Capacitance detection in electrochemical assay with improved sampling time offset
A method and a system are provided to determine fill sufficiency of an electrochemical biosensor test cell by determining capacitance of the electrochemical test cell. In particular, the system samples an output signal from the chamber at a second sampling-time interval different than a first sampling-time interval such that a magnitude of each sampled output signal is measured at each succession of the second sampling-time interval instead of at the first time interval. The system determines a phase angle between an output signal and the oscillating input signal from the chamber based on the sampled output signal of the sampling step. The system calculates a capacitance of the chamber from the phase angle. A method is also provided to perform similar functionalities of the system.
1. A method of determining capacitance of a biosensor chamber having two electrodes disposed in the chamber and coupled to a microcontroller, the method comprising:
initiating an electrochemical reaction of a sample upon deposition of the sample in the biosensor chamber;
applying an oscillating signal of a predetermined frequency to the chamber;
ascertaining a first sampling-time interval for measurement of an output signal based on a predetermined sampling rate per cycle of the output signal at the predetermined frequency;
sampling the output signal from the chamber at a second sampling-time interval different than the first sampling-time interval such that a magnitude of each sampled output signal is measured at each succession of the second sampling-time interval instead of at the first time interval;
determining a phase angle between an output signal and the oscillating input signal from the chamber based on the sampled output signal of the sampling step; and
calculating a capacitance of the chamber from the phase angle.
2. The method of claim 1 , in which the second sampling-time interval is based on a predetermined offset time with respect to the first sampling-time interval.
3. The method of claim 2 , in which the first sampling-time interval comprises a duration between each step change in magnitude of the output signal.
4. The method of claim 2 , in which the offset time comprises a percentage of the first sampling-time interval.
5. The method of claim 4 , in which the percentage comprises a range from about 5% to about 30% of the first sampling-time interval.
6. The method of claim 5 , in which the calculating comprises determining a difference in the angle between the oscillating input current and the output current as the phase angle.
7. The method of claim 1 , in which the ascertaining comprises:
determining a duration for one wave of the signal at the predetermined frequency;
dividing the duration over a number of measurement samples for each wave to obtain a time duration; and
setting the first sampling-time interval as being generally equal to the time duration.
8. The method of claim 1 , in which the ascertaining comprises:
evaluating the output signal to determine a time duration between each step change of the output signal; and
setting the first sampling-time interval as being generally equal to the time duration.
9. The method of one of claim 7 or claim 8 , in which the offset time comprises a percentage of the first sampling-time interval.
10. The method of claim 9 , in which the percentage comprises a range from about 5% to about 30% of the first sampling-time interval.
11. The method of claim 1 , in which the calculating comprises calculating capacitance with a phase angle compensation to account for phase shift in a circuit used to sample the output signal.
12. The method of one of claim 1 or claim 11 , in which the calculating comprises calculating capacitance with an equation of the form:
C=|i T sin(Φ+Φ COMP )|/2 πfV
where:
C≈capacitance;
i T ≈total current;
Φ≈phase angle between total current and resistor current;
Φ COMP ≈phase angle compensation;
f≈frequency; and
V≈voltage.
13. The method of claim 12 , in which the phase angle compensation comprises any value from about 3 degrees to about 20 degrees.
14. The method of claim 13 , in which the phase angle compensation comprises about 11 degrees.
15. The method of claim 12 , in which the calculating comprises:
sampling a plurality of current outputs from the chamber over one cycle of the frequency;
obtaining a mean of sampled current output;
subtracting the mean from each sampled current of the plurality of current outputs; and
extracting root-mean-squared value of all negative values from the subtracting to provide for the total current output.
16. The method of claim 15 , in which the calculating comprises:
determining from the sampling, at least one cross-over point of the current from negative to positive values; and
interpolating proximate the at least one cross-over point of the current to determine a first angle at which the current changes from positive to negative or negative to positive.
17. The method of claim 16 , in which the interpolating the at least one cross-over point of the current comprises:
interpolating another cross-over point from the sampling to determine another angle at which the current changes from positive to negative or negative to positive; and
subtracting from the another angle approximately 180 degrees to provide for a second angle.
18. The method of claim 17 , in which the subtracting further comprises calculating an average of the first and second angles.
19. An analyte measurement system comprising:
An analyte test strip including:
a substrate having a reagent disposed thereon;
at least two electrodes proximate the reagent a in test chamber of the test strip;
an analyte meter including:
a strip port connector disposed to connect to the two electrodes;
a power supply; and
a microcontroller electrically coupled to the strip port connector and the power supply, the microcontroller being programmed to:
(a) initiate an electrochemical reaction in the biosensor chamber; apply an oscillating voltage of a predetermined frequency to the chamber;
(b) ascertain a first sampling-time interval for measurement of an output signal based on a predetermined sampling rate per cycle of the output signal at the predetermined frequency;
(c) sample the output signal from the chamber at a second sampling-time interval different than the first sampling-time interval such that a magnitude of each sampled output signal is measured at each succession of the second sampling-time interval instead of at the first time interval;
(d) determine a phase angle between a current output and the oscillating voltage from the chamber based on the sampled output signal; and
(e) calculate a capacitance of the chamber based on the determined phase angle.
20. The system of claim 19 , in which the second sampling-time interval is based on a predetermined offset time with respect to the first sampling-time interval.
21. The system of claim 20 , in which the first sampling-time interval comprises a duration between each step change in magnitude of the output signal.
22. The system of one of claim 20 or claim 21 , in which the offset time comprises a percentage of the first sampling-time interval.
23. The system of claim 22 , in which the percentage comprises a range from about 5% to about 30% of the first sampling-time interval.