MEANS AND METHOD FOR SUBTRACTING DC NOISE FROM ELECTROCARDIOGRAPHIC SIGNALS

• US 3,580,243 A
• Filed: 10/21/1968
• Issued: 05/25/1971
• Est. Priority Date: 10/21/1968
• Status: Expired due to Term

• Current Assignee: Gilbert A. Johnson
• Original Assignee: Gilbert A. Johnson
• Inventors: Johnson, Gilbert A.
• Application Number:
  US04/769,314
• Time in Patent Office: 946 Days
• US Class Current: 600/508
• CPC Class Codes: A61B 5/30 Input circuits therefor
• CPC Class Codes: A61B 5/308 for electrocardiography [ECG]
• CPC Class Codes: H03F 1/303 using a switching device H03F1/305, H03F3/005, H03F3/38 take precedence
• CPC Class Codes: Y10S 128/902 Biological signal amplifier

Abstract

The electrocardiographic signal and a DC noise level are applied to a first of two input terminals to a differential amplifier and the output of the differential amplifier supplies the electrocardiographic signal to an output terminal without the DC noise level. To offset the DC noise level, a feedback loop from the output terminal to the second input terminal of the differential amplifier includes a sample and hold circuit that first applies the output from the differential amplifier through a nulling amplifier to a capacitor and then applies the potential on the capacitor to the second input terminal of the differential amplifier.

Claims

1. A method of obtaining an indication of an electrocardiographic signal comprising the steps of:

attaching electrodes to a subject for the generation of electrical potentials including a DC voltage level having a time varying electrocardiographic signal component imposed thereon;

receiving a first potential from said electrodes during a first period of time;

comparing said first potential with a ground reference and deriving an output signal representative of the DC voltage level of said first potential with respect to ground;

storing said output signal during said first period of time;

receiving a second potential from said electrodes during a second period of time;

subtracting from said second potential received during said second period of time said stored output signal to obtain the time varying component of said potential with respect to ground received during said second period of time; and

applying said time varying component of said potential to electrical-indicating equipment to obtain an indication of said electrocardiographic signal.

2. The method of claim 1 in which the step of storing said output signal includes the steps of applying the output signal to one plate of a capacitor until said capacitor is charged to a potential equal to that of the DC voltage level with respect to ground and removing the output signal from said plate of said capacitor after the signal stored on said capacitor is equal to the DC voltage level.

3. The method of claim 1 in which:

the steps of receiving and comparing said first potential and of storing said output signal include the steps of electrically connecting at least one input terminal of a differential amplifier to said electrodes, applying the output of said differential amplifier to the input of a comparator amPlifier, and applying the output signal of the comparator amplifier to one plate of a capacitor having the second plate thereof grounded for charging the capacitor; and

the step of subtracting the stored signal from said second potential comprising the steps of removing the output signal of said comparator amplifier from said one plate of said capacitor, and biasing the gate electrode of a field effect transistor connected in source follower configuration to the second input of said differential amplifier with the charge of the capacitor, whereby the output potential of said differential amplifier provides an indication of said electrocardiographic signal component.

4. An electrocardiograph input circuit having an input terminal adapted to receive an input potential signal comprised of an electrocardiographic signal and a DC voltage level and having an output terminal providing an electrocardiographic signal referenced to ground, comprising:

a differential amplifier (24) responsive to first and second input signals applied to first (26) and second (28) input terminals for providing a corresponding output signal at an output terminal, said first input terminal being connected to the input terminal of said electrocardiograph input circuit and said output terminal being connected to the output terminal (56) of the electrocardiograph input circuit;

a high gain comparator amplifier (32) for comparing signals applied to a pair of input terminals, one of said input terminals being connected to the output terminal of said differential amplifier for receiving the input potential signal, the other of said input terminals being connected to ground, said amplifier providing a ground referenced comparative output signal at an output terminal thereof;

a switch (36) having a first terminal electrically connected to the output terminal of said comparator amplifier (32) and a second terminal adapted to be connected to said first terminal when said switch is closed, said switch being periodically operable into a low resistance closed state during a first period of time and into a substantially infinite resistance open state during a second period of time;

means for storing a potential representative of the difference between said input potential signal and a ground reference comprising a capacitor (40) having one plate electrically connected to said second terminal of the switch for receiving said comparative output signal when said switch is closed, said capacitor having its other plate grounded; and

a field effect transistor (38) having a gate electrode connected to said second terminal of said switch, a drain electrode connected to a bias means, and a source electrode connected to said second input terminal of said differential amplifier for establishing the output of the differential amplifier at a ground potential reference during said first period of time by applying the comparative output signal of said comparator amplifier and for subtracting said stored potential from said input potential signal received on said circuit input terminal during said second period of time for providing said ground referenced electrocardiographic signal at the circuit output terminal.

5. An input circuit for an electrocardiograph in accordance with claim 4 and further including a means for electrically disconnecting said comparator amplifier when said switch is opened.

6. An input circuit for an electrocardiograph in accordance with claim 5 in which:

said means for electrically disconnecting said comparator amplifier includes a third terminal for said switch adapted to be connected to said first terminal when said switch is opened;

said third terminal of said switch being electrically connected to said input terminal of said comparator amplifier.