Potentiostat with offset calibration
A rail-to-rail potentiostat may require an offset current in order to support a bidirectional work electrode current at a work electrode. This offset current may improve measurements of the work electrode current made a dual-slope analog-to-digital converter, especially when the work electrode current is small, but can also lead to inaccuracies (e.g., due to a temperature coefficient) if it is not properly calibrated. Accordingly, bidirectional potentiostat is disclosed that can be configured in a normal configuration for measurement of a work electrode current or a calibration configuration for measurement (i.e., calibration) of an offset current. The reconfigurability allows calibrations to be taken as needed, on a schedule, or as specified by a user. The reconfigurability can also allow for maintaining a work electrode voltage and a work electrode current during calibration so that an electrochemical experiment using a cell coupled to the bidirectional potentiostat is unaffected by the calibration.
1. A method for measuring a work electrode current, the method comprising:
generating, using a potentiostat coupled to a work electrode, a first output current, the first output current including a work electrode current component and an offset current component, the offset current component generated by an offset current source of the potentiostat;
converting the first output current to a first digital word;
modifying the first digital word using a calibration factor stored in a memory to obtain a measurement of the work electrode current;
detecting a calibration event; and
performing a calibration to measure the offset current component by:
reconfiguring the potentiostat for the calibration;
generating, using the reconfigured potentiostat, a second output current, the second output current being a measurement of the offset current component;
converting the second output current to a second digital word; and
updating the calibration factor stored in the memory according to the second digital word.
2. The method for measuring a work electrode current according to claim 1 , wherein the calibration event corresponds to a change in an operating condition of the potentiostat.
3. The method for measuring a work electrode current according to claim 1 , wherein the calibration event corresponds to a schedule.
4. The method for measuring a work electrode current according to claim 1 , wherein the calibration event corresponds to a user input.
5. The method for measuring the work electrode current according to claim 1 , wherein performing the calibration includes:
maintaining a work electrode voltage and a work electrode current at the work electrode while performing the calibration.
6. The method for measuring a work electrode current according to claim 1 , wherein the calibration factor corresponds to a digital measurement of the offset current component generated by the offset current source.
7. The method for measuring a work electrode current according to claim 1 , wherein the reconfiguring includes adjusting circuitry of a front-end portion of the potentiostat to measure the offset current component generated by the offset current source without altering a work electrode voltage of the potentiostat.
8. The method for measuring a work electrode current according to claim 1 , wherein converting the first output current to a first digital word and converting the second output current to a second digital word is performed using a dual-slope analog-to-digital converter.
9. The method for measuring a work electrode current according to claim 8 , wherein:
the first digital word corresponds to a first number of clock cycles taken to charge a capacitor of the dual-slope analog-to-digital converter using the work electrode current component and the offset current component; and
the second digital word corresponds to a second number of clock cycles taken to charge the capacitor of the dual-slope analog-to-digital converter using the offset current component.
10. The method for measuring a work electrode current according to claim 9 , wherein the modifying the first digital word using a calibration factor stored in a memory to obtain a measurement of the work electrode current includes:
subtracting the second number of clock cycles from the first number of clock cycles to obtain a third number of clock cycles taken to charge the capacitor of the dual-slope analog-to-digital converter using the work electrode current component.
11. A method for configuring a potentiostat coupled to a cell, the method including:
coupling a front-end portion of the potentiostat to a work electrode of the cell, the front-end portion including a first feedback amplifier and a replica feedback amplifier that are substantially identical;
configuring the potentiostat in a normal configuration for measuring a work-electrode current, wherein the configuring the potentiostat in the normal configuration includes:
coupling the first feedback amplifier to a sensing and digitizing portion of the potentiostat; and
decoupling the replica feedback amplifier from the sensing and digitizing portion of the potentiostat;
triggering a calibration of an offset current, the offset current added to the work-electrode current by the potentiostat; and
configuring the potentiostat in a calibration configuration for measuring the offset current, wherein the configuring the potentiostat in the calibration configuration includes:
decoupling the first feedback amplifier from the sensing and digitizing portion of the potentiostat; and
coupling the replica feedback amplifier to the sensing and digitizing portion of the potentiostat.
12. The method according to claim 11 , wherein measuring the work-electrode current includes:
generating a first measurement of a charging period of a capacitor and a second measurement of a discharging period of the capacitor to digitize the work-electrode current.
13. The method according to claim 11 , wherein triggering the calibration of the offset current includes detecting a calibration event.
14. The method according to claim 11 , wherein:
configuring the potentiostat in the normal configuration further includes:
coupling an offset current source to the work electrode and the first feedback amplifier; and
coupling a replica offset current source to the replica feedback amplifier; and
configuring the potentiostat in the calibration configuration further includes:
coupling the offset current source to the replica feedback amplifier; and
coupling the replica offset current source to the work electrode and the first feedback amplifier.
15. The method according to claim 14 , wherein:
configuring the potentiostat in the normal configuration further includes:
maintaining a work electrode voltage and the work-electrode current at the work electrode of the cell using the first feedback amplifier and the offset current source; and
configuring the potentiostat in the calibration configuration further includes:
maintaining the work electrode voltage and the work-electrode current at the work electrode of the cell using the first feedback amplifier and the replica offset current source.
16. The method according to claim 14 , wherein:
configuring the potentiostat in the normal configuration further includes:
receiving a first output current including a work electrode current component and an offset current component at the sensing and digitizing portion; and
configuring the potentiostat in the calibration configuration further includes:
receiving a second output current including the offset current component at the sensing and digitizing portion.
17. The method according to claim 16 , wherein:
configuring the potentiostat in the normal configuration further includes:
configuring the sensing and digitizing portion to use a calibration factor to remove the offset current component from the first output current in order to generate a measurement of the work-electrode current; and
configuring the potentiostat in the calibration configuration further includes:
create/update the calibration factor corresponding to the offset current component.
18. The method according to claim 11 , wherein the potentiostat further includes:
a current-conveyer portion coupled between the front-end portion and the sensing and digitizing portion, the current-conveyer portion including a first current mirror and a second current mirror.
19. The method according to claim 18 , wherein the first current mirror and the second current mirror are each a cascode current mirror.
20. The method according to claim 18 , wherein the first current mirror and the second current mirror each include a chopper.
21. A method for calibrating a potentiostat coupled to a cell, the method including:
detecting a calibration event, the calibration event corresponding to a change in an offset current generated by an offset current source of the potentiostat;
configuring, in response to the calibration event, the potentiostat in a calibration configuration;
measuring the offset current at an output of a digitizer of the potentiostat in the calibration configuration;
determining a calibration factor corresponding to the offset current; and
storing the calibration factor for use by the potentiostat in a normal configuration to remove the offset current from the output of the digitizer of the potentiostat in the normal configuration to obtain a measurement of a work-electrode current.
22. The method according to claim 21 , wherein the calibration event includes a change in an operating condition of the potentiostat.
23. The method according to claim 21 , wherein the configuring, in response to the calibration event, the potentiostat includes enabling circuits of the potentiostat to measure the offset current without altering a work electrode voltage of the potentiostat.