Sigma-delta analog-to-digital converter
A potentiostat includes a voltage regulator, a current mirror, a capacitor, a comparator, a current source, and a counter. The voltage regulator maintains a voltage on a working electrode of an electrochemical sensor. The current mirror develops a mirror current that mirrors an input current from the working electrode. The capacitor is alternately charged by the mirror current, causing the capacitor voltage to increase at a rate related to the current's magnitude, and discharged by a control current, causing the capacitor voltage to decrease. The comparator outputs a waveform that includes upward and downward transitions based on the variations of the capacitor voltage. The current source produces the control current based on the waveform. The counter counts the number of upward or downward transitions in the waveform during a predetermined sampling period to produce a digital output. The digital output is representative of the magnitude of the input current.
1. A method, comprising:
receiving, by a current mirror, an input current from a working electrode of an electrochemical sensor, wherein the current mirror comprises a first current path between a voltage supply and the working electrode, a second current path between the voltage supply and a capacitor, a first field-effect transistor and a second field-effect transistor connected in series in the first current path, and a third field-effect transistor and a fourth field-effect transistor connected in series in the second current path;
developing, by the current mirror, a mirror current that mirrors the input current;
providing, by a current source, a control current based on a waveform;
developing, by the capacitor, a capacitor voltage in response to at least the mirror current and control current;
generating, by a comparator, the waveform based on variations of the capacitor voltage relative to a comparison voltage; and
providing, by a counter, a digital output based on a number of times a particular feature in the waveform occurs during a predetermined sampling period.
2. The method of claim 1 , further comprising:
maintaining, by a voltage regulator, a voltage on the working electrode based on a reference voltage.
3. The method of claim 2 , wherein the voltage regulator comprises a differential amplifier having an inverting input connected to the reference voltage, a non-inverting input connected to the working electrode, and an output connected to a gate of the first field-effect transistor and a gate of the third field-effect transistor.
4. The method of claim 1 , wherein developing, by the capacitor, a capacitor voltage in response to at least the current mirror and control current comprises:
developing, by the capacitor, an increasing capacitor voltage during a first period of time; and
developing, by the capacitor, a decreasing capacitor voltage during a second period of time.
5. The method of claim 4 , wherein the increasing capacitor voltage during the first period of time results from the mirror current charging the capacitor, and wherein the decreasing capacitor voltage during a second period of time results from the control current discharging the capacitor.
6. The method of claim 5 , wherein the waveform comprises transitions from a first voltage level to a second voltage and transitions from the second voltage level to the first voltage level, and wherein generating, by the comparator, the waveform based on variations of the capacitor voltage relative to a comparison voltage comprises:
transitioning, by the comparator, from the first voltage level to the second voltage level when the capacitor voltage increases to a level above the comparison voltage; and
transitioning, by the comparator, from the second voltage level to the first voltage level when the capacitor voltage decreases to a level below the comparison voltage.
7. The method of claim 6 , wherein providing, by a current source, a control current based on a waveform comprises:
providing, by the current source, the control current at a first current level when the waveform is at the first voltage level; and
providing, by the current source, the control current at a second current level when the waveform is at the second voltage level.
8. The method of claim 6 , wherein the particular feature in the waveform is one of an upward transition from the first voltage level to the second voltage level or a downward transition from the second voltage level to the first voltage level, and wherein providing, by the counter, a digital output based on a number of times the particular feature in the waveform occurs during a predetermined sampling period comprises:
counting, by the counter, the number of upward or downward transitions occurring in the waveform during the predetermined sampling period.