•
GPIB and Serial
mput/output ports
®
control circuitry
to integrate all these
facilities.
Although FRA connections
are shown
in the diagram for
completeness, the
SI1287
does not need one to make dc measurements.
DVM
SI1287 has two independent DVMs which have a
’5
x 9s’ output. The
DVMs
can be switched
to
measure
a
variety of different parameters
independently, and
these are indicated
in
Figure
1.1,
thus:
(are-bi
^
POLARIZATION CONTROL
Polarization of the cell is
a
combination of
dc, swept dc
and
an external voltage,
which
normally comes from a Frequency Response Analyzer (FRA). Internal
generators within the SI1287 generate an accurate and stable
dc
voltage
(POL)
which
is added to the external (ac) signal from the FRA, to give IPOL. This
signal is used to control the current through the cell, or the voltage
across the
reference
electrodes, depending on whether the ECI is in galvanostat (i.e., current
controlled) or potentiostat (voltage controlled) mode.
Feedback from the relevant cell parameter is used to control the polarization.
In
addition, the bandwidth
of the
feedback mechanism
can be selected by the user to
prevent unwanted oscillations from occurring. The polarization signal
is also
modified when IR compensation
is
used, either
by
adding
a compensation
voltage,
or by interrupting the polarization to the cell.
VOLTAGE
MEASUREMENT
The voltage across the
reference
electrodes (ARE) is measured
by a
differential
amplifier, and
this
voltage
is used to
control
the polarization when the SI12S7 is
in potentiostat mode.
In order to reject any
unwanted
steady dc level on the cell, a stable programmable
voltage, generated
within SI1287, can be subtracted from the measured voltage.
If sampled IR compensation is being used, the resultant voltage is passed through
a sample-and-hold
circuit. The signal
(ARE-Bi) is then buffered,
optionally
filtered,
and output to one channel of the FRA.
CURRENT MEASUREMENT
The
current from the working electrode is measured by passing it through
a
user-selectable resistor; this generates
a
voltage proportional
to
the
current which
is then measured
by a differential amplifier.
This allows
the working electrode to
be floating, and not connected to the instrument ground. If sampled IR
compensation is invoked, the output of the differential amplifier is
passed through
a
sample-and-hold
circuit, to yield I. A programmable rejection voltage,
representing a steady current through the cell which is to be offset, can
be
subtracted from the measured voltage to give (I-Bi) before it
is buffered,
optionally filtered
and output as a voltage (proportional to the current in the cell)
to the FRA.
RESULTS
STORE
The
History
file
is capable of storing up to
450 results,
depending on the number
of instrument set-ups stored
in
memory (each stored set-up reduces this number
by
16).
CSB/SI1287 User Guide/issue
AA 1.5
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