Bio-impedance spectroscopy system and method for bio-impedance measurement

1. An impedance spectroscopy system for bio-impedance measurement, comprising:

a signal generator configured to generate a signal with a broadband frequency spectrum and to generate an analog injection current from the signal with the broadband frequency spectrum,

wherein the analog injection current is filtered by a high-pass filter so that the analog injection current has a high-pass frequency characteristic, and

wherein the analog injection current has a greater magnitude at higher frequencies than at lower frequencies according to the high-pass frequency characteristic;

an amplifier configured to measure a voltage signal in response to the analog injection current and to simultaneously measure a biopotential signal, wherein the amplifier comprises a low-noise instrumentation amplifier with a variable gain;

a processor configured to analyze the voltage signal to derive a bio-impedance spectrum as well to derive further information from the biopotential signal; and

an additional high-pass filter, wherein the additional high-pass filter is configured to provide biasing to the amplifier and prevent saturation.

2. The impedance spectroscopy system according to claim 1 , further comprising a digital to analog converter configured to generate the analog injection current from the signal with the broadband frequency spectrum, wherein the signal generator comprises a pseudo-random sequence generator, wherein the high-pass filter comprises a differentiator configured to apply differentiation to result in the high-pass frequency characteristic, wherein the differentiator comprises a digital differentiator configured to apply digital differentiation before a digital to analog conversion, wherein the signal generator is controllable in a first mode to output a maximal length sequence and in a second mode to output a differentiated maximal length sequence, and wherein the maximal length sequence has a flat frequency spectrum.

3. The impedance spectroscopy system according to claim 1 , wherein the variable gain comprises three possible gain settings.

4. The impedance spectroscopy system according to claim 3 , wherein the three possible gain settings comprise gains of 4, 17, and 70.

5. The impedance spectroscopy system according to claim 1 , wherein the high-pass filter comprises a 22 nF capacitor and a 1 MΩ resistor, and wherein the additional high-pass filter comprises a 100 nF capacitor and a 10 MΩ resistor.

6. The impedance spectroscopy system according to claim 1 , wherein the analog injection current is a three level current signal with a zero level, a positive level, and a negative level, and wherein the broadband frequency spectrum is based on a chirped signal.

7. The impedance spectroscopy system according to claim 1 , wherein the high-pass filter comprises a digital filter having an order greater than first order.

8. The impedance spectroscopy system according to claim 1 , further comprising an analog to digital converter configured to convert an output of the amplifier to a digital signal, wherein the analog to digital converter comprises a power-efficient 12-bit successive approximation analog to digital converter.

9. The impedance spectroscopy system according to claim 8 , wherein the power-efficient 12-bit successive approximation analog to digital convertor is configured to sample up to 5 MS/s, wherein the power-efficient 12-bit successive approximation analog to digital convertor uses a monotonic switching scheme and metal finger capacitors, and wherein the power-efficient 12-bit successive approximation analog to digital convertor comprises a 2-mode comparator.

10. The impedance spectroscopy system according to claim 1 , further comprising two electrodes for current injection and two electrodes for capture of a signal for amplification by the amplifier.

11. The impedance spectroscopy system according to claim 1 , wherein the analog injection current has no component below 100 Hz, and wherein the impedance spectroscopy system further comprises an electrocardiography (ECG) monitor that comprises a low pass filter at an output of the amplifier.

12. A bio-impedance measurement method, comprising:

generating a signal with a broadband frequency spectrum;

generating an analog injection current from the signal,

filtering the analog injection current, using a high-pass filter, so that the analog injection current has a high-pass frequency characteristic, wherein the high-pass filter comprises a digital filter having an order greater than first order, and

wherein the analog injection current has a greater magnitude at higher frequencies than at lower frequencies according to the high-pass frequency characteristic;

measuring voltages in response to the analog injection current;

measuring biopotential signals from a subject simultaneously;

analyzing the voltages to derive a bio-impedance spectrum; and

analyzing the biopotential signals to obtain further information.

13. The bio-impedance measurement method according to claim 12 , further comprising:

generating a digital maximal length sequence signal;

applying digital differentiation; and

performing digital to analog conversion to result in the high-pass frequency characteristic.

14. The bio-impedance measurement method according to claim 13 , further comprising:

generating, in a first mode, a maximal length sequence; and

generating, in a second mode, a differentiated maximal length sequence.

15. The bio-impedance measurement method according to claim 12 , wherein the analog injection current has no component below 100 Hz, and the bio-impedance measurement method further comprises electrocardiography (ECG) monitoring by low pass filtering an amplifier output.

16. The bio-impedance measurement method according to claim 12 , wherein generating the analog injection current from the signal comprises generating a pseudo-random sequence.

17. The bio-impedance measurement method according to claim 12 , wherein filtering the analog injection current comprises applying digital differentiation before a digital to analog conversion to result in the high-pass frequency characteristic.

18. The bio-impedance measurement method according to claim 12 , wherein measuring the voltages in response to the analog injection current and measuring the biopotential signals from the subject simultaneously is performed by an amplifier.

19. The bio-impedance measurement method according to claim 18 , further comprising converting an output of the amplifier to a digital signal.

20. A bio-impedance spectroscopy circuit for bio-impedance measurement, comprising:

a signal generator configured to generate a signal with a broadband frequency spectrum and to generate an analog injection current from the signal with the broadband frequency spectrum,

wherein the analog injection current is filtered by a high-pass filter so that the analog injection current has a high-pass frequency characteristic, and

wherein the analog injection current has a greater magnitude at higher frequencies than at lower frequencies according to the high-pass frequency characteristic;

an amplifier configured to measure a voltage signal in response to the analog injection current and to simultaneously measure a biopotential signal;

a processor configured to analyze the voltage signal to derive a bio-impedance spectrum as well to derive further information from the biopotential signal; and

an additional high-pass filter, wherein the additional high-pass filter is configured to provide biasing to the amplifier and prevent saturation.