How does a potentiostat work?
A brief overview how a potentiostat work with reference to ADUCM355
The block diagram above is simplified, more details can be found in the user guide of ADUCM355 or the datasheet of AD5941.
From the perspective of a chemist a potentiostat is an electro-chemical measurement instrument to analyze chemical reactions in fluids, gases and particulate materials. Where the Emstat Pico device is focused on fluids and corrosion detection. The sensing elements connected to a potentiostat are 3x electrodes called Counter-Electrode (CE), Working-Electrode (WE) and Reference-Electrode (RE). The principle to identify substances in a solution is based on current vs voltage, current vs time, voltage vs time or impedance profiles. Each captured profile is unique for a certain substance.
From the perspective of an electronics engineer it is a circuit maintaining given voltages and measuring the resulting current. In the case of the Emstat Pico this circuitry consists of a dual channel DAC, a control amplifier and a transimpedance amplifier.
The amplifier connected to CE and RE is called in “potentiostat language” a control amplifier (CA). It controls the voltage at CE. By changing V_CE the measured V_RE is changed as well. The negative feedback loop indicates there is a control loop to maintain the voltage between the positive input and the negative input zero volts. Compared to a standard differential amplifier, the output is connected to the CE and the negative input is connected to the RE, so the solution is closing the loop and not a feedback resistor.
The TIA converts an input current into a proportional negative output voltage. It is put in a standard configuration as engineers are used to.
The dual output DAC provides two different voltages: Vbias for the CA positive input and Vzero for the TIA positive input. V_RE follows Vbias and V_WE follows Vzero. In this way the voltage difference between RE and WE is always controlled and measured (figure ADC input stages). The voltage V_RE_WE between RE and WE is called “DC potential range”. The “DC potential range” is always lower than the so called compliance voltage between CE and WE.
So the main task of a potentiostat is the flexible voltage adjustment of V_RE_WE. This is the key to do the different supported measurement techniques. As a result of the V_RE_WE adjustment the CE is adjusted as well. V_CE_WE is not of interest because the voltage is changing continually due to the regulation loop of CA which reacts on changes measured by RE during the chemical reaction.
Important to understand is that the chemical reaction causes the current flow through a redoxreaction (combination of reduction and oxidation), release and absorption of electrons which is depending on the applied V_CE potential. Both the CA and the transimpedance amplifier can source and sink current through their dedicated power supplies which are not shown in the block diagram, reason for mentioning this is that a chemical reaction can cause current flow in both directions.
Oxidation – donating electrons
Reduction – absorbing electrons