Noise reduction apparatus for low level analog signals

• US 5,724,967 A
• Filed: 11/21/1995
• Issued: 03/10/1998
• Est. Priority Date: 11/21/1995
• Status: Expired due to Term

• Current Assignee: Nellcor Puritan Bennett Incorporated (Medtronic Plc)
• Original Assignee: Nellcor Puritan Bennett Incorporated (Medtronic Plc)
• Inventors: Venkatachalam, Kalpathi Lakshminarayanan
• Primary Examiner(s): Bahr, Jennifer
• Assistant Examiner(s): WINAKUR, ERIC FRANK
• Application Number:
  US08/590,269
• Time in Patent Office: 840 Days
• Field of Search: 128/630, 128/633, 128/902, 128/635, 128/639, 128/696, 128/733, 128/734, 250/214 R, 250/214.1, 250/214 A, 250/214 C
• US Class Current: 600/310
• CPC Class Codes: A61B 2562/182 Electrical shielding, e.g. using a Faraday cage
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/30 Input circuits therefor
• CPC Class Codes: A61B 5/305 Common mode rejection
• CPC Class Codes: A61B 5/7203 for noise prevention, reduction or removal
• CPC Class Codes: Y10S 128/902 Biological signal amplifier

Abstract

Pairs of signal input leads with a low level signal imposed across them usually have noise induced by stray coupling capacitance which is typically unequal. This results in an unfavorable signal to noise ratio. Adding enough capacitance to the lead having the least capacitance, thereby equalizing the coupling capacitances to the noise services thereof, tends to reduces the noise effect considerably. This is accomplished by changing the relative sizes of the anode and/or cathode of a corresponding sensor device. The noise effect is further reduced by a negative feedback process which comprises extracting the induced noise component from the leads, inverting the noise component, and coupling the inverted noise component back to the input leads.

Claims

1. Apparatus for improving the signal-to-noise ratio of an analog signal having a noise component, comprising:

a. a sensor device for providing the analog signal between an anode and a cathode thereof, at least a portion of the noise component of the analog signal being produced by a stray impedance to the anode and a stray impedance to the cathode, said anode and said cathode being sized such that the stray impedance to the anode and the stray impedance to the cathode are substantially balanced;

b. a differential amplifier coupled to said anode and said cathode of said sensor device for amplifying said analog signal, said differential amplifier having a common-mode rejection capability thereby at least partially rejecting the common mode noise component from the analog signal.

2. Apparatus according to claim 1 wherein said sensor device comprises a photodiode and said analog signal is an analog current signal.

3. A method for improving the signal-to-noise ratio of an analog signal having a noise component in a sensor, the sensor having an anode with a cross sectional area and a cathode with a cross sectional area, the noise component being produced by stray impedance coupled to the anode and the cathode, comprising the steps of:

a. balancing the stray capacitance between the anode and the cathode by changing the size of the cross sectional area of the anode with respect to the cross sectional area of the cathode; and

b. providing the analog signal to a differential amplifier having a common-mode rejection capability.

4. A method for improving the signal-to-noise ratio of an analog signal having a noise component in a sensor, the sensor having an anode with a cross sectional area and a cathode with a cross sectional area, the noise component being produced by stray impedance coupled to the anode and the cathode, comprising the steps of:

a. balancing the stray capacitance between the anode and the cathode by changing the size of the cross sectional area of the cathode with respect to the cross sectional area of the anode; and

b. providing the analog signal to a differential amplifier having a common-mode rejection capability.

5. A for improving the signal-to-noise ratio of an analog signal having a noise component in a sensor, the sensor having an anode with a cross sectional area and a cathode with a cross sectional area, the noise component being produced by stray impedance coupled to the anode and the cathode, comprising the steps of:

a. balancing the stray capacitance between the anode and the cathode by changing the relative sizes of the cross sectional area of the anode and the cross sectional area of the cathode; and

b. providing the analog signal to a differential amplifier having a common-mode rejection capability.

6. A method according to claim 5 wherein said sensor is an oximeter sensor.

7. A method according to claim 6 wherein said oximeter sensor comprises a photodiode.