Lowering Impedance

Good electrode-skin contact is essential for high-quality EEG/ExG recordings. Impedance reflects how well the electrodes conduct electrical activity from the scalp to the amplifier. Lower and more stable impedance values reduce noise and increase signal reliability. In general, the lower the impedance, the higher the signal quality.

Verify good impedance values before every recording. You can visualize impedance values in Explore Desktop or Explore Mobile. To learn how to stream impedance data via explorepy, see the explorepy documentation.

Impedance values are measured as the sum of the impedance on the REF electrode and each individual channel. For example, if your target impedance is 15 kΩ per channel, aim for approximately 30 kΩ total impedance, because the displayed value includes REF and the channel contribution.

Recommended Impedance Levels

  • For dry electrodes, values below 50–80 kΩ are usually acceptable, although they are typically higher than gel electrode impedances.

  • For gel electrodes, aim for values below 10–20 kΩ.

  • Reference and ground electrodes should always have the lowest possible impedance.

  • Poor reference quality can degrade all channels.

Checking Impedance

  • Use the impedance section in Explore Desktop or Explore Mobile to check impedance values before each recording.

  • In explorepy, run the impedance check via the command line or Python scripts.

  • Correct channels with unstable or excessively high impedance before recording.

  • If you are having trouble identifying which channel you are testing, move a single electrode. This will allow you to identify the corresponding signal.

  • Re-check impedance periodically during long recordings, especially when using dry electrodes.

  • Document impedance values at the start of the session in your logbook for quality control.

General Techniques to Improve Impedance

Always verify that there is good contact between the electrode and the scalp. Ensure that all electrodes are being pulled onto the scalp by the cap and that there are no creases in the cap causing electrodes to float.

A thin, elastic cap placed over the electrode cap can further reduce impedance values. It applies gentle pressure to all electrodes, particularly in occipital positions.

Keep cables and electrode leads tidy and away from power supplies. Ensure that the subject is seated comfortably and instructed to minimize movement.

Gel Electrodes

  1. Begin with the reference electrode. Position the reference electrode and apply gel. Make sure the reference electrode does not move. If it moves, consider using an adhesive medical electrode or an ear clip electrode.

  2. To further improve impedance, gently apply a medical abrasive scrub to the skin, such as Nuprep® or a similar product. Make sure to disinfect the skin after applying the scrub.

  3. Apply conductive gel through the openings at the back of the electrodes using a blunt syringe.

  4. Move the syringe in a circular motion to push hair away from the electrode. This allows the gel to form a conductive bridge between the electrode and the scalp.

  5. Apply more gel as needed, but avoid gel pools, as they can introduce cross-talk between channels.

  6. Good gel electrode impedances are approximately 10 kΩ.

Dry Electrodes

  1. Use flat electrodes for bare skin areas, such as the mastoid and forehead. Ensure that there are no stray hairs between the electrode and the skin.

  2. Use brush electrodes for areas covered in hair. For thick hair, XL brush electrodes can penetrate the hair more effectively.

  3. Twist brush electrodes back and forth while gently pushing them onto the scalp. This helps the bristles penetrate the hair. Ideally, all bristles should touch the scalp.

  4. Good dry electrode impedances are below 100 kΩ for flat electrodes placed on bare skin and below 300 kΩ for brush electrodes placed on hair.

  5. Dry electrode impedance can often be lowered by applying a small amount of gel or paste to the REF electrode or by applying a skin scrub before electrode placement.

Environmental Factors

  • Reduce sources of electromagnetic interference, such as nearby smartphones, Wi-Fi routers, fluorescent lights, or old electronic devices.

  • Keep recording equipment and electrode cables away from power supplies where possible.

  • Minimize subject movement during impedance checks and recordings.

  • Replace worn or damaged electrodes promptly to avoid persistent impedance issues.

Notes

  • Unlike raw ExG data, impedance measurements are fully online and are therefore more prone to Bluetooth-related errors.

  • Bluetooth uses the same frequency band as Wi-Fi and ISM devices, which can introduce interference.

  • In the lower-left corner of Explore Desktop, black text indicates whether the connection is stable during impedance measurement. If the connection is unstable, consider the impedance measurement unreliable.

  • If there is too much noise, impedance measurements may be incorrect. Reduce noisy signals before relying on the displayed impedance values.