How can I obtain the best quality data with SAGA?

This article provides tips for obtaining optimal quality data.

Placement of device

Mains interference (50/60 Hz) is coming from external sources. This can be many things. It is a common mistake to state that battery powered devices are not susceptible to mains interference and devices powered from mains are. Mains interference can get into the measurement system via multiple ways, for example if the device is placed on a metal table with other electrical equipment on it.

Optimal placement of systems will increase the quality of the acquired data. The ideal measurement setup is on a non-conducting (wood) or well-earthed table. Mains cables and mains power adapters should be placed on the floor or at least not on the same table as the measurement system.

Patient Ground connection

From all electrodes that are connected for a measurement, the Patient Ground electrode is the most important one. TMSi recommends to use a wetted wristband, or to clean/prepare the skin as good as possible for optimal skin contact when using electrode patches as Patient Ground electrode. Higher Patient Ground electrode impedance may cause more mains interference in all of the measurement electrodes.

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 electrode materials without amplifier channels going out of range. Contact sales@tmsi.com 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 the cable from also pulling 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. In Average Reference mode 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 signal of (2) from each electrode.

These steps are called rereferencing: a recalculation of the reference signal.