Capacitance detection in electrochemical assays

Abstract

A method and system are provided to determine fill sufficiency of a biosensor test chamber by determining capacitance of the test chamber.

1 . A method of determining capacitance of a biosensor chamber having a two electrodes disposed in the chamber and coupled to a microcontroller, the method comprising:
 initiating an electrochemical reaction in the biosensor chamber;   applying an oscillating voltage of a predetermined frequency to the chamber;   determining a phase angle between a current output and the oscillating voltage from the chamber; and   calculating a capacitance of the chamber based on a product of the current output and a sine of the phase angle divided by a product of two times pi times the frequency and the voltage.   
     
     
         2 . The method of  claim 1 , in which the calculating comprises calculating capacitance with an equation of the form:
     C =|( i   T sinΦ)|÷2 πƒV  
   where:
 C≈capacitance; 
 i T ≈total current; 
 Φ≈phase angle between total current and resistor current; 
 ƒ≈frequency; and 
 V≈voltage. 
   
     
     
         3 . The method of  claim 2 , 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.   
     
     
         4 . The method of  claim 3 , 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.   
     
     
         5 . The method of  claim 4 , 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.   
     
     
         6 . The method of  claim 5 , in which the subtracting further comprises calculating an average of the first and second angles. 
     
     
         7 . 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. 
     
     
         8 . An analyte measurement system comprising:
 An analyte test strip including:
 a substrate having a reagent disposed thereon; 
 at least two electrodes proximate the reagent in test chamber; 
   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) determine a phase angle between a current output and the oscillating voltage from the chamber; and 
 (c) calculate a capacitance of the chamber based on a product of the current output and a sine of the phase angle divided by a product of two times pi times the frequency and the voltage. 
 
   
     
     
         9 . An analyte measurement system comprising:
 An analyte test strip including:
 a substrate having a reagent disposed thereon; 
 at least two electrodes proximate the reagent in test chamber; 
   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 such that a percent error in capacitance measurement of the test strip across a range of capacitance as compared to a referential parallel R-C circuit is less than about 3%.