5-5
Ru/
Ω
measured RC
time constant/
µ
s
measured Ru/
Ω
error of Ru
measurement/ %
50.3 38 29 -42
100.4 94 92 -8.4
150 146 145 -3.3
200 198 198 -1.0
250 250 249 -0.4
300 302 301 +0.3
347 350 349 +0.6
401 406 404 +0.7
452 360 449 -0.7
Table 5-1. Measured resistance and time constant via exponential extrapolation
(P. He and L.R. Faulkner, Anal. Chem. 58 (1986) 523).
Table 5-1 shows the results of resistance measurements for various dummy cells
using exponential extrapolation. There is very good agreement when the time
constant is greater than 200
µ
s; however, the error in the extrapolation is large if
the time constant is below 100
µ
s, since the current has decayed significantly by
the time the response is sampled. Therefore, the Auto iR Compensation may not
work when the uncompensated resistance is low, and/or the time constant is
small.
Compensation and Circuit Stability Testing
Compensation is achieved by positive feedback into the potentiostat. However,
problems due to circuit instability can arise, even when the degree of
compensation is significantly less than 100%. Therefore, the positive feedback is
applied in increments of the calculated uncompensated resistance: 5% increments
up to 80%, 2% increments from 80% to 90%, and 1% increments thereafter. The
stability of the circuit is tested after each increment. The degree of compensation
is defined by the user (default = 100%).
In the stability test, a 50 mV potential step around Test E is again applied, and the
current data are collected at a rate of 20000 points/sec for 50 ms. A baseline is
measured by collecting data shortly before the potential step is applied, and these
baseline values are subtracted from the potential step data to yield net current
values.
As the percent compensation increases, the current response at first exhibits a
"ringing effect" following exponential decay (Figure 5-4) which would eventually
go into oscillation. The degree of pre-oscillation ringing can quantitated by a term
defined as Overshoot, which the ratio of a minimum (net negative) current value
(I
min
) to a maximum current value (I
max
) expressed as a percentage: that is,
Overshoot = (I
min
/I
max
) x 100. The maximum allowable Overshoot value is
defined by the user (Default = 10%). If the measured Overshoot value is less than
the maximum allowable value, then compensation is continued. If it is greater,
and the desired level of compensation has not yet been achieved, a capacitor is
