USER MANUAL SAGA
Page 36 of 47
Electrode materials
SAGA is a DC-coupled amplifier with a high input range of +/-150 mV. This is more
than sufficient for common measurement setups. The electrode-skin interface will
act as a battery which causes a DC-shift in the signal of several millivolts up to a few
hundred. If all electrodes, including Patient Ground, are made of the same material,
this will not be a problem at all. If materials are mixed (for example gold electrodes
or platinum electrodes are used in combination with Ag/AgCl electrodes, the DC shift
can cause that channels go out of range of the amplifier.
Different electrode materials have different DC characteristics. Some materials may
drift a lot, whereas others are fairly stable over time. Ag/AgCl is known to be a very
DC-stable material and is used in most of the commercially available electrodes for
contacting intact skin.
SAGA is also available in other configurations with a higher input range (up to +/-
600 mV) which allows the use of different materials without saturations. Contact
for information about the availability of these configurations.
Electrode movement artefacts
When cables pull on the electrode, this is visible in the data as a DC-shift, because
of the change in electrode-skin interface. This can be prevented by making a strain
relief loop in the cabling. If for some reason the cable is pulled, the strain relief
prevents that the cable also pulls on the electrode, causing artefacts. Cable
movement in itself is not a problem thanks to SAGA’s active shielding technology.
Rereferencing/Artefacts
If during a measurement an electrode is causing artefacts in the data it may be
necessary to exclude this electrode from the measurement data. SAGA measures
its signals against the average of all connected electrodes. This means that if an
electrode becomes disconnected, this electrode is immediately taken out of the
average calculation. If a channel needs to be excluded from the measurement
ensemble, follow the steps below:
1. Identify the channels that are to be excluded from the measurement
ensemble.
2. In the measurement data processing create a new signal by taking the
average of all electrodes except those that are to be excluded.
3. Subtract the values of (2) from each electrode.
These steps are called rereferencing: a recalculation of the reference signal.
