Measurement of Small-current Signals – Hints for System and Cell Design
77
Worse yet, surface contamination often lowers the effective resistivity of good insulators. Water films can be a
real problem, especially on glass.
Shunt capacitance and resistance also occur in the potentiostat itself. The Reference 3000 Potentiostat mode
specifications in Appendix A contain equivalent values for the potentiostat’s
R
shunt
and
C
shunt
. These values can
be measured by an impedance measurement with no cell.
In most cases, the cell’s shunt resistance and capacitance errors are larger than those from the potentiostat.
Hints for System and Cell Design
The following hints may prove helpful.
Faraday Shield
A Faraday shield surrounding your cell is
mandatory
for very low-level measurements. It reduces both current
noise picked up directly on the working electrode and voltage noise picked up by the reference electrode.
A Faraday shield is a conductive enclosure that surrounds the cell. The shield can be constructed from sheet
metal, fine-mesh wire screen, or even conductive paint on plastic. It must be continuous and completely
surround the cell. Don’t forget the areas above and below the cell. All parts of the shield must be electrically
connected. You will need an opening in the shield large enough to allow a cell cable to enter the shield.
The shield
must be electrically connected
to the Reference 3000’s floating-ground terminal.
An additional connection of both the shield and the Reference 3000 floating ground to an earth ground may
also prove helpful.
Avoid External Noise Sources
Try to keep your system away from electrical noise sources. Some of the worst are:
Fluorescent lights
Motors
Radio transmitters
Computers and computer monitors
Try to avoid AC powered or computerized apparatus within your Faraday shield.
Cell Cable Length and Construction
The Reference 3000 is shipped with a 60 cm shielded cell cable. We also offer extended length cables and
unshielded cables as options at extra cost.
Cell cables longer than 1 meter
will
result in degraded instrument performance. Increased noise and decreased
stability both can occur. However, with most cells, the instrument will work acceptably with an extended cell
cable, so our advice is to try it. As a rule, do not attempt to use current-interrupt
IR
-compensation with cell
cables longer than 5 m.
We do not recommend that you use the Reference 3000 with any cables not supplied by Gamry Instruments.
The Reference 3000 cable is not a simple cable like a typical computer cable. The Reference 3000 cable
Only connect the Reference 3000 ground to earth ground if all conductive cell components
are well-isolated from earth ground. A glass cell is usually well isolated. An autoclave is generally not well
isolated.
