Respiratory CO.sub.2 detector circuit with high quality waveform

• US 4,955,946 A
• Filed: 02/02/1989
• Issued: 09/11/1990
• Est. Priority Date: 12/11/1986
• Status: Expired due to Fees

• Current Assignee: Marquette Gas Analysis
• Original Assignee: Marquette Gas Analysis
• Inventors: Becker, Douglas P., Mount, Bruce E.
• Primary Examiner(s): Cohen, Lee S.
• Assistant Examiner(s): Hanley, John C.
• Application Number:
  US07/306,234
• Time in Patent Office: 586 Days
• Field of Search: 128/716, 128/719, 128/725
• US Class Current: 600/532
• CPC Class Codes: G01J 2001/182 with SH sample and hold circuits
• CPC Class Codes: G01J 2001/444 Compensating; Calibrating, e.g. dark current, temperature drift, noise reduction or baseline correction; Adjusting
• CPC Class Codes: G01N 21/27 using photo-electric detection G01N21/31 takes precedence; circuits for computing concentration logarithmic circuits G06G7/24; photometric circuits in general G01J
• CPC Class Codes: G01N 21/3504 for analysing gases, e.g. multi-gas analysis
• CPC Class Codes: G01N 2201/069 Supply of sources
• CPC Class Codes: G01N 2201/127 Calibration; base line adjustment; drift compensation
• CPC Class Codes: G01N 33/497 of gaseous biological material, e.g. breath

Abstract

A respiratory CO₂ detector (10) comprising an infrared lamp source (44) and an infrared detector (50) responsive thereto forming an optical path for detecting the change in CO₂ concentration, or an obstruction in a cuvette (42). The output of the infrared detector (50) provides a high and low voltage signal to be applied to a feedback control loop (12) and to an output circuit (14).

The feedback control loop (12) includes a peak detector (22), a contamination detector (24), a pulse-width modulator (26) and a low pass filter (28), the latter providing a DC bias on the infrared lamp (44). The peak detector (22) is connected to the pulse-width modulator (26) to maintain the lamp voltage constant and is connected to comparators (56,62) to compare both outputs of the peak and contamination detectors (22,24). The contamination detector (24) will respond to blockage in the cuvette (42).

The output control circuit (14) includes a sample-and-hold circuit (30) and a subtractor (32) connected to the output of said infrared detector to receive the high and low voltage signal from the infrared detector, the outputs of which produce an output signal without the DC bias which is then inverted to provide a high quality waveform.

Claims

1. A CO₂ detector for monitoring a patient'"'"'s breath inhalation and exhalation directed through a cuvette, comprising,

an infrared lamp having an input and responsive to a lamp voltage applied thereto,

a circuit including a pulse-width modulator and a lowpass filter connected to said lamp input to provide said lamp input voltage,

an infrared detector,

a preamplifier having an input and an output, said input being connected to said infrared detector,

said lamp being positioned to provide an optical path through a cuvette and impinge on said infrared detector,

said infrared detector and said preamplifier being constructed and arranged so that the output of the infrared detector and preamplifier will produce an electrical waveform representative of the quantity of CO₂ in said cuvette, said waveform having high voltage peaks representing low CO₂ and low voltage valleys representing high CO₂,

said infrared lamp being constructed and arranged to be responsive to the output of said pulse-width modulator and lowpass filter which causes the preamplifier to have a reference (baseline) DC level,

a peak detector including

a first RC network having an input and an output, said first RC network having a predetermined discharge time constant,

a first switch connected between said preamplifier and said first RC network so that when said first switch is closed the output of said preamplifier will charge said first RC network and when said first switch is open said first RC network will discharge, and

a first comparator having an output connected to operate said switch and having a first input connected directly to the output of said preamplifier and having a second input connected to the output of said first RC network to compare the voltage level at the output of said preamplifier with the voltage level at the output of said RC network so that said first switch will be opened or closed depending upon the compared voltages, wherein the output from said first RC network will apply a voltage so said pulse-width modulator and said lowpass filter to control the lamp input voltage,

a contamination detector including

a second comparator having an output and two inputs,

a second RC network having an input connected to the output of said preamplifier and having an output connected to one input of said second comparator, said second RC network having a discharge time constant shorter than said discharge time constant of said first RC network,

a DC voltage source having a positive terminal and a negative terminal, said negative terminal being connected to a second input of said second comparator and the positive terminal being connected to said second input of said first comparator, the output of said second comparator being connected to operate said first switch to connect the input of said first RC network with the preamplifier output at a preselected value of compared voltages,

whereby said contamination detector will override the peak detector and raise the voltage on the lamp in the event of contamination of the cuvette.

2. The CO₂ detector as claimed in claim 1,

further including a subtractor having a first input coupled to said preamplifier output, and having a second input,

a sample-and-hold circuit having an input connectable to said preamplifier output through a second switch and an output connected to said second input of said subtractor,

said second switch being constructed and arranged to open and close concurrently with said first switch, whereby the waveform output from said subtractor will produce a waveform representative of the concentration of CO₂ in said cuvette, but inverted.

3. The CO₂ detector as claimed in claim 2 further including means for inverting said subtractor output waveform for producing a high quality waveform output.

4. The CO₂ detector as claimed in claim 3 wherein said lamp input voltage is utilizable for triggering an alarm representative of an obstruction in said cuvette.

6. The improved detector as claimed in claim 1, wherein said lamp input circuit includes a pulse-width modulator and a lowpass filter between the output of said peak detector and said lamp.

7. The improved detector as claimed in claim 6, wherein said lowpass filter has an output connection utilizable as a contamination warning signal.

5. In a CO₂ detector including a cuvette, a circuit including an infrared lamp responsive to a voltage applied to an input thereof, a lamp input circuit, and an infrared detector and detector preamplifier coupled thereto,

said preamplifier having an output with a reference (baseline) voltage level,

said lamp being positioned to illuminate an optical path through said cuvette so that the infrared detector and detector preamplifier will produce an output signal voltage representative of the quantity of CO₂ in said cuvette, that is, said reference voltage level for minimum CO₂ concentration and a second voltage level for maximum CO₂ concentration,

the improvement comprising

a feedback gain control loop and an output circuit,

said feedback control loop being defined by first means and second means, and said output circuit being defined by third means,

said first means comprising a peak detector having

a first input connected to said detector preamplifier output so as to be responsive to said output signal voltage, and,

a second input connectable to said detector preamplifier output through a first switch, and an output connected to said infrared lamp input circuit,

said first means further including

a first capacitive timing means including a ground connection and having an input connected to said switch and having an output defining said output of said peak detector, said timing means having a predetermined discharge time constant,

whereby said first capacitive timing means is changed by said preamplifier output signal voltage when said first switch is closed and discharges through said ground connection when said first switch is open, and,

comparing means having a first input connected to said detector preamplifier output and a second input connected to the output of said first capacitive timing means for comparing said preamplifier output signal voltage and the voltage at the output of said first capacitive timing means, said comparing means having an output constructed and arranged to open and close the first switch in response to the compared voltages so that said first capacitive timing means will discharge when said first switch is open and thus apply its voltage to said lamp input circuit to maintain the reference (baseline) voltage of the detector preamplifier output at a nearly constant value,

said second means comprising a contamination detector having an output coupled to operate said first switch,

said contamination detector further including

a second capacitive timing means having an input charged by said preamplifier output signal voltage,

said second capacitive timing means having a shorter discharge time constant than said first capacitive timing means, and further having an output,

a second comparing means with one input coupled to the output of said second capacitive timing means and with a second input coupled to the output of said first capacitive timing means for comparing the output voltages of said first and second capacitive timing means,

said second comparing means having an output defining said output of the contamination detector which is constructed and arranged to close the first switch in response to compared voltages indicative of blocking of said cuvette, wherein said first capacitive timing means causes said lamp input circuit to drive the lamp with a higher voltage level to maintain the reference voltage level substantially constant;

and said third means including

a subtracter having one input coupled to the output of said detector preamplifier so as to be responsive to its output signal voltage,

a sample-and-hold circuit having an input coupled to said detector preamplifier through a second switch,

said second switch being constructed and arranged so as to be opened and closed simultaneously with said first switch, said sample and hold circuit having an output connected to a second input of said subtracter for eliminating the reference (baseline) voltage in said preamplifier output signal voltage,

thereby providing an output signal waveform corresponding to the CO₂ concentration changes occurring within said cuvette and of such quality that the output signal may be useful for diagnostic purposes.