Q
What is the “half-cell potential” and how does it affect the design of
biopotential measurement front-ends?
A
Biopotential signals are small signals (mV or below) riding on top of a high
voltage level (V or hundreds of mV) at dc or near dc. This slow changing
component is due to what is known as the half-cell potential.
In quite a simplistic way, it could be said that this potential is created
where an electrode (where electrons flow) makes contact with skin or tissue
(where ions flow) and the concentration of charges changes. In other words,
there is a polarization of the skin-electrode interface.
What matters here is that this voltage depends on the material the electrodes
are made of. For instance, the so common Ag-AgCl electrodes are specified at
+0.223V, silver electrodes +0.799V and gold electrodes +1.680V.
This voltage is the main factor that limits the gain of the first stage. A 5V
supply traditional analog front end could not even amplify by two the +1.680V
gold electrodes create. Without early amplification the noise introduced by
this first stage can be as high as the biopotential signal we are aim to
measure.
The AD8232 is fully specified to remove offsets up to ±300mV; its innovative
implementation allows the amplification of the input biopotential signal by a
factor of 100 without saturation of the output (please note the uncommitted
amplifier can add extra gain in next stage if required).
In many cases, ac-coupling is used to remove the offsets to increase the signal
amplification. Alternatively, dc-coupled signal paths can be used at the
expense of a higher resolution ADC.