The Electrometer is Differential.
Because the working electrode is not at virtual ground a second input has been added to the electrometer. (See the blue
arrow.) This input is called the Sense, Working
Sense, or Reference 2 input by various manufacturers. The
electrometer output reflects only the difference between its
inputs. Both inputs must be high impedance inputs if small currents are to
The I/E Converter amplifier must only report the iR drop across the
current sense resistor. It is is not involved in the control of the
working electrode and it does not have to be a high current amplifier. The
frequency response of this style of I/E converter can be faster than the
"active" design because of this. Its frequency response
does not have to be sacrificed for potentiostat stability since the I/E
converter is not involved in the "control loop."
Because the electrometer is differential, a potentiostat
of this design can be used to control the voltage across a membrane
or across the interface between two immiscible liquids. In this
application two reference electrodes are used. One is connected to the
Reference input, and the other (on the other side of the membrane or
interface) is connected to the Working Sense input.
The Common Mode Rejection Ratio (CMRR) of the differential electrometer
can have a profound influence on the stability of this
potentiostat. There are two feedback paths through the differential
electrometer. The path through the reference electrode is a net negative
feedback path if you follow it from the reference electrode to the
counter electrode via the electrometer and control amplifier. This path
gives stability. The path through the Wrk Sense
input, however, is a positive feedback
path and is destabilizing.
The overall 'gain' of the control amplifier is higher with this scheme.
This is another way of saying that the control amplifier must work harder
to keep the working-reference voltage correct. Not only must it overcome
the counter-reference resistance, but the voltage drop across the current
measuring resistor as well. In many commercial designs, the voltage drop
across this resistor is 100 mV or less, even at 'full scale'
current. The differential amplifier often amplifies the voltage
level (to a nominal 1V at full scale current) as well as buffering it.