eDAQ Picostat, User Manual

Page: 97 / 122

Appendix A — Technical Aspects 93
which is applied to the Counter electrode (CE) in such a way as to
minimise the error voltage.
The QuadStat, and the electrochemical cell to which it is connected,
form a composite feedback control system in which the value of the
Counter Electrode (CE) voltage is controlled in such a way as to make
the Reference voltage with respect to the Working electrode voltage
equal to the desired Command voltage. The current flow (I) in the
Working Electrode (WE) is the quantity being measured.
When the QuadStat is in Standby mode the CE and RE are
disconnected from the cell and connected to an internal dummy cell.
This prevents current flowing through the electrochemical cell and
allows a simple test of circuit operations.
A steady state, or transient, overload condition is indicated when the
error voltage exceeds a small non-zero value. This indicates that the
system could not establish or sustain a balanced state. The Overload
light will be reset when scanning is stopped or when the QuadStat
controls are next accessed.
The RE voltage is measured by a high input impedance amplifier whose
output is summed with the WE potential to provides a low impedance
output to the e-corder unit proportional to the actual potential difference
between the RE and WE.
Overload
Auxiliary
Reference
Working
Online
Dummy Cell
Hi Z
Ref Amp
Gain
Power Amp
Error Voltage
Subtractor
Overload Detector
Front Panel
Back Panel
E In
(Command
voltage)
E Out
(Reference
voltage)
I Out
I
2
C Input
I
2
C Control
Interface
Low–pass Filter
I to V
+
–
I
2
C Output
Shield
Shield
I Offset
E Offset
+
–
+
–
To QuadStat channels 2, 3, and 4
∑
∑
+
–
DAC
DAC
PGA
Figure A–3
Block diagram of the
QuadStat, Channel 1.