Method and apparatus for demodulating signals in a pulse oximetry system

• US 7,003,339 B2
• Filed: 11/03/2003
• Issued: 02/21/2006
• Est. Priority Date: 04/14/1997
• Status: Expired due to Term

• Current Assignee: Masimo Corporation
• Original Assignee: Masimo Corporation
• Inventors: Weber, Walter M., Al-Ali, Ammar, Diab, Mohamed K.
• Primary Examiner(s): WINAKUR, ERIC FRANK
• Application Number:
  US10/700,324
• Publication Number:
  US 20040152965A1
• Time in Patent Office: 841 Days
• Field of Search: 600/310, 600/322, 600/323, 600/336
• US Class Current: 600/336
• CPC Class Codes: A61B 2560/0242 adapted to measure environmental factors, e.g. temperature, pollution
• CPC Class Codes: A61B 2562/0238 Optical sensor arrangements for performing transmission measurements on body tissue
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/14552 Details of sensors specially adapted therefor
• CPC Class Codes: A61B 5/1495 Calibrating or testing of in-vivo probes
• CPC Class Codes: A61B 5/7203 for noise prevention, reduction or removal
• CPC Class Codes: A61B 5/7217 of noise originating from a therapeutic or surgical apparatus, e.g. from a pacemaker
• CPC Class Codes: A61B 5/7228 Signal modulation applied to the input signal sent to patient or subject; demodulation to recover the physiological signal modulation H03C
• CPC Class Codes: A61B 5/7232 involving compression of the physiological signal, e.g. to extend the signal recording period
• CPC Class Codes: H04J 14/08 Time-division multiplex systems
• CPC Class Codes: H04J 3/10 Arrangements for reducing cross-talk between channels

Abstract

A method and an apparatus measure blood oxygenation in a subject. A first signal source applies a first input signal during a first time interval. A second signal source applies a second input signal during a second time interval. A detector detects a first parametric signal responsive to the first input signal passing through a portion of the subject having blood therein. The detector also detects a second parametric signal responsive to the second input signal passing through the portion of the subject. The detector generates a detector output signal responsive to the first and second parametric signals. A signal processor receives the detector output signal and demodulates the detector output signal by applying a first demodulation signal to a signal responsive to the detector output signal to generate a first output signal responsive to the first parametric signal. The signal processor applies a second demodulation signal to the signal responsive to the detector output signal to generate a second output signal responsive to the second parametric signal. The first demodulation signal and the second demodulation signal both include at least a first component having a first frequency and a first amplitude and a second component having a second frequency and a second amplitude. The second frequency is a harmonic of the first frequency. The second amplitude is related to the first amplitude to minimize crosstalk from the first parametric signal to the second output signal and to minimize crosstalk from the second parametric signal to the first output signal.

Claims

1. In a system that includes:

a sampling frequency generator generating a sampling frequency f_s;

a detector;

a modulation signal generator, said modulation signal generator generating a sequence of pulses having a pulse repetition frequency, said pulses having a duty cycle of Q sample periods of said sampling frequency f_s;

a transmitter which emits electromagnetic energy at a wavelength in response to said pulse, said detector receiving said electromagnetic energy to generate a detector output signal responsive to the received electromagnetic energy, said detector output signal including noise caused by ambient electromagnetic energy detected by said detector;

a digital to analog converter which produces digital samples of said detector output signal at said sampling frequency; and

a demodulator which demodulates said digital samples to produce a desired output signal, a method for minimizing said noise due to ambient electromagnetic energy in said desired output signal comprising the steps of;

identifying all undesired frequency components of said ambient electromagnetic energy detected by said detector;

using said undesired frequency components to compute a set of acceptable modulation cycle times T; and

using said acceptable modulation cycle times to select said f_s and said Q using an equation T=4Q/f_s.