Low I DC Calibration

Low I DC Calibration fixes a subtle problem in Gamry’s normal calibration procedure. The cause is complex but the following attempts to describe the problem in simple terms.

All voltage-measurement devices draw some input current from the system being measured. The reference lead and the counter sense lead on Gamry Instruments’ potentiostats each have an input current that is typically between 0.2 and 3 pA. This current comes from the circuit point with the lowest impedance. If no current source is available, the measurement over-ranges.

During normal DC Calibration, these input currents are measured as part of the I/E Converter current error. The cell switch is off (with an “off” impedance that is typically tens of TΩ), so the reference and counter sense current is sourced through dummy-cell resistor and the I/E Converter.

When you measure a high-impedance electrochemical cell with the cell switch on, the counter-electrode lead is the lowest impedance source for the reference and counter sense current. The sum of the reference and counter sense currents no longer flows through the I/E converter, so the current measured during a test differs from the calibration by a small amount. The most sensitive current scale on a Reference™ 600 (after I Channel gain) is 0.6 pA, so a 1 pA error is huge.

When you measure EIS on cells with a very high impedance , the EIS optimization routine can be confused by the calibration difference and take several measurements at each frequency before it finds the correct settings. This can use up a lot of time on tests at mHz frequencies.

Low I DC Calibration makes a special measurement of the offset current on one or two sensitive current ranges. The reference and counter sense leads are connected to ground (earth) during this special calibration, so their input currents are not measured as cell current. The offset errors on these ranges overwrite the values stored in normal Instrument Calibration and vice versa, so run the Low I DC Calibration last.

NOTE:Low I DC calibration is only needed on the Reference™ 600, Reference™ 600+, and Reference™ 3000 instruments. Other instruments are not sensitive enough to be confused by the error currents. If you try to run this routine on other instruments (such as an Interface™ 1000), it will run but do nothing.

Run the Low I DC Calibration script by selecting Experiment > Utilities > Low I DC Calibration on the Framework™ menu bar. Please note the connection information in the dialog boxes. This script does not use the resistive dummy cell provided with your system.