ELECTRICALLY ISOLATED SIGNAL PATH MEANS FOR A PHYSIOLOGICAL MONITOR

• US 3,690,313 A
• Filed: 10/09/1970
• Issued: 09/12/1972
• Est. Priority Date: 10/09/1970
• Status: Expired due to Term

• Current Assignee: Benjamin H. Weppner, Leo P. Lefebvre, Victor R. Miller
• Original Assignee: Benjamin H. Weppner, Leo P. Lefebvre, Victor R. Miller
• Inventors: Miller, Victor R., Weppner, Benjamin H., Lefebvre, Leo P.
• Primary Examiner(s): Kamm, William E.
• Application Number:
  US05/079,516
• Time in Patent Office: 704 Days
• Field of Search: 128/2.5R,2.6B,2.8R,2.1A,2.1R 307/92 317/9AC,9B,9R
• US Class Current: 600/508
• CPC Class Codes: A61B 5/301 providing electrical separation, e.g. by using isolating transformers or optocouplers
• CPC Class Codes: A61B 5/308 for electrocardiography [ECG]
• CPC Class Codes: G01R 19/18 using conversion of DC into AC, e.g. with choppers
• CPC Class Codes: H03F 3/387 with semiconductor devices only
• CPC Class Codes: Y10S 128/902 Biological signal amplifier
• CPC Class Codes: Y10S 128/908 Patient protection from electric shock

Abstract

A circuit for providing electrical isolation between a patient and physiological monitoring apparatus comprising an amplifier, having applied to the input thereof a physiological signal from the patient, and a first circuit branch including an isolation transformer connected between a modulator and a demodulator for connecting a d.c. voltage source of relatively low magnitude to the amplifier. Connected to the amplifier output is a second circuit branch comprising an isolation transformer connected between a modulator and a demodulator, and the output of the demodulator is coupled to the input of physiological monitoring apparatus. Each of the isolation transformers includes an extremely low capacitance between the primary and secondary windings thereof to present an extremely high reactance to low frequency, including line frequency, alternating current and each is capable of withstanding relatively high voltages without breakdown.

Claims

1. A circuit for providing electrical isolation between a patient and physiological monitoring apparatus comprising:

a. an amplifier;

b. means for applying a physiological signal from a patient to the input of said amplifier;

c. means defining an electrical path for connecting said amplifier to a d.c. source of relatively low magnitude;

d. first isolation means in said electrical path;

e. means defining a signal transmission path for connecting the output of said amplifier to the input of physiological monitoring apparatus;

f. second isolation means in said signal transmission path; and

g. said first and second isolation means each including an extremely low capacitance and no other leakage current path between the patient and earth ground to present a relatively high reactance to line frequency alternating current and each being capable of withstanding relatively high voltages applied thereacross without breakdown for isolating the patient from leakage currents.

2. A circuit according to claim 1 wherein each of said first and second isolation means comprises a transformer having primary and secondary windings, the capacitance between said primary and secondary windings in each transformer being the only stray capacitance providing a path for leakage currents and being sufficiently low without earth ground shielding to present an extremely high reactance to low frequency and line frequency alternating current so as not to permit a patient hazard from leakage currents at low and line frequency.

3. A circuit according to claim 1 wherein said means defining an electrical path comprises:

a. modulator means for converting a d.c. voltage into a periodic signal, the output of which is connected to said first isolation means;

b. means for connecting the input of said modulator to said d.c. source; and

c. demodulator means having an input connected to said isolation means and an output connected to said amplifier. ,4 A circuit according to claim 3 wherein the frequency of said periodic signal is greater than about 1,000 c.p.s.

5. A circuit according to claim 3 wherein said first isolation means comprises a transformer having a primary winding connected to the output of said modulator means and a secondary winding connected to the input of said demodulator means, the capacitance between said primary and secondary windings being the only stray capacitance providing a path for leakage currents and being sufficiently low without earth ground shielding to present an extremely high reactance to low frequency and line frequency alternating current so as not to permit a patient hazard from leakage currents at low and line frequency.

6. A circuit according to claim 3 further including means connecting the output of said modulator means to said signal transmission path for synchronizing the operation of said signal transmission path with the operation of said electrical path.

7. A circuit according to claim 1 wherein said means defining a signal transmission path comprises:

a. modulator means for producing a periodic signal, the output of which is connected to said second isolation means;

b. means for connecting the input of said modulator to the output of said amplifier;

c. demodulator means having an input connected to said second isolation means and an output; and

d. means for coupling the output of said demodulator means to said physiological monitoring apparatus.

8. A circuit according to claim 7 wherein said second isolation means comprises a transformer having a primary winding connected to the input of said demodulator means and a secondary winding connected to the output of said modulator means, the maximum capacitance between said primary and secondary windings being the only stray capacitance providing a path for leakage currents and being sufficiently low without earth ground shielding to present an extremely high reactance to low frequency and line frequency alternating current so as not to permit a patient hazard from leakage currents at low and line frequency.

9. A circuit according to claim 7 wherein the frequency of said periodic signal is greater than about 1,000 c.p.s.

10. A circuit according to claim 7 further including means connecting said modulator means and said demodulator means to said electrical path for synchronizing the operation of said modulator means and demodulator means with that of said electrical path.

11. A circuit for providing electrical isolation between a patient and physiological monitoring apparatus comprising:

a. an amplifier having an input and an output;

b. means for applying a physiological signal from a patient to the input of said amplifier;

c. a first circuit branch comprising a modulator, an isolation transformer connected to said modulator, and a demodulator connected to said transformer, the output of said demodulator being connected to said amplifier;

d. means for connecting the input of said modulator to a d.c. voltage source of relatively low magnitude;

e. a second circuit branch comprising a modulator, an isolation transformer connected to said modulator, and a demodulator connected to said transformer, the input of said modulator being connected to the output of said amplifier;

f. means for coupling the output of said demodulator in said second branch to the input of physiological monitoring apparatus; and

g. each of said isolation transformers including an extremely low capacitance between the primary and secondary windings thereof without earth ground shielding to present a relatively high reactance to low frequency, including line frequency, alternating current and each being capable of withstanding relatively high voltages applied across the windings thereof without breakdown.

12. A circuit according to claim 11 wherein the capacitance between said primary and secondary windings is the only stray capacitance providing a path for leakage currents and is sufficiently low so as not to permit a patient hazard from leakage currents at low and line frequency.

13. A circuit according to claim 11 wherein said modulators in said first and second circuit branches each operate at a frequency substantially greater than line frequency.

14. A circuit according to claim 11 further including means connecting the output of said modulator in said first circuit branch to said modulator and demodulator in said second circuit branch whereby the operation of the latter are synchronized with the frequency of modulation in said first branch.

15. A circuit according to claim 11 wherein said modulator in said second branch comprises a switching transistor connected between said modulator input and a ground or reference potential and having a control terminal connected to said first circuit branch whereby amplified physiological signals applied to the input of said modulator are chopped at a rate equal to the frequency of modulation of said modulator in said first branch.

16. A circuit according to claim 11 wherein said coupling means comprises an amplifier having an RC network connected across the input and output thereof and wherein said demodulator in said second branch comprises a transistor switch connected between said isolation transformer and said coupling means and having a control terminal connected to said modulator in said first circuit branch.