Bio-electrical impedance analyzer

• US 6,631,292 B1
• Filed: 03/23/2001
• Issued: 10/07/2003
• Est. Priority Date: 03/23/2001
• Status: Active Grant

• Current Assignee: RJL Systems, Inc.
• Original Assignee: RJL Systems, Inc.
• Inventors: Liedtke, Rudolph J.
• Primary Examiner(s): Jeffery, John A.
• Application Number:
  US09/816,880
• Time in Patent Office: 928 Days
• Field of Search: 600/547, 600/506, 324/713, 324/600
• US Class Current: 600/547
• CPC Class Codes: A61B 5/053 Measuring electrical impedance or conductance of a portion of the body

Abstract

An apparatus for measuring the resistance and reactance of a subject, or a segment of a subject, includes a constant current source supplying a periodic high frequency input current to the subject. The constant current source controls the input current to a targeted value by measuring the actual current through the subject and comparing that actual current to the target current. The resulting error signal controls the input current. The input current is supplied to the segment through a coupling transformer, and another coupling transformer coupled to a resistor in the subject path supplies the value of the actual current. The apparatus also includes an impedance measuring circuit that detects an output voltage across the segment, preferably using a detection coupling transformer, and produces at least one of a reactance output signal and a resistance output signal using the output voltage. In one aspect, an automatic shut down circuit is included.

Claims

1. An apparatus for measuring an impedance of a segment of a subject, comprising:

a constant current source wherein a level of an input current is controlled by a feedback loop using an error signal representing a difference between an actual current measured through the segment and a target current; and

an impedance measuring circuit detecting an output voltage across the segment and adapted to produce at least one of a reactance output signal and a resistance output signal using the output voltage; and

wherein neither one of the constant current source and the impedance measuring circuit share a common ground with the subject.

2. The apparatus according to claim 1, further comprising:

reporting means for reporting at least one of the reactance output signal and the resistance output signal.

3. The apparatus according to claim 1, wherein the constant current source supplies the input current through first external leads connectable to the segment and the impedance measuring circuit measures the output voltage through second external leads connectable to the segment.

4. An apparatus for measuring an impedance of a segment of a subject, comprising:

a constant current source wherein a level of an input current is controlled by a feedback loop using an error signal representing a difference between an actual current measured through the segment and a target current; and

an impedance measuring circuit detecting an output voltage across the segment and adapted to produce at least one of a reactance output signal and a resistance output signal using the output voltage; and

wherein the constant current source includes;

a direct power supply providing a voltage source to an oscillating current source;

a first amplifier connected to the oscillating current source wherein the gain of the first amplifier is controlled by the feedback loop; and

an input coupling transformer receiving the output of the first amplifier and adapted to supply the input current through first external leads connectable to the segment.

5. The apparatus according to claim 4, wherein the oscillating current source comprises:

a single field-effect transistor configured as a Colpitts oscillator.

6. An apparatus for measuring an impedance of a segment of a subject, comprising:

a constant current source wherein a level of an input current is controlled by a feedback loop using an error signal representing a difference between an actual current measured through the segment and a target current; and

an impedance measuring circuit detecting an output voltage across the segment and adapted to produce at least one of a reactance output signal and a resistance output signal using the output voltage; and

wherein the feedback loop of the constant current source includes;

a reference resistor coupled to a reference coupling transformer for measuring a constant current signal corresponding to the actual current through the segment;

means for producing the error signal by comparing the constant current signal to a reference signal corresponding to the target current; and

a modulator for controlling the gain of an amplifier of the constant current source using the error signal.

7. The apparatus according to claim 6, wherein the modulator is an optocoupler that changes resistance based on a value of the error signal.

8. The apparatus according to claim 6, wherein the means for producing the error signal comprises:

a buffer amplifier connected to the reference transformer and adapted to produce an amplified constant current signal;

a rectifier connected to the buffer amplifier and adapted to rectify the amplified constant current signal; and

a comparator receiving an output of the rectifier and the reference signal and producing the error signal.

9. An apparatus for measuring an impedance of a segment of a subject, comprising:

a constant current source wherein a level of an input current is controlled by a feedback loop using an error signal representing a difference between an actual current measured through the segment and a target current; and

an impedance measuring circuit detecting an output voltage across the segment and adapted to produce at least one of a reactance output signal and a resistance output signal using the output voltage; and

wherein the impedance measuring circuit comprises;

a detection coupling transformer adapted to detect the output voltage;

an output amplifier connected to the detection transformer adapted to produce an amplified output signal; and

means for converting the amplified output signal into the at least one of the reactance output signal and the resistance output signal.

10. The apparatus according to claim 9, wherein the amplified output signal is capacitively coupled to the means for converting the amplified output signal into the reactance output signal and the resistance output signal.

11. The apparatus according to claim 9, wherein the detection coupling transformer is designed for a common mode rejection ratio of greater than 90 dB at its operating frequency.

12. The apparatus according to claim 9, wherein the feedback loop comprises:

a reference resistor coupled to a reference transformer for measuring a constant current signal corresponding to the actual current through the segment; and

a buffer amplifier connected to the reference transformer and adapted to produce an amplified constant current signal; and

wherein the means for converting the output voltage into the reactance output signal and the resistance output signal includes;

an integrator connected to receive the amplified constant current signal;

a first balanced synchronous demodulator adapted to produce a first and a second direct current differential signal using a first reference vector provided by the integrator;

a second balanced synchronous demodulator adapted to produce a third and a fourth direct current differential signal using a second reference vector provided by the buffer amplifier;

a first instrumentation amplifier adapted to convert the first and second direct current differential signals into the reactance output signal; and

a second instrumentation amplifier adapted to convert the third and fourth direct current differential signals into the resistance output signal.

13. The apparatus according to claim 12, wherein the first and the second balanced synchronous demodulators are two passive analogue phase detectors.

14. An apparatus for measuring an impedance of a segment of a subject, comprising:

a constant current source wherein a level of an input current is controlled by a feedback loop using an error signal representing a difference between an actual current measured through the segment and a target current;

an impedance measuring circuit detecting an output voltage across the segment and adapted to produce at least one of a reactance output signal and a resistance output signal using the output voltage; and

an automatic shut down circuit adapted to shut down operation of at least one of the constant current source and the impedance measuring circuit after at least one predetermined time period.

15. The apparatus according to claim 14, wherein the automatic shut down circuit comprises:

isolating means to isolate a power supply from at least one of the constant current source and the impedance measuring circuit.

16. An apparatus for measuring an impedance of a segment of a subject, comprising:

a constant current source supplying a level of an input current to an input coupling transformer through first external leads connectable to the segment;

a detection coupling transformer detecting an output voltage across the segment through second external leads connectable to the segment; and

a circuit adapted to produce at least one output signal using the output voltage.

17. The apparatus according to claim 16, further comprising:

a reference resistor coupled to a reference coupling transformer for measuring a constant current signal corresponding to an actual current through the segment and providing an input to the constant current source.

18. The apparatus according to claim 16, wherein the level of the input current is controlled by an error signal representing a difference between an actual current measured through the segment and a target current.

19. The apparatus according to claim 18, further comprising:

an optocoupler controlling the level of the input current using the error signal.

20. The apparatus according to claim 16, wherein the detection coupling transformer is designed for a common mode rejection ratio of greater than 90 dB at its operating frequency.