Multi-bit ADC with sigma-delta modulation

• US 20050184895A1
• Filed: 02/25/2004
• Published: 08/25/2005
• Est. Priority Date: 02/25/2004
• Status: Active Grant

• Current Assignee: Covidien LP (Medtronic Plc)
• Original Assignee: Nellcor Puritan Bennett Incorporated (Medtronic Plc)
• Inventors: Petersen, Ethan, Chew, Bradford B., Shea, William
• Granted Patent: US 7,142,142 B2
• Time in Patent Office: Days
• US Class Current: 341/143
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• 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: H03M 3/424 the quantiser being a multiple bit one
• CPC Class Codes: H03M 3/456 the modulator having a first order loop filter in the feedforward path
• CPC Class Codes: H03M 3/464 Details of the digital/analogue conversion in the feedback path

Abstract

An oximeter uses a sigma-delta modulator and a multi-bit ADC with PWM feedback enabling high precision multi-bit conversion. Demodulation is done in software, thus requiring only a single hardware path for both red and IR. Multiple capacitors are switched into the integrator in the sigma-delta modulator, with different capacitors for red, IR and dark signals, thus enabling the use of the single hardware path. A switching circuit at the input of the sigma-delta modulator acts as a sample and hold, controlled by the PWM feedback.

Claims

1. An oximeter apparatus comprising:

a sigma-delta modulator having an input coupled to receive an analog sensor signal;

a first switching circuit, coupled to said input, for switching between connecting first and second reference voltages to said input; and

a feedback circuit, coupled to an output of said sigma-delta modulator, for controlling the switching between said first and second reference voltages by said switching circuit.

2. The oximeter apparatus of claim 1 further comprising:

a multiple bit analog-to-digital converter coupled to said output of said sigma-delta modulator to provide a digital output; and

said feedback circuit including a digital-to-analog converter having an input coupled to said digital output, and an output coupled to said switching circuit.

3. The oximeter apparatus of claim 1 further comprising:

a resistor coupled between said switching circuit and said input of said sigma-delta modulator.

4. The oximeter apparatus of claim 1 wherein said sigma-delta modulator comprises first and second integrators, and further comprising:

a second switching circuit coupled to an input of said second integrator and controlled by said feedback circuit, for switching to said second voltage reference when said first switching circuit connects to said first voltage reference, and switching to said first voltage reference when said first switching circuit switches to said second voltage reference.

5. The oximeter apparatus of claim 1 further comprising:

wherein said sigma-delta modulator includes an operational amplifier configured as an integrator;

a second switching circuit configured to function as a sample and hold by alternately couple said input to inverting and non-inverting inputs of said operational amplifier; and

a control circuit, coupled to said feedback circuit, for controlling the switching of said second switching circuit.

6. The oximeter apparatus of claim 5 wherein said control circuit comprises a flip-flop.

7. The oximeter apparatus of claim 1 wherein said sigma-delta modulator includes an integrator, and further comprising:

at least first and second capacitors; and

a capacitor switching circuit for connecting said first capacitor into said integrator upon receipt of a first sensor signal, and connecting said second capacitor into said integrator upon receipt of a second sensor signal.

8. The oximeter apparatus of claim 7 wherein said first sensor signal is a red signal and said second sensor signal is an infrared signal.

9. The oximeter apparatus of claim 8 further comprising a third capacitor for connecting into said integrator upon receipt of a dark signal between said red and infrared signals.

10. The oximeter apparatus of claim 9 further comprising a fourth capacitor for connecting into said integrator for a second dark signal between said infrared signal and said red signal.

11. The oximeter apparatus of claim 1 wherein a single signal path through said sigma-delta modulator is used for both a red and an infrared signal.

12. The oximeter apparatus of claim 11 further comprising:

a multiple bit analog-to-digital converter coupled to said output of said sigma-delta modulator to provide a digital output; and

a memory containing a program for demodulating said digital output into red and infrared digital signals.

13. An oximeter apparatus comprising:

a sigma-delta modulator having an input coupled to receive an analog sensor signal;

a first switching circuit, coupled to said input, for switching between connecting first and second reference voltages to said input;

a multiple bit analog-to-digital converter coupled to said output of said sigma-delta modulator to provide a digital output; and

a feedback circuit, coupled to an output of said multiple bit analog-to-digital converter, for controlling the switching between said first and second reference voltages by said switching circuit, said feedback circuit providing a pulse width modulated signal to said switching circuit; and

a resistor coupled between said switching circuit and said input of said sigma-delta modulator.

14. An oximeter apparatus comprising:

a sigma-delta modulator having an input coupled to receive an analog sensor signal, wherein said sigma-delta modulator includes an operational amplifier configured as an integrator;

a switching circuit configured to function as a sample and hold by alternately couple said input to inverting and non-inverting inputs of said operational amplifier;

a feedback circuit coupled between an output and said input of said sigma-delta modulator; and

a control circuit, coupled to said feedback circuit, for controlling the switching of said switching circuit.

15. An oximeter apparatus comprising:

a sigma-delta modulator having an input coupled to receive an analog sensor signal, wherein said sigma-delta modulator includes an integrator;

at least first and second capacitors; and

a capacitor switching circuit for connecting said first capacitor into said integrator upon receipt of a first sensor signal, and connecting said second capacitor into said integrator upon receipt of a second sensor signal.

16. The oximeter apparatus of claim 16 wherein said first sensor signal is a red signal and said second sensor signal is an infrared signal.

17. The oximeter apparatus of claim 17 further comprising a third capacitor for connecting into said integrator upon receipt of a dark signal between said red and infrared signals.

18. The oximeter apparatus of claim 18 further comprising a fourth capacitor for connecting into said integrator for a second dark signal between said infrared signal and said red signal.

19. An oximeter apparatus comprising:

a sigma-delta modulator having an input coupled to receive an analog sensor signal;

a multiple bit analog-to-digital converter coupled to said output of said sigma-delta modulator to provide a digital output;

a feedback circuit, coupled between an output of said analog-to-digital converter and said input of said sigma-delta modulator; and

a memory containing a program for demodulating said digital output into red and infrared digital signals

wherein a single signal path through said sigma-delta modulator is used for modulated red and infrared signals.