IMPEDANCE MEASUREMENT CIRCUIT AND METHOD

• US 20110208458A1
• Filed: 10/08/2009
• Published: 08/25/2011
• Est. Priority Date: 10/16/2008
• Status: Active Grant

• Current Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Original Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Inventors: Schmidt, Ralf, Pinter, Robert
• Granted Patent: US 8,831,898 B2
• Time in Patent Office: Days
• US Class Current: 702/65
• CPC Class Codes: A61B 5/053 Measuring electrical impedance or conductance of a portion of the body

Abstract

An impedance measurement circuit comprises a current source arrangement, a voltage measurement arrangement and a processor. The circuit is operable in a two-point measurement mode and a four-point measurement mode and the processor as is adapted to derive the impedance to be measured by combining the measurement voltages from the two-point and the four-point measurement modes. This combines the results of both two-point and four-point measurement techniques to provide improved accuracy. In particular, the two results enable the effect of the electrode resistance to be cancelled.

Claims

1. An impedance measurement circuit comprising:

a current source arrangement (12);

a voltage measurement arrangement (14); and

a processor (60),

wherein the circuit is operable in a two-point measurement mode and a four-point measurement mode, and

wherein the processor is adapted to derive the impedance to be measured (Z) by combining the measurement voltages from the two-point and the four-point measurement modes.

2. A circuit as claimed in claim 1, wherein:

the two-point mode comprises a mode in which a first voltage across an impedance to be measured is obtained using a pair of measurement terminals (2,3) which are used to route current to and from the impedance to be measured; and

the four-point mode comprises a mode in which a second voltage across the impedance to be measured is obtained using the pair of measurement terminals (2,3), and with another pair of terminals (1,4) used to route current to and from the impedance to be measured (Z).

3. A circuit as claimed in claim 1, comprising a switching arrangement (50) for switching a current output of a current source (12) arrangement between a first pair of terminals (2,3) and a second pair of terminals (1,4), the first pair of terminals (2,3) comprising two-point impedance measurement terminals, and the first and second pair of measurement terminals (1,2,3,4) together comprising four-point impedance measurement terminals.

4. A circuit as claimed in claim 3, wherein the switching arrangement has a first pair of inputs connected to opposite sides of the current source arrangement, and two pairs of outputs, the first pair of outputs comprising the two-point impedance measurement terminals and the second pair of outputs comprising the four-point measurement drive terminals.

5. A circuit as claimed in claim 1, wherein the processor is adapted to derive the impedance to be measured by calculating:

6. A biosensor for measuring skin impedance, comprising:

a set of four contact electrodes for making contact with the skin; and

an impedance measuring circuit as claimed in claim 1.

7. An impedance measurement method comprising:

obtaining a voltage measurement in a two-point measurement mode;

obtaining a voltage measurement in a four-point measurement mode;

deriving the impedance to be measured (Z) by combining the measurement voltages from the two-point and the four-point measurement modes.

8. A method as claimed in claim 7, wherein:

the two-point mode comprises obtaining a first voltage across an impedance to be measured using a pair of measurement terminals (2,3) which are used to route current to and from the impedance to be measured; and

the four-point mode comprises obtaining a second voltage across the impedance to be measured using the pair of measurement terminals (2,3), and using another pair of terminals (1,4) in order to route current to and from the impedance to be measured (Z).

9. A method as claimed in claim 8, comprising switching a current source arrangement (12) between a first pair of terminals (2,3) and a second pair of terminals (1,4), the first pair of terminals (2,3) comprising two-point impedance measurement terminals, and a second pair of terminals, the first and second pair of measurement terminals (1,2,3,4) together comprising four-point impedance measurement terminals.

10. A method as claimed in claim 7 wherein the processor derives the impedance to be measured by calculating:

11. A method as claimed in claim 10, wherein the internal impedance Ri is obtained by performing the impedance measurement method using a known reference impedance to be measured.

12. A skin impedance measuring method, comprising:

applying a set of four contact electrodes to make contact with the skin; and

carrying out an impedance measuring method as claimed in claim 7 using two of the electrodes for the two-point measurement and using all four electrodes for the four-point measurement.