HEKA PG 310, Руководство пользователя

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5.2 Galvanostatic Mode 43
External Input, are provided as a voltage, whereby a voltage magnitude of
10 V means a cell current of 100 percent of the selected current range.
Note: in the galvanostatic mode the output of the differen-
tial amplifier also includes a contribution from uncompensated
resistance. Compensation of this potential error is performed
by IR compensation with positive feedback as described in the
section above.
5.2.1 IR Compensation
Whenever currents are passing through the electrochemical cell, there is
always a potential control error
IR
u
because of the uncompensated resis-
tance R
u
. If a cathodic current flows, the true working electrode potential
will be less negative than the nominal value. The opposite holds for an
anodic current. Usually, this error is small, or can be made insignificant by
a proper choice of the cell parameters. As long as this error is proportional
to the cell current, it can be compensated by using a signal of equal mag-
nitude but of opposite polarity. The magnitude of the compensation signal
is determined by the setting of the R
u
compensation control on the current
amplifier (positive feedback compensation). In the PG 300 series potentio-
stat/galvanostat, a voltage compensation based on this positive feedback
compensation scheme is implemented. When
IR Compensation is enabled
in the POTPULSE program, a fraction of the voltage (opposite in sign)
developed by the I-U converter or resistance cascade in the current mea-
suring circuit is fed back through a digital-to-analog converter (DAC) as
an additional input to the control amplifier. This internal signal is effective
at the working electrode terminal.
The working electrode potential (vs. reference electrode) differs from the
desired value defined by the sum of the software and external settings by
the control error I(R
u
-R
compensation
).
In galvanostatic operation the differential amplifier is not part of the control
circuit, and the voltage drop error due to uncompensated resistance should
only be corrected, if the cell potential is monitored. IR compensation will
act as an additional voltage to the
U-Cell Monitor ; it is determined by the
R
compensation
setting and the output signal from the current amplifier.
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