Impedance Measurement; Bias; Potentiostat/Galvanostat Bandwidth - Solartron Analytical Power Booster System EXTBOOST50V5A User Manual

Booster User Guide

5. Impedance Measurement

5.1

BIAS

For voltage control applications, the DC bias (polarization voltage) is usually
applied relative to the open circuit cell voltage, for example:
±100mV relative to open circuit may be used to charge or discharge the cell
•
during the impedance test.
0V relative to open circuit runs impedance tests at open circuit potential.
•
The DC conditions should be chosen to operate within the limits of the cell, since
cells may overheat or explode if incorrect voltages are applied. The applied AC
voltage should be selected to operate within the linear range of the cell. An
excessively high AC voltage will stimulate non-linearities within the cell, and an
excessively low AC voltage will lead to noisy results. In any case, the peak AC
voltage should not be greater than the open circuit cell voltage, otherwise a
negative voltage will be applied to the cell, leading to possible cell damage.
For current control applications, the DC current polarization may be set to zero to
measure impedance effectively under open circuit conditions, or a particular DC
polarization can be applied to charge or discharge the cell during the impedance
test. Again, the DC conditions should be chosen with care, to avoid overheating
or damaging the cell. The AC current should also be selected according to
requirements which are similar to the voltage control case, with regard to
linearity, stability, noise, and the requirement to avoid driving the cell to a
negative voltage during the impedance test.
5.2

POTENTIOSTAT/GALVANOSTAT BANDWIDTH

The choice of a particular bandwidth setting on the potentiostat, for example
800Hz, does not mean measurements are invalid above 800Hz. The cell alone
determines the relationship between the voltage applied to it and the resulting
current. The voltage subsystem scales the voltage across the cell and the current
subsystem scales the current through the cell. The bandwidth selection has no
effect on either of these systems; it affects only the control loop, which tries to
make the stimulus on the cell the same as the signal applied to the ECI
Polarization Input, scaled by the appropriate scaling factor. Above the loop
bandwidth point, all that happens is that the stimulus to the cell attenuates with
increasing frequency in relation to the required ac level, and for stability reasons a
lower bandwidth may be necessary. The only downside to this is that as the
stimulus attenuates, so do the voltage and current signals. Consequently, these
signals may become noisier at high frequencies, leading to a noisier impedance
plot.
14906001_AA 12-09-07
5-1