Low power measurement of skin electrical properties

1. A measurement system for measuring an electrical property of skin, comprising:

an excitation circuit comprising a digital waveform generator to generate a periodic excitation signal that, when provided to the skin, generates a response signal in the skin indicative of the electrical property;

a receiver circuit comprising an analog-to-digital converter (ADC); and

a sequencer circuit configured to perform operations comprising:

activate the excitation circuit to provide the excitation signal to the skin;

while the excitation circuit is activated to provide the excitation signal to the skin, activating the receiver circuit to execute a first sample cycle to generate a first plurality of samples of the response signal, wherein the receiver circuit oversamples the response signal during the first sample cycle;

after the receiver circuit executes the first sample cycle, deactivating the receiver circuit;

after the deactivating of the receiver circuit and while the excitation circuit is activated to provide the excitation signal to the skin, reactivating the receiver circuit to execute a second sample cycle to generate a second plurality of samples of the response signal;

determining a first value for the electrical property of the skin based at least in part on the first plurality of samples of the response signal; and

determining a second value for the electrical property of the skin based at least in part on the second plurality of samples of the response signal.

2. The measurement system of claim 1 , wherein the receiver circuit is configured to perform operations comprising:

capturing a first sample of the first plurality of samples when the excitation signal is at a first position of a response signal period;

capturing a second sample of the first plurality of samples when the excitation signal, is at a second position of the response signal period;

capturing a first sample of the second plurality of samples when the excitation signal is at the first position of the response signal period; and

capturing a second sample of the second plurality of samples when the excitation signal is at the second position of the response signal period.

3. The measurement system of claim 1 , wherein the first sample cycle is executed after an excitation setup time has elapsed since the activating of the excitation circuit.

4. The measurement system of claim 1 , wherein the sequencer circuit is further configured to perform operations comprising:

deactivating the excitation circuit after the receiver circuit executes the first sample cycle; and

reactivating the excitation circuit, wherein the reactivating the receiver circuit is after the reactivating of the excitation circuit by an excitation setup time.

5. The measurement system of claim 1 , further comprising a discrete Fourier transform (DFT) circuit configured to determine a DFT of the response signal based at least in part on the first plurality of samples.

6. The measurement system of claim 5 , wherein the DFT circuit is further configured to generate a real component of the DFT of the response signal corresponding to an ohmic resistance of the skin and an imaginary component of the DFT corresponding to a reactance of the skin.

7. The measurement system of claim 1 , wherein a sampling frequency of the ADC is more than twice a frequency of the excitation signal.

8. The measurement system of claim 1 , wherein a sampling frequency of the ADC is at least four times a frequency of the excitation signal.

9. The measurement system of claim 1 , wherein the receiver circuit draws a first current level when deactivated, a second current level when activated and not actively sampling the response signal, and a third current level when activated and actively sampling the response signal, wherein the third current level is higher than the second current level, and wherein the second current level is greater than the first current level.

10. A method for measuring an electrical property of skin, comprising:

activating an excitation circuit to provide a periodic excitation signal to the skin; wherein the excitation signal, when provided to the skin, generates a response signal in the skin indicative of the electrical property;

while the excitation circuit is activated to provide the excitation signal to the skin, activating a receiver circuit to execute a first sample cycle to generate a first plurality of samples of the response signal;

oversampling the response signal during the first sample cycle;

after the receiver circuit executes the first sample cycle, deactivating the receiver circuit;

after the deactivating of the receiver circuit and while the excitation circuit is activated to provide the excitation signal to the skin, reactivating the receiver circuit to execute a second sample cycle to generate a second plurality of samples of the response signal;

determining a first value for the electrical property of the skin based at least in part on the first plurality of samples of the response signal; and

determining a second value for the electrical property of the skin based at least in part on the second plurality of samples of the response signal.

11. The method of claim 10 , further comprising:

capturing a first sample of the first plurality of samples when the excitation signal is at a first position of a response signal period;

capturing a second sample of the first plurality of samples when the excitation signal is at a second position of the response signal period;

capturing a first sample of the second plurality of samples when the excitation signal is at the first position of the response signal period; and

capturing a second sample of the second plurality of samples when the excitation signal is at the second position of the response signal period.

12. The method of claim 10 , further comprising waiting for an excitation setup time after the activating of the excitation circuit before executing the first sample cycle.

13. The method of claim 10 , further comprising:

deactivating the excitation circuit after the receiver circuit executes the first sample cycle; and

reactivating the excitation circuit, wherein the reactivating the receiver circuit is after the reactivating of the excitation circuit by an excitation setup time.

14. The method of claim 10 , further comprising determining a DFT of the response signal based at least in part on the first plurality of samples.

15. The method of claim 14 , further comprising determining a real component of the DFT of the response signal corresponding to an ohmic resistance of the skin and an imaginary component of the DFT corresponding to a reactance of the skin.

16. The method of claim 10 , wherein executing the first sample cycle comprises operating an ADC at more than twice a frequency of the excitation signal.

17. The method of claim 10 , wherein executing the first sample cycle comprises operating an ADC at greater than four times a frequency of the excitation signal.

18. The method of claim 10 , wherein the receiver circuit draws a first current level when deactivated, a second current level when activated and not actively sampling the response signal, and a third current level when activated and actively sampling the response signal, wherein the third current level is higher than the second current level, and wherein the second current level is greater than the first current level.

19. A computing device for measuring an electrical property of skin, comprising:

a first electrode;

a second electrode; and

a measurement system unit in communication with, the first electrode and the second electrode, wherein the measurement system unit comprises:

an excitation circuit comprising a digital waveform generator to generate a periodic excitation signal that, when provided to the skin, generates a response signal in the skin indicative of the electrical property;

a receiver circuit comprising an analog-to-digital converter (ADC); and

a sequencer circuit configured to perform operations comprising:

activate the excitation circuit to provide the excitation signal to the skin;

while the excitation circuit is activated to provide the excitation signal to the skin, activating the receiver circuit to execute a first sample cycle to generate a first plurality of samples of the response signal, wherein the receiver circuit oversamples the response signal during the first sample cycle;

after the receiver circuit executes the first sample cycle, deactivating the receiver circuit;

after the deactivating of the receiver circuit and while the excitation circuit is activated to provide the excitation signal to the skin, reactivating the receiver circuit to execute a second sample cycle to generate a second plurality of samples of the response signal;

determining a first value for the electrical property of the skin based at least in part on the first plurality of samples of the response signal; and

determining a second value for the electrical property of the skin based at least in part on the second plurality of samples of the response signal.

20. The computing device of claim 19 , wherein the receiver circuit is configured to perform operations comprising:

capturing a first sample of the first plurality of samples when the excitation signal is at a first position of a response signal period;

capturing a second sample of the first plurality of samples when the excitation signal is at a second position of the response signal period;

capturing a first sample of the second plurality of samples when the excitation signal is at the first position of the response signal period; and

capturing a second sample of the second plurality of samples when the excitation signal is at the second position of the response signal period.

21. The computing device of claim 19 , wherein the receiver circuit draws a first current level when deactivated, a second current level when activated and not actively sampling the response signal, and a third current level when activated and actively sampling the response signal, wherein the third current level is higher than the second current level, and wherein the second current level is greater than the first current level.