION OF THF PRFFFRRFn
source of electrical current. l™DS™,nJr
Certain techniques have been proposed to provide EMBODIMENT
this isolation, including instruments wherein the ampli- 30 Referring now to the drawing, the incoming bio-
fiers are DC isolated by employing transformer isola- signal is applied to one input 11 of a first integrated cir-
tion in a DC-to-DG converter consisting of an input cuit operational amplifier 12 via resistor Rl, the other
preamplifier driving a low level modulator-transformer input 13 of the amplifier 12 being coupled via resistor
or a light coupler-demodulator at a given frequency. R2 to ground Gl, the patient ground which is isolated
The systems use either amplitude modulation or fre- 35 from the main current ground G2. The output of ampli-
quency modulation techniques for transmitting the DC fier 12 is coupled to the base of transistor Ql which
information. The isolation occurs at a low level and the serves as a current amplifier to transmit current via its
DC input signals are restored at the demodulator out- emitter to the light-emitting diode 14 of a first optic
put. Such systems have undesirable features or charac- coupler 15, the transistor Ql being coupled in series
teristics including special shielded transformers to limit 40 with diode 14 and resistor R3. The gain of amplifier 12
stray leakage capacitance to 95 picofarad or less, ques- is determined by the ratio of the values of resistor Rl
tionable reliability with complex modulating- and resistor R4 in the feedback from the emitter of Ql'-
demodulating techniques, electrical noise typically lim- to the inverting input of the operational amplifier 12.
iting system accuracy to 5 percent, and excess power Capacitor Cl provides frequency stability and sets the
for FM light-pipe-coupled open loop systems. 45 low-pass frequency response for amplifier 12 and for
« I I M M A B V nc- THF PRFSEMT FMVFMTinN the bi°-'solated °C operational amplifier.SUMMARY OF THE PRESENT INVENTION
 The ^ QT ana)og signa]s afe CQUp,ed tnrQUgh ^  jn_
The present invention provides a simple stable bio- tegrated circuit optic coupler 15 comprising the em it-
isolated DC amplifier system for monitoring bio- ting diode 14 and the detector circuit comprising the
electrical and physical activities of a' patient wherein phototransistor Q2 and/or associated amplifier transis-
the patient is isolated from the power line or earth tor Q3. The optic coupler 15 provides isolation resis-
ground. The amplifier is capable of directly isolating tance of the order of 10U ohms and coupling capaci-
DC and AC and amplifying the same with the superior tance of 1 .5 picofarad or less. The output of Q3 is cou-
operating characteristics typical of a simple directly pled to the non-inverting input of the integrated circuit
connected operational amplifier. operational amplifier 16 which produces the desired
A first operational amplifier receives at its input the output to the subsequent utilization circuits. The gain
incoming bio-signal from the patient, e.g. ECG, EEG, of operational amplifier 16 is set by the ratio of resis-
EMG or analog voltage in an instrument monitoring tors R5 and R6. Capacitor C2 determines its frequency
organ, dimension, the output of the operational ampli- , response and stability..
fier being coupled to the light emitter of a first optic The output of amplifier 16 is coupled in a feedback
coupler. The photodetector of the optic coupler is cou- loop to the input 11 of amplifier 12' comprising a cur-
pled to the input of a second operational amplifier, the rent amplifier Q4 and a second optic coupler 17. The
output of this second amplifier being transmitted to current amplifier Q4 drives the light-emitting diode 18
subsequent stages or read-outs: A feedback circuit is
 65 of integrated circuit coupler 17, the phototransistor Q5
coupled from the output of the second amplifier to the and amplifier transistor Q6 transmitting the DC and
input of the first operational amplifier, the feedback AC feedback signals to amplifier 12 via feedback resis-
. circuit comprising a second optic coupler: The first op- tor R7 which determines the gain.
3,811,094
Resistors R8 and R9 provide DC bias to the detector
of optic coupler 17 to zero DC balance the operational
amplifier 12 output to prevent signal distortion. Resis-
tors RIO and Rll perform a similar function for optic
coupler 15 and operational amplifier 16.
Batteries BT1 and BT2 supply power to the electron-
ics left of dividing line A—A' and batteries BT3 and
BT4 supply power to the electronics on the right side.
Current amplifiers Ql and Q4 are not required for
current amplification if the integrated operational am-
plifiers 12 or 16 have high output current capability or
the optic couplers utilized are sufficiently efficient in
conversion and transmission of light.
I claim: .
1. An amplifier circuit for electrically isolating an
input signal from an output signal which is an amplified
version of the input signal comprising
a first operational amplifier having an input and an
output,
a signal input terminal coupled to said amplifier in-
put,
a first light emitter, and a first photodetector capable
of generating a signal when energized, said first
light emitter being optically coupled to said first
photodetector and electrically coupled to said out-
put of said first operational amplifier,
a second operational amplifier having an input and
an output, said output signal being generated at
said output of said second operational amplifier,
said input of said second operational amplifier
being coupled to said first photodetector,
a negative voltage feedback circuit for coupling the
output of said second operational amplifier to the
input of said first operational amplifier, said feed-
back circuit comprising a second light emitter, a
second photodetector capable of generating a sig-
nal when energized, and a resistor, said second
light emitter being optically coupled to said second
photodetector and electrically coupled to said out-
put of said second operational amplifier, said resis-
5 tor being connected between said photodetector
and said input of said first operational amplifier,
said feedback circuit applying a signal to said input
of said first operational amplifier which is always of
opposite polarity to the signal at said signal input
10 terminal,
a first power supply coupled to said first operational
amplifier, said first light emitter and said second
photodetector,
a second power supply coupled to said first photode-
15 tector, said second operational amplifier and said
second light emitter, said first power supply being
electrically isolated from said second power supply
and said signal input terminal being electrically iso-
lated from said output 'of said second operational
20 amplifier.
2. A circuit as claimed in claim 1 comprising a first
current amplifier coupled between the output of said
first operational amplifier and said first light emitter
and a second current amplifier coupled between the
25 output of said second operational amplifier and said
second light emitter.
3. A circuit as claimed in claim 2 wherein each of
said current amplifiers includes a transistor having a
base coupled to the output of the associated opera-
30 tional amplifier and a collector-emitter circuit coupled
to the associated light emitter.
4. A circuit as claimed in claim 1 wherein each of
said photodetectors comprises a phototransistor.
5. A circuit as claimed in claim 4 wherein each pho-
35 todetector further includes a transistor amplifier cou-
pled to said phototransistor.
40
45
50
55
60
65


Bio-isolated dc operational amplifier

Possibility of shocks from leakage currents can be reduced by use of isolated preamplifiers. Amplifier consists of battery-powered operational amplifier coupled by means of light-emitting diodes to another amplifier which may be grounded and operated from ac power mains or separate battery supply

August 1974	 B74-10112 
NASA TECH BRIEF 
Ames Research Center 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Bio-Isolated DC Operational Amplifier 
. 
is
The amplification of bioelectrical potentials ob-
tained from catheterized patients is always attendant 
with the possibility of electrical shock, for if line-
operated amplifying systems have ground leakage 
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currents as low as 20 microamperes, ventricular fibril-
lation can set in and be fatal unless corrected within 
minutes. The shock pathway generally involves the 
patient ground traditionally connected to a local 
powerline or earth ground. 
The possibility of occurence of shocks from leakage 
currents can be reduced by the use of isolated pre-
amplifiers. Prior methods for achieving isolation usu-
ally involved transformers in DC-to-DC converter 
circuits, for example, an input preamplifier driving a 
low-level modulator-transformer or light coupler-de-
modulator at a given frequency; the isolation occurs 
This document was prepared under the sponsorship o f the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
at low levels and the DC input signals are restored at 
the demodulator ouput. Either amplitude- or FM-
modulation techniques are used for transmitting the 
DC information. 
The isolated amplifier circuit shown in the diagram 
consists of a battery-powered operational amplifier 
coupled by means of light-emitting diodes to another 
amplifier in the right of the line A-A' which may be 
grounded and operated from AC power mains or a 
separate battery supply. The important feature of this 
arrangement is that the circuit and its ground on the 
left of line A-A' is completely separate from the 
circuit and the ground on the right side of the line. 
Bio signals from ECG, EEG, or other analog mon-
itoring instruments are applied to the input of opera-
tional amplifier Al, the gain of which is determined 
by the ratio of Rl and R2. If Al does not have suf-
ficient power to operate the light-emitting diode Dl 
in the optic coupler assembly OC1, amplifier Qi is 
included in the circuit as shown (with amplifiers of 
sufficient power, the overall device is quite simple). 
The amount of light emitted by diode Dl is propor-
tional to the signal fed to the amplifier Al.. 
The optic coupler OC1 provides an isolation resis-

tance of about 1011 ohms with a coupling capacitance
of about 1.5 pf; the coupler consists of the light-

emitting diode attached to the output of Al and a
phototransistor which is in the input circuit of a non-

inverting operational amplifier A2. The gain of am-

plifier A2 is set by the ratio of R3 and R4 along
with Q2. Amplifier 02 drives the light-emitting diode
in optic coupler 0C2; the system operates to couple
a DC feedback signal from amplifier A2 to the photo-

transistor detector in 0C2. The output of 0C2 it
coupled through resistor R5 to the input of amplifier
Al, thus controlling the gain of Al. Resistor R6 pro-

vides DC bias to the detector of 0C2 and serves to
(continued overleaf) 
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be tree from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19740000112 2020-03-17T14:04:27+00:00Z
balance Alto a DC zero so that signals are not dis-
torted; similarly, R7 is used to balance A2. 
Note: 
No additional documentation is available. Specific 
questions, however, may be directed to: 
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: TSP 74-10112 
Patent status: 
This invention has been patented by NASA (U.S.
Patent No. 3,811,094). Inquiries concerning nonex-
clusive or exclusive license for its commercial devel-
opment should be addressed to: 
NASA Patent Counsel 
Mail Code 200-11 A 
Ames Research Center 
Moffett Field, California 94035 
Source: Robert D. Lee 
Ames Research Center 
N	 (ARC-10596)
. 
B74-10112
	
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Bio-isolated dc operational amplifier

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